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Magmatism and Ore Deposit Formation in SW Pacific Island Arcs

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MAGMATISM AND ORE DEPOSIT FORMATI ON IN SW PACIFIC ISLAND ARCS By GEORGE DIMITROV KAMENOV A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLOR IDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY UNIVERSITY OF FLORIDA 2004

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Copyright 2004 by George Dimitrov Kamenov

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iii ACKNOWLEDGMENTS I wish to thank the members of my committee for their support, patience and guidance during the project. This disserta tion would not be possible without the insights, expert guidance, and support of Dr s. Michael Perfit and Paul Mueller. Dr. Ann Heatherington and Howard Scher provided invaluable help during the clean lab sample preparation and TIMS analyses. I am grateful to Drs. David Foster, Joseph Delfino, and Ian Jonasson for providing insights and reviewing this work. I am grateful to Drs. Michael Perfit, Brent McInnes, Mark Hannington, and Peter Herzig, and the crew of the RV SONNE for obtaining the samples from the TLTF area. Dr. David Foster provided the bi otite Ar-Ar data on Tubaf seamount. The accomplishment of this research would not be possible without the major and trace element analyses at the Canadian Geologica l Survey under the supe rvision of Dr. Ian Jonasson. I am grateful to Rahul Chopra fo r separating the xenolith samples from the Tubaf lavas. I am thankful to Tom Bisley for providing technical assistance with the microprobe analyses at the Florida Intern ational University. Field and laboratory investigations were supported by an NSF OCE-9403773 grant awarded to Dr. Michael Perfit and a GSA 7430-03 student gr ant. This study would not be possible without the NSF ARI grant 96-01872 for the purchase of the Nu-plasma MC-ICPMS. Many thanks go to Dr. Jamie Williams for his invaluable assistance during the set up of the MC-ICP-MS lab. I am grat eful for the fina ncial support by the

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iv University of Florida Alumni Fellowship dur ing the course of this study. This work benefited from the expertise provided by Drs. Ray Russo, and Phil Neuhoff. Many thanks go to Ray Thomas and Kevin Hartl fo r their invaluable t echnical support. The staff of the geology department, Ron, Mary, a nd Jodie, have been very helpful during my study in the University of Florida. I w ould like to thank all of my friends in the Department of Geological Sciences for th e friendship and sup port throughout the 4 years of graduate study. Finall y, I would like to thank my wife, Katrin, for her love, patience, and encouragement during this study.

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v TABLE OF CONTENTS page ACKNOWLEDGMENTS.................................................................................................iii LIST OF TABLES............................................................................................................vii LIST OF FIGURES.........................................................................................................viii ABSTRACT....................................................................................................................... xi CHAPTER 1 INTRODUCTION........................................................................................................1 2 OPTIMIZATION OF MIXED LEADTHALLIUM SOLUTIONS FOR HIGH PRECISION ISOTOPIC ANALYSES BY MC-ICP-MS............................................6 Introduction................................................................................................................... 6 Analytical Procedures...................................................................................................7 Results........................................................................................................................ ...9 Wet Plasma Experiments.......................................................................................9 Dry Plasma Experiments.......................................................................................9 Discussion...................................................................................................................15 Accuracy and Precision of the Pb and Tl Isotope Measurements.......................15 Production of Tl3+................................................................................................16 Mass Discrimination Behavior of Pb and Tl.......................................................18 Conclusions.................................................................................................................22 3 HIGH-PRECISION PB ISOTOPE MEASUREMENTS REVEAL MAGMA RECHARGE AS A MECHANISM FOR ORE DEPOSIT FORMATION: EXAMPLES FROM LIHIR ISLAND AND CONICAL SEAMOUNT, PAPUA NEW GUINEA...........................................................................................................24 Introduction.................................................................................................................24 Geological Settings.....................................................................................................27 Samples and Analytical Methods...............................................................................30 Results........................................................................................................................ .34 Discussion...................................................................................................................39 Physical Constraints on Magmatism and Pb Isotopic Variations in the Volcanic Rocks................................................................................................................39 Phase Chemical Constraints on Petrogenesis......................................................42

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vi Pb and Sr Isotopic Variations in th e Conical and Lihir Mineralizations.............45 Mechanism for Ore Formation............................................................................50 Conclusions.................................................................................................................52 4 DECIPHERING MANTLE AND CRUSTA L CONTROLS IN AN ISLAND ARC ENVIRONMENT: A SR, ND, AND PB ISOTOPIC STUDY OF SW PACIFIC ISLAND ARCS AND SUB-ARC XENOLITHS.......................................................55 Introduction.................................................................................................................55 Geological Settings.....................................................................................................56 Samples and Analytical Methods...............................................................................59 Results........................................................................................................................ .63 Discussion...................................................................................................................80 Major and Trace Element Variations in Seamount and Lihir Lavas...................80 Major and Trace Element Variations in Mafic and Ultramafic Xenoliths..........85 Sr and Nd Isotopic Variations in Xenolith and Lava Samples............................87 Pb Isotopic Variations in the Mafic and Ultramafic Xenoliths...........................89 Indian Ocean-Type Mantle Versus Australian Subcontinental Mantle Incorporation in the Mantle Wedge Beneath TLTF........................................92 Relationships between Compositions of Xenoliths and Regional Volcanism ...........95 Model for the Alkaline Magmatism in the TLTF Area............................................101 5 SUMMARY AND CONCLUSIONS.......................................................................112 6 FUTURE RESEARCH.............................................................................................117 APPENDIX A MC-ICP-MS ANALYSES OF NBS 981 LEAD ISOTOPIC STANDARD MIXED WITH QCD ICP-MS THALLIUM ST ANDARD. TABLE INCLUDES WET PLASMA AND DSN FRESH AND AGED, EXPOSED TO LIGHT MIXED PB-TL SOLUTIONS ANALYSES......................................................................................119 B LEAD EXTRACTION PROCEDURE (MODI FICATION OF THE TIMS HBR PB SEPARATION OF MANHES ET AL., (1978), SIMPLIFIED AND ADAPTED FOR MC-ICP-MS MEASUREMENTS OF PB ISOTOPES BY TL SPIKING......126 C MIXING CALCULATIONS WITH 207PB/204PB BETWEEN SEDIMENTARY AND MAFIC SOURCES.........................................................................................127 LIST OF REFERENCES.................................................................................................128 BIOGRAPHICAL SKETCH...........................................................................................139

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vii LIST OF TABLES Table page 2-1 Measured SRM 981 Pb and QCD Tl isotopic compositions and Pb/Tl intensity ratios............................................................................................................9 3-1 Location, general description, and strontium and lead isotopic compositions of samples in the study area..........................................................................................31 3-2 Microprobe analyses on seamount s clinopyroxene phenocrysts............................40 4-1 Major and trace element data for lavas and xenoliths..............................................64 4-2 Pb, Sr, and Nd isotopic analyses of lava and xenolith samples...............................78

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viii LIST OF FIGURES Figure page 1-1 Regional map of the Western Pacific O cean, showing some of the Western Pacific Gold Province deposits (in italic)...............................................................................2 2-1 (A) SRM 981 206Pb/204Pb DSN measurements of fresh and aged, exposed to sunlight mixtures. (B) QCD 205Tl DSN measurements of fresh and aged, exposed to sunlight mixtures....................................................................................11 2-2 (A) Changes in measured Pb/Tl intensity ratios in mixed solutions with time and exposure to sunlight. All solutions prepared with Pb/Tl=6 (B) Changes in measured 205Tl in mixed solutions with time and exposure to sunlight..................................13 2-3 Chelex-100 resin elution be havior of Pb and Tl in fresh and aged, exposed to sunlight mixed solutions...........................................................................................14 2-4 (A) Fractionation factors obtained for lead and thallium in Tl3+-bearing solutions, fresh mixtures, and wet plasma experi ments. (B) Comparison of fractionation factors for several Tl3+-solutions analyses collected w ithin a few days with the true Pb and Tl fractionation f actor line (dashed line)......................................................19 3-1 Regional map of Northwest Papaua Ne w Guinea showing the Tabar-Lihir-TangaFeni island chain, modified after Taylor, 1979........................................................28 3-2 206Pb/204Pb vs Pb concentrations of Coni cal seamount lavas and highand lowtemperature mineralized zones.................................................................................35 3-4 Comparison of Pb isotopic compositions of TLTF volcanic rocks with sedimentary and mafic xenoliths, an d Pacific MORB and sediments.....................38 3-5 Representative compositional changes from core to rim in Conical clinopyroxene shown in figure 3-6...........................................................................42 3-6 Photomicrograph of Conical seamount clinopyroxene. Numbers correspond to the analysis numbers pr esented in Table 3-2...........................................................44 3-7 87Sr/86Sr vs Sr concentrations for mineralized samples in comparison with Conical fresh lavas and sediments...........................................................................46

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ix 3-8 MgO vs Cl content of seamounts a nd Lihir island. Arrows show general direction of changes during magma degassing, fractionation, and fractionation accompanied with degassing....................................................................................51 3-9 Cartoon depicting the inferre d ore formation mechanism.......................................53 4-1 Regional map of the study area, modified after Hall (2001). Se e chapter 3 for more detailed map of the TLTF area.................................................................................57 4-2 Classification of the C onical, Edison, Tubaf, and Li hir lavas based on their SiO2 vs Na2O+K2O concentrations.......................................................................................72 4-3 Comparison of MgO vs CaO, Fe2O3, Na2O, and Al2O3 between Lihir and seamount lavas.........................................................................................................73 4-4 Comparison of MgO vs Ni, V, Ba, a nd Rb between Lihir and seamount lavas.......74 4-5 Normalized REE patterns of TLTF lava s and mafic and ultramafic xenoliths........75 4-6 Primitive Mantle normalized trace element patterns of TLTF lavas and mafic xenoliths...................................................................................................................76 4-7 Plot CaO/Al2O3 vs MgO indicating clinopyroxene as a major fractionation phase. Symbols and data sources are th e same as in figure 4-2..........................................81 4-9 Primitive Mantle normalized trace elemen t patterns of mafic xenoliths compared with MORB, altered MORB, and oceanic gabbros..................................................84 4-10 Primitive Mantle normalized trace elem ent patterns of ultramafic xenoliths compared to TLTF lavas..........................................................................................86 4-11 Nd vs Sr isotopic compositions of the mafic and ultramafic xenoliths relative to possible source in the area........................................................................................88 4-12 Pb isotopic compositions of mafic and u ltramafic xenoliths relative to possible sources in the area....................................................................................................90 4-13 Pb isotopic compositions of xenoliths relative to Indian, Pacific MORB, and Eastern and North Easter n Australia basalts............................................................94 4-14 Nd vs Sr isotopic compositions for island arc and back-arc basins lavas in the region, compared to Indian and P acific MORB and Pacific sediments...................96 4-15 Pb isotopic compositions of lavas from TLTF, New Britain, Solomon Islands, Manus and Woodlark back-arc basins.....................................................................98 4-14 Cartoon summarizing the suggested origin of the TLTF volcanism......................102

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x 4-17 Comparison of K2O/Ba vs Ba/La between lavas and mafic xenoliths and pelagic and carbonate sediments.........................................................................................105 4-18 Interaction of slab melt (SCHARM) with ultramafic mantle and formation of the TLTF magmas........................................................................................................108

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xi Abstract of Dissertation Pres ented to the Graduate School of the University of Florida in Partial Fulfillment of the Requirements for the Degree of Doctor of Philosophy MAGMATISM AND ORE DEPOSIT FORMATI ON IN SW PACIFIC ISLAND ARCS By George Dimitrov Kamenov December 2004 Chair: Michael R. Perfit Major Department: Geological Sciences Majority of the richest base and precious metal deposits on Earth are located in the Circum-Pacific region and are found either in a close proximity or within subductionrelated volcano-magmatic complexes. It is not clear, however, why some of the volcanic centers contain ore deposits but others do not. This st udy utilizes a novel method for high-precision Pb isotopic measurements by multi-collector-ICP-MS and reveals a genetical connection between the ore-form ation and magmatism in the Tabar-LihirTanga-Feni (TLTF) island arc, SW Pacific. Th e developed model sugge sts that injection of a volatile-rich magma into an evolving magma body near the surface is the triggering event that ultimately results in the ore mineralization. Isotope data for mantle xenoliths sugg est that three possible end-members, including Pacific and Indian mantle, and Pacific sediments, can control the chemistry of the magmas in the region. Comparison between the isotopic compos itions of lavas, and crustal and mantle xenoliths indicate that the incompatible elements in the TLTF

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xii magmas, including a major part of the ore metals, were ultimately derived from a subducted oceanic slab with Pacific mantle a ffinity. Regional and loca l isotopic trends in the volcanic rocks in the region indicate that subducted oceanic sl abs overall control the composition of the island-arc magmas. Once the contribution from the subducting slab decreases, incorporation of a mantle component with Indian affinity can be identified in the isotopic composition of th e lavas in the region.

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1 CHAPTER 1 INTRODUCTION The most significant geological pheno mena on Earth, such as powerful earthquakes, widespread volcanism, and island -arcs and continents growth, occur in the zones of subduction. In addition, these zones are of great economic importance because most of the worlds richest base and preci ous metal deposits are associated with subduction-related volcanism. A number of subduction zones and back-arc basins are located in the western Pacific (Fig. 1-1) a nd a subject of this study is the Tabar-LihirTanga-Feni (TLTF) island chain, part of th e Bismarck Archipelago. The TLTF islands are part of the Western P acific gold province (Fig. 1-1), which extends from Japan, through the Philippines, Papua New Guinea, So lomon Islands, and Fiji to New Zealand (Sillitoe 1989). The formation of the Bismarck Archipelago is related to the subduction of the Pacific plate under the I ndo-Australian plate and most of the arcs are dominated by calc-alkaline volcanism, although the TLTF ch ain is dominated by alkaline volcanoes (Wallace et al. 1983). It is generally believed that the widespread magmatic activity in the island arcs is due to lowering the mantle wedge peridotite melting point as a result of dehydration of the subducted slab (Tatsumi 1989). Ho wever, some fundamental geological processes, such as what controls the chem istry of the magmas and why there are ore deposits associated with some of the volcan ic systems but none with others, remain unclear. The TLTF island chain provides an outstanding opportunity to investigate these processes in detail because a number of xenoliths were recovered from a submarine

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2 Figure 1-1. Regional map of the Western Paci fic Ocean, showing some of the Western Pacific Gold Province depos its (in italic). Note that only about 1/3 of the known deposits are shown on the map for clarity purposes, for more details see Sillitoe (1989). Deposits are ch aracterized by porphyry and epithermal gold mineralizations and are young, ranging from Middle Miocene to Quaternary (17-0 Ma). The study area (enc losed in the dashed field) includes Bismarck Archipelago (New Britain, New Ireland, TLTF, Bougainville), the Solomon Islands, and Manus and Woodlar k back-arc basins, more detailed maps are available in Chapters 3 and 4.

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3 volcano, located on the flanks of Lihir island, hos t of one of the richest gold deposits on Earth (Moyle et al. 1990). The xenoliths provide us with rare samples from the crust and mantle underlying the islands, and detailed ge ochemical and isotopic study of xenoliths, lavas, and ores will allow us to look at de tails of the processes responsible for the magmatism and ore deposit formation in the region. A number of existing and novel techniques, such as petrographic observations, major and trace element analyses, Sr and Nd isotope data, and high-precision Pb isotope measurements, are utilized throughout this study. Comparison of existing data from the area (Wallace et al. 1983; Kennedy et al. 1990a,b; McInnes and Cameron 1994; Stracke and Hegner 1998; Mller et al. 2001, 2003) and new major and trace element analyses of lavas allow us to look in detail at the nature of the magmas in the area and processes that led to their formati on. Previous studies i ndicate that the lavas erupted on the surface are not common for isla nd arc settings because of their high alkalinity, and was hypothesized that they are products of adiabatic decompression melting of the subduction-modified mantle wedge (McInnes and Cameron 1994). The mantle wedge beneath TLTF islands is com posed of depleted peridotites, probably a residue from a partial melting event at Midocean ridge settings (McInnes et al. 2001, Gregoire et al. 2001, Franz et al. 2002). Visual and chemical evidences, however, indicated that the ultramafic xenoliths experienced strong metasomatism by fluids released probably from the subducted Pacific slab (McInnes et al. 2001, Gregoire et al. 2001, Franz et al. 2002). Detailed Sr, N d, and Pb isotopic comparison between the metasomatized xenoliths and TLTF lavas, theref ore, will provide evid ence if the magmas in the region are a result of partial melting of the metasomatized mantle wedge.

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4 Sr, Nd, and Pb isotopic comparison is a ro utine technique utili zed in geochemical studies (Faure 1986) however, some recent studies indicate that significant analytical errors can be generated during the tradit ional thermal ionization mass-spectrometry (TIMS) measurements of Pb isotopes ( Woodhead et al. 1995, Kamenov et al. 2003). The problems stem from uncompensated Pb isotopic fractionation behavior during the TIMS analyses. For example, during the Sr isotopic analyses a non-radiogenic pair (86Sr/88Sr) is used to correct for the fractio nation behavior of Sr during the TIMS analyses. Such correction is not possible during the Pb analyses due to the existence of only one non-radiogenic lead isotope (204Pb). A novel technique utilizing 205Tl/203Tl to serve as a non-radiogenic pair for mass-bias correction during Pb isotope measurements was adopted with the devel opment of the multi-collector ICP-MS (Rehkmper and Halliday 1998). Although some studies have questioned the technique (Thirwall, 2002), our recent experiments demonstrated that highl y precise Pb isotope measurements can be obtained by preventi ng the thallium photoxidation to 3+ state (Kamenov et al. 2004a,b). In addition to deciphering the relationships between magmas and xenoliths, Pb isotopic measurements provide valuable in formation for the formation of the ore mineralizations in the area. Lead is the onl y ore metal other than Os which shows large natural isotopic variability and has similar ge ochemical behavior to other ore metals, such as zinc, copper, and silver. Therefore, compar ison of ore lead isotope ratios with those of host rocks and other units in a particular mi ning district will suggest and/or rule out possible ore metal sources. Combining the hi gh-precision Pb isotopic measurements with

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5 petrographic and geochemical data will lead to development of a model integrating the magmatism and the ore-formation in the area. On a larger scale, the observations and the wealth of data collected during the course of this study provide valuable information on the composition of the mantle in the area. Local and regional trends observed in the arc lavas from the region can be related to a particular source or sources, which can be either a subduction-derived oceanic crust and/or sediment component, or the local ma ntle. This will shed new light on the island arc petrogenesis in the region and will further contribute to our understanding of the subduction zone magmatism and associated ore deposit formation.

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6 CHAPTER 2 OPTIMIZATION OF MIXED LEAD-T HALLIUM SOLUTIONS FOR HIGH PRECISION ISOTOPIC ANALYSES BY MC-ICP-MS Introduction The introduction of the MC-ICP-MS has le d to a novel technique for Pb isotope ratio measurements that utilizes 205Tl/203Tl as an internal standard to correct for massdependant fractionation in mixed Pb-Tl solu tions ( Belshaw et al. 1998, Rehkmper and Halliday 1999, Collerson et al. 2002). C onversely, precise Tl isotope measurements can be obtained by mixing a Tl-bearing soluti on with Pb of known isotopic composition (e.g., 208Pb/206Pb) as an internal standard to correct for mass-dependant fractionation of Tl (Rehkmper and Halliday 1999). Thirwall ( 2002), however, suggested that the Pb-Tl isotope measurements determined by MC-IC P-MS are subject to large analytical uncertainties and that this t echnique is not suitable for obt aining highly precise data for Pb or Tl isotopes. Thirwalls contributi on constituted part of an on-going debate concerning the reliability of Pb isotope analyses corrected by Tl-normalization (Rehkmper and Mezger 200,; White et al. 2000, Woodhead 2002). For example, Rehkmper and Mezger (2000) adjusted the 205Tl/203Tl normalization ratio on a daily basis to obtain results closes t to the DS (double spike) TIMS SRM 981 values of Todt et al. (1996). The need for such adjustments im plies inconsistent mass-fractionation of Pb and Tl during the analytical procedure. In addition, Thirwall (2002) and Collerson et al. (2002) observed that Tl intensity was lower th an expected in some mixed Pb-Tl solutions and suggested that it resulted from unspecifi ed and anomalous behavior of thallium in

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7 solution. Rehkmper and Halliday (1998) also observed anomalous behavior of Tl in mixed Pb-Tl solutions and suggested that it re sulted from variations in the complexation of Tl that could be avoided if the solutions were allowed to age for a period of several days. In this chapter I report results of an investigation of the interaction between Pb and Tl in mixed, dilute nitric and hydroch loric acid solutions and its effects on the elemental and isotopic ratio measurements. I show that very precise MC-ICP-MS isotope analyses of lead and thallium can be obtaine d using this system by controlling the analyte conditions to minimize the formation of Tl3+. Analytical Procedures Solutions were prepared in Teflon bottles or vials by mixing SRM 981 Pb standard solution with commercially available ICP Tl stock solutions (QCD Analysts, USA and Aldrich, USA). The mixed solutions were prep ared with Pb-Tl elemental ratios ranging from 2 to 6 and were diluted to 100 to 150 ppb Pb and 25 to 35 Pb ppb for the wet and dry plasma experiments, respectively (Appendix A) Most of the experiments were conducted on mixed solutions diluted in either 0.6 2, or 5% HNO3 (percent dilution of concentrated Optima-brand HNO3); severa l experiments were conducted in 2% HCl solutions and in 2% HNO3 solutions with tr aces of HCl. A Nu-Plasma MC-ICP-MS (Nu Instruments, UK) in the Department of Geologi cal Sciences at the University of Florida was used for this study. The instrument de scription, typical operating conditions, and analytical protocol during Pb-Tl analysis ar e reported in Belshaw et al. (1998). Sample and standard solutions were aspirated eith er through a Nu Instruments desolvating nebulizer (DSN-100) (dry plasma mode) or di rectly into the plas ma source through a Micromist nebulizer with GE spray chamber (wet plasma mode). Measured uptake rate

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8 for both sample introduction methods was about 100 L min-1. The instrument settings were carefully tuned to maximi ze the signal intensities on a daily basis. Preamplifier gain calibrations were run before the beginning of each analytical session. Typical sensitivities for Pb were about 300 V ppm-1 and about 40 V ppm-1 for dry and wet plasma modes, respectively. Sampler and skimmer cones (N i) were thoroughly cleaned after 3 or 4 analytical sessions. All analys es reported in this paper we re conducted in static mode by directing 202Hg on low-2, 203Tl on low1, 204Pb on Axial, 205Tl on high-1, 206Pb on high-2, 207Pb on high-3, and 208Pb on high-4 (a ll Faraday detectors). The measured 204Pb beam was corrected for isobari c interference from 204Hg using 204Hg/202Hg=0.2290 (before mass-bias correction) although always negligible. Data were acquired in blocks of 20 ratios w ith 10 s integration times. Background measurements of 30 s durations preceded each block. All of the reported SRM 981 ratios were normalized using the exponential law for mass-bias correction and 205Tl/203Tl=2.3875 after Belshaw et al. (1998) All Tl isotope analyses were normalized using the exponential law and SRM 981 208Pb/206Pb=2.1664. This ratio represents the averag e of all fresh SRM 981 runs conducted during this study and is identical to the average of all MC-ICPMS SRM 981 208Pb/206Pb analyses reported by Reuer et al. (2003). Tl -notation values are calcu lated relative to NIST 997 205Tl/203Tl=2.3871, following Rehkmper et al (2002). Note that the 205Tl/203Tl (2.3875) used throughout these experiments is 1.68 epsilon units greater than the value of NIST 997 (205Tl/203Tl=2.3871), which is why our average 205Tl results for QCD are greater than zero (Table 2-1).

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9 Results Wet Plasma Experiments Forty two wet plasma analyses of SRM 981 mixed with QCD Tl were conducted between April and June, 2003 and gave the following results: 206Pb/204Pb=16.9369 (+/-0.0039, 2 ), 207Pb/204Pb=15.4904 (+/-0.0034, 2 ), and 208Pb/204Pb=36.6949 (+/-0.0087, 2 ) (Table 2-1). The reported 2-sigma error (2 ) is the external precision, calculated on the basis of these 42 runs. The internal pr ecision obtained during a single analysis is usually lower, on the order of +/0.001 to 0.002 (2 ). Measured 205Tl in the QCD standard during the wet plasma experiments was 0.9 (+/-1.2, 2 ) (Table 2-1). Measured Pb and Tl beam intensities for these solutio ns showed intensity ratios similar to the elemental ratios in the original mixtures (Table 2-1, Appendix A). Table 2-1. Measured SRM 981 Pb, QCD Tl isotopic compositions, Pb/Tl intensity ratiosexperiment Pb/Tl (mixed) Pb/Tl (measured) 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 205Tl (QCD) wet plasma (fresh and Tl3+ solutions) n=42 2 to 6 2 to 6 16.9369 (+/-0.0039, 2 ) 15.4904 (+/-0.0034, 2 ) 36.6949 (+/-0.0087, 2 ) 0.9 (+/-1.2, 2 ) dry plasma (only fresh mixtures) n=29 2 to 6 2 to 6 16.9373 (+/-0.0011, 2 ) 15.4907 (+/-0.0012, 2 ) 36.6935 (+/-0.0039, 2 ) 1.5 (+/-0.8, 2 ) dry plasma (only Tl3+ solutions) n=39 2 to 6 variable up to 36 16.921 (+/-0.039 2 ) 15.469 (+/-0.052 2 ) 36.630 (+/-0.160 2 ) from .9 to +30.1 dry plasma (fresh mix) 20 20 16.9375 (+/-0.0018, 2 ) 15.4910 (+/-0.0018, 2 ) 36.6979 (+/-0.0052, 2 ) 0.4 (+/-1.0, 2 ) dry plasma (fresh mix) 40 40 16.9359 (+/-0.0020, 2 ) 15.4886 (+/-0.0020, 2 ) 36.6908 (+/-0.0062, 2 ) 1.4 (+/-1.0, 2 ) Dry Plasma Experiments Experiments using the DSN-100 were also conducted between April 2003 and January 2004. During the initial experiments, Pb and Tl from single stock solutions were mixed in 2% HNO3. Solutions were then measured from 48 hours to 60 days after preparation (mixing), as suggested by Rehk mper and Halliday (1999). Contrary to the

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10 results of Rehkmper and Halliday (1999), however, our initial results with these solutions revealed poor overall precision and accuracy with 206Pb/204Pb=16.921 (+/-0.039 2 ), 207Pb/204Pb=15.469 (+/-0.052 2 ), and 208Pb/204Pb=36.630 (+/-0.160 2 ) (Table 2-1, Fig. 2-1A). Similarly, using SRM 981 as a fi xed Pb isotopic composition for mass bias corrections of the Tl isot opes yielded relatively large 205Tl variations (i.e., from .9 to +30.1) for our QCD standard (Table 2-1, Fig. 2-1B). Comparison of measured Pb and Tl beam in tensities for these solutions over this period showed essentially constant Pb ion beam intensities, but lower Tl intensities (i.e., higher Pb:Tl intensity ratios compared to the known elemental ratios in the original mixtures) (Fig. 2-2A). Measured intensiti es in Pb-only (30 ppb) and Tl-only (5 ppb) control solutions did not show changes in intensity during the experiments, indicating that the decrease in the Tl signal intensity occurred only in aged, mixed solutions. In addition, the wash out times were consistently longer for the aged compared to the fresh mixtures. Condensate (i.e., wa ste solution) from the DSN collected during analyses of the aged solutions showed lower Pb-Tl inte nsity ratios, from 0.6 to 1, indicating a net increase in Tl over Pb in the waste (i.e., during the combined desolvation and washout processes). Overall, it appear ed that extending the interval between mixing the Pb and Tl solutions and analysis produced significant, but irregular Pb-Tl elemental decoupling along with poorer precision and accuracy in th e isotope ratio measurements for both Pb and Tl. To address this problem, we prepared new mixed solutions (also in 2% HNO3) and analyzed them immediately (<1 h) after mixing. Analyses of these mixed solutions (n=29) yielded consistent and highly precise results with 206Pb/204Pb=16.9373 (+/-0.0011,

PAGE 23

11 2 ), 207Pb/204Pb=15.4907 (+/-0.0012, 2 ), and 208Pb/204Pb=36.6935 (+/-0.0039, 2 ) (Table 2-1, Fig. 2-1A). The average of the measured 205Tl in these fresh solutions Figure 2-1. (A) SRM 981 206Pb/204Pb DSN measur ements of fresh and aged, exposed to sunlight mixtures. (B) QCD 205Tl DSN measurements of fresh and aged, exposed to sunlight mixtures (for mo re details see the text, Table 2-1, and Appendix A).

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12 prepared with the QCD standa rd and SRM 981 was 1.5 (+/-0.8, 2 ) (Table 2-1, Fig. 2-1B). In addition, measured Pb and Tl ion beam intensities always reflected those calculated for the mixtures. Analyses of fresh 2%HNO3 mixtures prepared with high Pb/Tl ratios yielded Pb and Tl isotopic values within error with the rest of the fresh mixtures, suggesting that the observed discre pancies during the analyses of the aged mixtures is not a result of the decrease in the Tl signal (i.e., hi gh Pb/Tl, Table 2-1). Several experiments were also cond ucted with 2%HCl and a range of HNO3 concentrations. Aging of mixed Pb:Tl solutions in 2%HCl, 5%HNO3, and 2%HNO3+0.05%HCl showed similar behavior to the 2%HNO3 experiments, i.e., decoupling of the Pb-Tl elemental abundan ces and imprecise and inaccurate isotope ratios (Fig. 2-2A and B). Us ing solutions with a lower HNO3 concentration (0.6%) showed less Pb-Tl elemental decoupling and more precise reproducibility of isotope ratios over time compared to the solutions with higher HNO3 concentrations (Fig. 2-2A and B). Subsequent testing of the same mi xed solutions noted above (0.6 to 5% HNO3) after several months of storage, howev er, showed that the solutions reverted to behave as fresh mixtures; that is, they did not show the Pb-Tl elemental decoupling and poor precision of Tl and Pb isotope ratio measurements that had ch aracterized them previ ously (Fig. 2-2A and B). The source of the freshening of the solutions was ultimately traced to the fact that the solutions were stored in cabinets and were not exposed to sunlight, suggesting a photochemical effect. To test this hypothesis, the solutions were again moved to their initial storage lo cation, which allowed exposure to di rect sunlight. The subsequent analyses showed the strong decoupling in the Pb/Tl ratios and the relatively imprecise Pb

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13 Figure 2-2. (A) Changes in measured Pb/Tl inte nsity ratios in mixed solutions with time and exposure to sunlight. All solutions prepared with Pb/Tl=6 (B) Changes in measured 205Tl in mixed solutions with time and exposure to sunlight. Note that all (with the exception of 2%HCl) solutions revert to behave as fresh mixtures after prolonged storage without exposure to sunlight.

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14 and Tl isotope ratio measurements again characterized these solutions. Repeated experiments confirmed that exposure to sunlight plays a major role in the interaction of Pb and Tl in the solutions (Fig. 2-2A and B). The nature of this reaction was cons trained using chromatographic columns (Chelex-100 resin) designed (Lin and Nriagu, 1999) to separate Tl+ from Tl3+. The Tl from the fresh mixtures and Pb from both fr esh and aged, exposed to sunlight mixtures elute similarly in 2% HNO3. Tl from solutions containing Pb and exposed to sunlight, however, eluted much later, and ultim ately required the use of 10% and 20% HNO3 for complete elution (Fig. 2-3). These results strongly suggest that Tl+ is oxidized to Tl3+ in solutions containing Pb when exposed to sunl ight. The reaction is reversible when the solutions are stored without exposure to sunlight. Figure 2-3. Chelex-100 resin elutio n behavior of Pb and Tl in fresh and aged, exposed to sunlight mixed solutions, for more details see the text.

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15 Discussion Accuracy and Precision of the Pb and Tl Isotope Measurements Data for SRM 981 wet plasma experime nts provide a significant improvement over unspiked Pb isotopic analyses by TIMS, but are not generally as precise as double spike results. Analyses, however, complete d using the DSN and fresh Pb-Tl mixtures revealed excellent precisi on, equivalent or even bette r than commonly reported for double spike methods via TIMS (Todt et al. 1996, Thirwall 2000). The generally lower precision of wet plasma vs dry plasma expe riments is probably related to less stable plasma conditions during wet plasma m ode. The absolute value of the SRM 981, however, is dependant on the 205Tl/203Tl ratio used for normalization (Belshaw et al. 1998, Collerson et al. 2002). All of the reporte d SRM 981 data in this paper, summarized in Table 2-1, were normalized using 205Tl/203Tl=2.3875, adapted from Belshaw et al. (1998). The values observed during wet plasma and DSN fresh mixtures analyses (Table 2-1) are in excellent agreement with the DS TIMS data of Todt et al. (1996). In terms of Tl isotopes, Rehkmper and Halliday (1999) measured the 205Tl/203Tl ratios in commercially available Alfa/AES AR, Aldrich, and NBS 997 Tl solutions and concluded that they all have indistingui shable isotopic compositions. They reported 205Tl=-1.38 (+/-1.48 3 ) for Aldrich and 205Tl=0.19 (+/-1.28 3 ) for Alfa/AESAR solutions, relative to NIST 997 205Tl/203Tl=2.3871. Fresh mixture measurements of our aliquots of commercially available QCD and Aldrich Tl solutions yielded 205Tl=1.5 (+/-0.8 2 ) and 205Tl=0.1 (+/-0.8 2 ) respectively, values that are within error of the values measured by Rehkamper and Halliday (1999) Conversely, the measured Tl and Pb

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16 isotopic compositions in the aged Pb-Tl mixtur es reveal large discrepancies (Fig. 2-1A and 2-1B) that I attribute to the presence of Tl3+ in these solutions. Production of Tl3+ Thermodynamic considerations indicate that only monovalent thallium is stable in the solutions measured for th is study (Lin and Nriagu 1999) and this expectation was confirmed by our experiments on the elution be havior of fresh mixtures with the Chelex100 resin (Fig. 2-3). The observed change in th e thallium elution behavior for the mixed solutions exposed to sunlight in the laboratory, however, indicates that Tl+ was oxidized to Tl3+ (Fig. 2-3). The fact that measured ion beam intensities in unmixed Pb and Tl control solutions did not exhibi t this behavior suggests that the photo-oxidation of Tl was catalyzed by the presence of Pb. Photoelect rochemical reactions can be driven in an uphill direction by exposure to ultraviolet radiation (Bard 1980). In particular, Switzer et al. (1982) demonstrated that Tl+ can be oxidized to Tl3+ in a liquid pho tovoltaic cell by exposure to UV light in the presen ce of semiconductors such as TiO2, WO3, and ZnO. It has been proposed that ultrav iolet irradiation induces a Tl+ Tl3+ electron transfer by production of a short-lived Tl2+ chain carrier (Stranks a nd Yandell 1969). Because Tl+ was converted to Tl3+ upon exposure to solar UV radiati on and only in the presence of Pb2+ during our experiments, it may be that Pb2+ acts as a short-lived electron carrier in a possible one or two stage reaction, e.g.: Tl++Pb2+ Tl3++Pb or Tl++2Pb2+ Tl3++2Pb+ It is difficult to confirm the formation of either of the two lead species noted above due to the very low concen trations and the fact that Pb and Pb+ are not stable in nitric acid and should be rapidly oxidized to Pb2+. Consequently, the ultimate storage of

PAGE 29

17 the electrons liberated during the oxidation is likely to be accommodated by small changes in the NO2 -/NO3 ratio (i.e., amount of nitric aci d reduction). This hypothesis is supported by the observation that after storag e without exposure to sunlight, all of the solutions returned to thei r initial state (Fig. 2-2A and B); that is, they behave as Tl+-bearing mixtures. The exception is the 2%HCl solution, probably because Tl3+ is stabilized in presence of Cl (Cotton et al. 1999). In a ddition, it appears that af ter aging and exposure to sunlight, HNO3 solutions with added traces of Cl tend to produce less variable 205Tl/203Tl isotopic compositions, although very different than the 205Tl defined by the fresh and wet plasma experiments (Fig. 2-2B and 2-4B). The association between Tl3+ and the generation of increased Pb/Tl ratios, increased wash-out time for Tl, and imprecise isotopic ratios in the post-desolvation fraction of the analyte suggests that Tl3+ does not behave in the same manner as Tl+ or Pb2+ during the desolvation. The di fferential behavior of these two Tl species may be related to more than one factor or process. For example, Tl+ is much more soluble than Tl3+ and Tl3+ (or complex ions/molecules containing Tl3+) may precipitate or adsorb onto surfaces inside the DSN more readily than Tl+ during the concentration that occurs as the sample is evaporated. Tl3+ is more likely to be hydrol yzed and form colloids than Tl+, even at pH 1 to 2.5 (Cotton et al. 1999). If formed, these colloidal particles would be small enough to be transported downstream from the spray chamber and may adhere, therefore, to a variety of surfaces inside the DSN, including the membrane. Similarly, Tl3+ (or a complex ion containing Tl3+) may interact differently with the desolvation membrane. Although composed of relatively inert fluorocarbon, the membrane has a

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18 relatively large surface area compared to the other components of the DSN and is, therefore, a likely zone of retention. Mass Discrimination Behavior of Pb and Tl The observed elemental fractionation th at occurs during desolvation of Tl3+bearing solutions appears to be accompanie d by fractionation of Tl isotopes. Figure 2-4 shows the fractionation factors derived for Tl and Pb for the experiments reported here. The calculated Pb and Tl frac tionation factors for fresh mixt ures and wet plasma show very similar behavior and define a common line (indisti nguishable slopes) indicating identical mass fractionation behavior for the Pb and Tl isotopes (Fig. 2-4A). Data from the wet plasma experiments incl ude data generated from both Tl+ and Tl3+-bearing solutions (Table 2-1), which also lie on this common fractionation line. This suggests that even though thallium is oxidized to Tl3+, it does not experience differential mass-bias behavior in the plasma comp ared to Pb nor does the oxida tion process itself appear to cause isotopic fractionation. No decoupling in the Pb/Tl ratios was observed in wet plasma mode (Table 2-1) and furtherm ore, scanning at lower masses (68.3, 102.5, etc.) failed to detect any peaks where Tl3+ or Tl2+ would be present, suggesting that Tl3+ is efficiently transformed to Tl+ in the plasma. Fractionation factors for the Tl3+-bearing solutions analyzed by DSN, however, deviate from the factors calculated for the Tl+-bearing solutions and wet plasma experiments, which suggests mass-dependant fractionation of Tl during desolvation. The variations in the isotopic composition of Tl produced during desolvation can be represented by fractionation lines offset fr om, but parallel to, the fractionation line defined by the fresh and wet plasma experime nts (Fig. 2-4B). These results indicate

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19 Figure 2-4. (A) Fractionati on factors obtained for lead and thallium in Tl3+-bearing solutions, fresh mixtures, and wet plasma experiments. The data for wet and DSN fresh mixtures show very similar behavior and lie on a line expected for identical mass-bias behavior experien ced by Pb and Tl (dashed line). (B) Comparison of fractionation factors for several Tl3+-solutions analyses collected within a few days with the tr ue Pb and Tl fractio nation factor line (dashed line). Solid lines represent calculated fractionation factors using different Tl isotopic compositions (the offset from the wet and fresh analyses is shown in epsilon notation).

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20 that Pb and Tl continue to experience simila r mass-bias behavior, but that a net change in the isotopic composition of thallium in the desolvated Tl3+-bearing solutions occurred. Changes in the isotopic composition of Tl are not, however, consistent. For example, a 0.6% HNO3 Pb-Tl solution exhibited relatively small changes in 205Tl, but at the same time, a 5%HNO3 Pb-Tl solution exhibited a range of almost 10 205Tl units over a several days time period (Fig. 2-2B and 2-4B). Over the same period a 2%HNO3+traces of HCl Pb-Tl solution showed relatively stable 205Tl, although the values were very different than the value obtained for the fres h mixtures (Fig. 2-4B ). These observations suggest that the measured 205Tl in the Tl3+-bearing solutions is influenced by several factors, including the acid matrix, the condi tions during desolvati on, and the dose of UV (sunlight) received (Fig. 2-2 and 2-4). The exact process or processes which produce differential mass fractionation of Tl+3-bearing solutions during desolvation is not clear. Mass-dependant isotopic fractionation during redox reac tions involving Fe has been suggested by Zhu et al. (2002). Nielsen et al. (2004), however, demonstrated that mass-fractionation of Tl does not occur during laboratory handling that includes ion exchange chromatography and oxidation of Tl+ to Tl3+. Our experiments also show no evidence for mass fractionation during photo-oxidation reaction. It seems most likely, therefore, that the observed isotopic fractionation (enrichment in 205Tl) occurs during desolvat ion. The fact that the analyte is enriched in the heavier isotope suggests that adsorption (Rehkmper et al. 2002) is not the controlling process, though we cannot rule out its occurrence. A more likely process that must be consider ed for the preferential removal of 203Tl from the analyte during desolvation is diffusion rela ted mass-dependant fractionation. During the

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21 diffusion process the lighter isotopes have higher average velocities and so diffuse proportionally faster than the heavier isotop es through a porous membrane (Chopin et al. 2002). This process, known for almost a centu ry, is commercially utilized in uranium enrichment plants using UF6. The isotope enrichment is accomplished in a series of diffusion devises consisting of cells divided by a porous me mbrane (commonly made of fluorocarbon), which can maintain differen tial gas pressure (Chopin et al. 2002). The basic construction of gaseous diffusion devices, therefore, is similar to the desolvators utilized in this and other studi es referred to herein. By an alogy with the enrichment of uranium, it may be that the 203Tl3+ ions, molecules, or complex ions (e.g., hydrated Tl3+) pass through the membrane at a greater rate than the 205Tl3+ species as they pass along the length of the desolvating membrane, wh ich results in an enrichment in 205Tl in the desolvated analyte. The greater rate of transf er of Tl under these conditions may also lead to greater interaction with the membrane to the point that more Tl is ultimately held on the membrane, which results in the longer washout times observed. The net effect of these two processes (adsorption and diffusive fractionation) appears to favor diffusion as the primary process that determines the isotopic composition of the analyte. Fractionation during diffusion, however, shoul d affect all species in the analyte stream to some degree, including Pb. If, however, the fractionation behavior of the elements of interest is governed by a common fractionation law during the passage through the DSN and in the plasma, then pr ecise isotopic ratios can be obtained by applying a single correction fact or (e.g., exponential law), as is the case for our fresh PbTl mixtures. If the fractionation behavior of the elements of intere st does not result from processes adequately described by a single fr actionation law, then such a correction will

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22 not result in precise isotopic measurements as is observed in our aged, exposed to sunlight mixtures. At this stage, however, it is not clear why Tl3+ species experience different fractionati on behavior than Tl+ and Pb2+. Possible explanations include 1) Tl3+ ions have lower hydration en thalphy (-4184) compared to Tl+ (-326) and Pb2+(-1480) (Burgess 1978) and, therefore, form stronger bonds with wate r molecules (or with Ar, N) that may persist through the desolvation pro cess and interact with the membrane to a greater extend; 2) by analogy w ith the use of the volatile UF6 for U-enrichment, it may be that the formed Tl3+ species are more volatile than Tl+ species and, therefore, are more susceptible to gaseous diffusion enrichment; a nd 3) it is also possi ble that the emerging from the DSN Tl3+-bearing dry aerosols experience di fferent mass-bias than the Tl+bearing dry aerosols in the plasma, although the wet plasma experiments argue against such possibility. Fractionation may occur also during the vaporization of solvent, which leads to the formation of compounds and/or complex molecules that may preferentially incorporate heavier isotopes. In addition, the so formed compounds, molecules, and/or hydrous ions may have different vapor pressure s, and consequently, one species (e.g., Tl) may be lost preferentially to another (e.g., Pb ) and also isotopically fractionated. Further experiments are required to test these hypotheses. Conclusions High-precision Pb isotopic measurem ents provide valuable geochemical information, however, significant analytical errors can be introduced during MC-ICP-MS isotopic analyses of desolvated Pb and Tl-b earing solutions because different species appear to exhibit different behavior during the liquid-vapor transition and subsequent desolvation process inside the DSN. My e xperiments demonstrate that significant Tl

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23 isotopic variations, equivalent to the range observed in na tural samples by Rehkmper et al. (2002) can be generated by varying experimental conditions, as reported here. The interaction of Pb and Tl is ther modynamically unexpected and appears to result from photoelectric driven oxidation of Tl+ to Tl3+. The reaction occurs only after the addition of Pb, which appears to act as an intermediate (short-lived) electron acceptor. Although no detailed experiments were conducted to constrain the kinetics of the reaction in detail, this phenomenon was observed in mixe d solutions exposed to sunlight for more than several hours. The de monstrated production of Tl3+ in solution, which is related to a complex combination of factors (e.g., differe nces in the acid matrix and molarity, desolvation conditions, and durat ion of UV light exposure), gr eatly affects the precision and accuracy of Pb and Tl isotopic measurements. Spiking samples intended for Pb isotope measurements with Tl just before th e analysis and/or preventing the mixed Pb-Tl solutions from being exposed to sunlight ( UV light) are critical for achieving the most precise Pb isotope ratio measurements fr om desolvated nitric acid solutions.

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24 CHAPTER 3 HIGH-PRECISION PB ISOTOPE ME ASUREMENTS REVEAL MAGMA RECHARGE AS A MECHANISM FOR ORE DEPOSIT FORMATION: EXAMPLES FROM LIHIR ISLAND AND CONICA L SEAMOUNT, PAPUA NEW GUINEA. Introduction Ore deposits associated with subducti on-related volcanism are among the most important sources of base and precious metals in the world. Although many of these deposits have received extensive study, seve ral major questions re garding their genesis remain unanswered. Such major questions in clude why base metal mineralization is associated with some magmatic systems, but not with others? What is the mechanism of incorporation of ore metals in to hydrothermal fluids, have these components been derived from active sub-volcanic melts as a result of magma degassing, or have they been extracted from the host rocks via meteoric or seawater hydrotherm al leaching driven by thermal energy from magmas? If the latter is the case, magmatic heat will drive the hydrothermal circulation and, consequently, is th e most critical factor for the formation of an ore deposit. Relatively few magmatic complexes, however, contain world-class deposits, suggesting that other mechanisms must also play important roles in the formation of ore deposits associated with ma gmatic systems. Such mechanisms include efficient release of metal-b earing fluids from solidifying magmas (Hedenquist and Lowenstern 1994). Suitable conditions for magm atic fluid saturation and release can be reached during magma cooling accompanied by crystallization or during recharge by volatile-rich magm a (Sillitoe 1997). Studies s uggest that during degassing, magma chambers maintain rela tively oxidizing conditions that i nhibit formation of immiscible

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25 sulfide liquids that can sequester ore meta ls (McInnes and Evans 1996) and thereby enhance the ore formation potential of the ex solving fluids. Providing clear evidence that the exsolution of ore-bearing fluids from magmas is a major mechanism for ore formation remains elusive. Studies utilizing stable isotope tracers, such as oxygen and sulfur, have shown that the magmatic signature in the ore-forming fluid is often obscured due to mixing with meteoric fluids in the near surface environment and so considerable disagreement exists concerning which of the fl uids (i.e., magmatic or meteoric) transport and deposit ore metals (Bodnar 1995). Another important isotopic tracer that can be directly related to the ore metal sources are the lead isotopes. Lead is a common ore metal and shares similar geochemical behavior with other ore metals such as Ag, Cu, and Zn. Numerous studies have utilized Pb isotopes to investigate th e relationship between ores and their possible sources (Doe and Delevaux 1972, T ilton et al. 1981, Macfarlane et al. 1990, Richards et al. 1991, Kamenov et al. 2002). Historically, the use of Pb isotopes suffered from the fact that due to isotopic fractiona tion, lead isotopic data obtained by thermal ionization mass spectrometry (TIMS) lack th e precision and accuracy readily obtained for Sr, Nd, Hf, and Os isotopic analyses. For ex ample, for Sr a non-radiogenic isotope pair, usually 86Sr and 88Sr, is used to correct for the obse rved isotopic fractionation and thus highly precise data can be obtai ned. A similar correction is not possible for lead because there is only one non-radiogenic isotope (204Pb). So, in the conven tional TIMS analysis, a set of samples is corrected using the fractionation measured in a well-characterized standard, usually NBS 981, by assuming similar mass-bias behavior in the standard and the unknowns. It has been shown, however, that samples may exhibit different

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26 fractionation behavior than the pure standard s due to the presence of other metals and varying amounts of sample loaded and the ex tent of evaporation for individual samples (Woodhead et al. 1995, Kamenov et al. 200 3). In order to deal with this problem some studies have adopted double (DS) and tr iple (TS) spike met hods to correct for instrumental mass bias and ha ve successfully produced high -precision Pb isotopic data (Todt et al. 1996, Abouchami et al. 2000, Thirwall 2000). Recent studies have demonstrated that important geochemical co rrelations and trends among Pb isotopes, not readily observable in the traditional TIMS-der ived data, can be revealed by utilizing high-precision Pb isotopic analyses (Abouchami et al. 2000, Woodhead 2002). With the development of the MC-ICP-MS a novel technique for Pb isotopic measurements was adopted that uses 205Tl/203Tl as an internal standard to correct for mass-dependant fractionation of Pb ( Rehkmper and Hallida y 1998, Collerson et al. 2002, Woodhead 2002). Although some studies have questioned th e applicability of Tl isotopes for instrumental mass fractiona tion corrections of Pb isotopes (Thirwall 2002, Baker et al. 2004), our recent experiments have s hown that highly precise Pb isotopic ratios, equivalent to the DS and TS TIMS Pb isotope measurements, can be obtained using MC-ICP-MS. The key issue in this application is to control the interaction of Pb and Tl in solution so that thallium doe s not oxidize to the 3+ state (Kamenov et al. 2004). Here I present high-precision Pb isot opic data (measured by MC-ICP-MS) from subaerial and submarine alkaline lavas and gol d-bearing ores from Papua-New Guinea to address the origin of these valuable mine ral deposits. A number of large epithermal gold and copper deposits genetically associated with alkaline ro cks have recen tly attracted

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27 considerable interest (Sil litoe, 2002), including the giant Ladolam gold deposit in the Luise caldera, located on Li hir Island, Papua-New Guinea (M oyle et al. 1990; Rytuba et al. 1993; Mller et al. 2001, 2003). A group of seamounts (Edis Daughter, Edison, Tubaf, and Conical) were r ecently discovered on the flanks of Lihir Island and the presence of gol d mineralization was discovere d at the summit of Conical seamount in 1998 (Herzig et al. 1999). In th is Chapter, I investig ate the sources and processes that may have led to the developm ent of the deposits at both Lihir Island and Conical seamount. Geological Settings Lihir island is one of a group of islands in Papua-New Gu inea that lie northeast of the islands of New Britain a nd New Ireland in the Southwest Pacific. The Tabar, Lihir, Tanga, and Feni (TLTF) island chain is locat ed in the former fore-arc region of New Ireland, part of the Bismarck archipelago (Fig. 3-1). Volcanism on New Ireland and on most of the islands in the archipelago is dom inantly calc-alkaline to high K/calcalkaline, and was generated as a result of subduction of the Pacific plate under the Indo-Australian plate along the Manus-Kilinailau trench (Johnson 1979). Subduction ceased about 10 Ma ago, when the thick and relatively buoyant Ontong-Java plateau collided with the subduction zone (Coleman and Kroenke 1981). Subsequent to the collision, regional relocation of stress caused re versal of the subduction dir ection, formation of the northnorhtwest facing New Britain-San Cristobal trenc h, initiation of back-a rc spreading in the Manus Basin at about 3.5 Ma, and developm ent of several microplates (Taylor 1979). The TLTF island chain extends for about 250 km parallel to the presently inactive Manus-Kilinailau trench (Fig. 3-1). The isla nds are equally spaced at about 75 km, and

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28 seismic reflection surveys have delineated horst and sediment-filled grabens bounding the chain, suggesting extension a nd crustal thinning (Exon et al. 1986). Volcanism in the Figure 3-1. Regional map of Northwest Papa ua New Guinea showing the Tabar-LihirTanga-Feni island chain, modified af ter Taylor, 1979. The inset shows the location of Lihir Island and the seamounts, where samples for this study were recovered. TLTF area began on Simberi island, about 3.7 Ma ago (Johnson et al., 1976, McInnes, 1992), coeval with the initiati on of back-arc spreading in the Manus Basin, and migrated southward to Feni island (2300 y), as the island of New Br itain was transported to the southeast. Taylor (1979) suggest s that the local extension an d volcanism are related to the opening of the Manus back-arc spreading cente r. TLTF islands are characterized mainly by high-K calc-alkaline rocks with similar to arc lavas trace element and isotopic characteristics (Wallace et al. 1983; Kennedy et al. 1990a,b; Stracke and Hegner 1998). These lavas were probably derived from a number of different parental magmas

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29 generated in a mantle source enriched in incompatible elements during earlier periods of subduction of the Pacific plate (McInnes a nd Cameron 1994, Stracke and Hegner 1998). During a cruise of the RV SONNE in 1994 (SONNE 94) Edis Daughter, Edison, TUBAF, and Conical volcanic seamounts were discovered on the flanks of Lihir Island (Fig. 3-1, Herzig et al. 1994). Sampling (dredge and TV-assisted grab) and observations using a remote camera indicated the seamounts we re volcanically active in the recent past and that Conical seamount had recently been hydrothermally active (Herzig et al. 1994). Ultramafic, gabbroic, basaltic and sedimentary xenoliths we re recovered in submarine lavas from Tubaf and Edison seamounts. The abundance of xenoliths in Tubaf lavas is remarkable; one TV-grab sample weighing 200 kg contained more than 70 cm-sized xenoliths (McInnes et al. 2001) During a cruise of the RV SONNE in 1998 (SO-133) the existence of epithermal-style gold mineralizat ion at Conical seamount was confirmed by extensive sampling (Herzig et al. 1998). The seamount is a submarine volcanic cone underlain by a thick (about 5 km ) layer of Miocene to Recent sediments (McInnes et al. 2001). The recovered lavas ar e porphyritic vesicular tr achybasalts to basaltic trachyandesites (Mller et al. 2003). The minera lization is confined to the top of the seamount along a main eruptive fissure and sh ows zonal distribution (Herzig et al., 1999). Three styles of mineralization can be iden tified (Petersen et al 2002): (a) intensely altered clay-silica rich zone containing semi-m assive and stockwork-like pyrite, (b) goldrich, disseminated, polymetallic sulfides in feldspathic and siliceous veins, and (c) lowtemperature, late-stage, fracture-hosted As-S b mineralization. Here I have divided the mineralized samples analyzed in this study into two groups: 1) hi gh-temperature goldrich samples (including (a) and (b)) and 2) lo w-temperature samples (c) (Table 3-1).

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30 Samples and Analytical Methods General descriptions and locations of sa mples analyzed in the study are presented in Table 3-1. Fresh volcanic rocks and altere d volcanic rock samples exhibiting various degrees of mineralization were recovered from the seamounts during the 1998 R/V Sonne cruise. Conical seamount trachybasalts are composed of about 20 to 25% clinopyroxene (Cpx), 5% plagioclase (Plag), and rare ol ivine and phlogopite phenocrysts embedded in a slightly vesicular, fine-gra ined groundmass which consists of glass, plagioclase, clinopyroxene, spinel, and apatite microcryst s (Mller et al. 2003). The clinopyroxene phenocrysts are large (up to about 6mm), euhe dral to subhedral, and exhibit distinct zoning. Plagioclase phenocrysts are much smaller, up to about 1mm long. Tubaf and Edison seamounts lavas are also trachybasalts and contain phenocryst s of clinopyroxene, as well as phlogopite and amphibole embe dded in a groundmass containing about 20% vesicles and glass (McInnes et al. 2001). Clinopyroxenes in these samples are smaller (up to 1mm) and more homogeneous th an those from Conical lavas. Mineralized samples were collected from the Ladolam mine. Two unaltered monzonite samples were obtained from a drill core taken from different depths in the Ladolam mine. Sedimentary (limestones and carbonaceous muds tones) and mafic (gabbro and basaltic) xenoliths were extr acted from Tubaf seamount lavas using a microdrill (Table 3-1). Mineral analyses in seamount lavas were conducted at the Florida Center for Analytical Electron Microscopy (Florida International University) using a JEOL EPMA JXA-8900-R equipped with 5 wave length dispersive spectrometers. Mineralized samples

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31 Table 3-1. Location, general description, and st rontium and lead isotopic compositions of samples in the study area. Table includes also Pb isotope data for USGS rock standards prepared and analyzed together with the samples. Sample # Location Description Sr 87Sr/86Sr Pb 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 15GTVA 2DSp Conical High temperature mineralization (Gn*, Sp) 25 0.70401 29300 18.764 15.546 38.372 15GTVA 2DSp Duplicate 18.765 15.549 38.374 23GTVA Sp Conical High temperature mineralization (Sp) 18.768 15.548 38.382 25GTVA 6BSp Conical High temperature mineralization (Cp, Sp, Stbn, Plag, Kfs) 490 0.70411 13500 18.763 15.549 38.380 25GTVA 6BSp Duplicate 18.761 15.547 38.371 25GTVA 8C2 Conical High temperature mineralization (Gn, Sp, Cp, Chd, Plag) 172 0.70457 34100 18.767 15.551 38.379 53GTVA 2AGn Conical High temperature mineralization (Gn, Py, Cp, Kfs, Qtz) 212 0.70575 25700 18.764 15.546 39.372 53GTVA 2Agn Duplicate 18.766 15.547 38.371 39GTVA 2Xpy Conical Low temperature mineralization (Py, Mrc, Qtz, Kfs, Rlg, Jar) 846 0.70745 298 18.756 15.554 38.377 40GTVA 2Py2 Conical Low temperature mineralization (Chl, Am-Si, Py, Anat) 66 0.70493 563 18.742 15.543 38.350 42GTVA 2Py Conical Low temperature mineralization (Qtz, Chd, Py, Hm, Plag) 928 0.70421 286 18.764 15.546 38.366 53GTVA 1Bpy Conical Low temperature mineralization (Py, Hm, Qtz, Plag, Smec) 411 0.70863 33 18.754 15.547 38.359 7DR Conical Fresh lava 4 18.742 15.542 38.341 12DR2A Conical Fresh lava 4 18.733 15.551 39.351 12DR2A Duplicate 18.732 15.549 38.344 13DR4 Conical Fresh lava 6 18.734 15.539 38.335 36GTVA 1 Conical Fresh lava 5 18.737 15.548 38.364 50DR1C Conical Fresh lava 4 18.746 15.548 38.360 52GTVA 1 Conical Fresh lava 5 18.721 15.547 38.332 52GTVA 1 Duplicate 18.720 15.544 38.327 54GTVA 4-2 Tubaf Fresh lava 1504 0.70397 10 18.759 15.554 38.391 56GTVA 2C Tubaf Fresh lava 1698 0.70397 10 18.756 15.550 38.377 56GTVA 2M Tubaf Fresh lava 11 18.766 15.550 38.366 10GTVA 5A2 Edison Fresh lava 5 18.761 15.547 38.371 11GTVA 2A1 Edison Fresh lava 4 18.762 15.546 38.373 33GTVA 2J1 Edison Fresh lava 5 18.766 15.550 38.384 34GTVA 3-1 Edison Fresh lava 5 18.765 15.549 38.378 LH98-3 Lihir, Ladolam Mineralization (Kfs, Py, Cp) 0.70395 18.718 15.545 38.339

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32 Table 3-1. Continued LH98-6 Lihir, Ladolam Mineralization (Qtz, Py, Cp, Gn) 0.70405 18.715 15.544 38.337 LH98-7 Lihir, Ladolam Mineralization (Qtz, Py) 0.70409 18.726 15.543 38.342 LH98-7 Duplicate 18.726 15.542 38.336 LH98-8 Lihir, Ladolam Mineralization (Qtz, Py) 18.727 15.543 38.340 LH98-9 Lihir, Ladolam Mineralization (Qtz, Py, Cp, Gn) 18.720 15.539 38.331 LIH-10 Lihir, Londolov it Fresh lava 18.765 15.546 38.357 LIH-11 Lihir, Londolov it Fresh lava 18.766 15.554 38.377 LIH-18 Lihir, Luise Fresh lava 18.759 15.549 38.368 LIH-21 Lihir, Luise Fresh lava 18.762 15.548 38.367 LIH-23 Lihir, Luise Fresh lava 18.757 15.549 38.368 LIH-25 Lihir, Luise Fresh lava 18.747 15.553 38.366 LIH98-1 Lihir, Luise Fresh lava 18.748 15.543 38.347 LIH98-2 Lihir, Luise Fresh lava 18.745 15.541 38.341 471-265 Lihir, Luise Monzonite 18.737 15.553 38.369 471-33950 Lihir, Luise Monzonite 795 0.70396 18.727 15.544 38.342 471-33950 Duplicate 18.728 15.544 38.344 56GTVA 3O Tubaf Basaltic xenolith 144 0.70297 0.33 18. 692 15.509 38.163 54GTVA 3E Tubaf Gabbro xenolith 120 0.70302 18.747 15.509 38.196 55GTVA 2E Tubaf Gabbro xenolith 18.722 15.533 38.280 56GTVA 3I Tubaf Gabbro xenolith 130 0.70293 0.26 18. 647 15.529 38.191 54GTVA 6B Tubaf Sedimentary xenolith 892 0.70751 2 18.627 15.557 38.302 54GTVA 6G Tubaf Sedimentary xenolith 820 0.70828 6 18.640 15.568 38.325 54GTVA 6E Tubaf Sedimentary xenolith 818 0.70759 2 18.901 15.560 38.495 54GTVA 6F Tubaf Sedimentary xenolith 456 0.70669 4 18.771 15.567 38.441 56GTVA 4D Tubaf Sedimentary xenolith 1221 0.70791 5 18.712 15.528 38.307 56GTVA 4F Tubaf Sedimentary xeno lith 2 18.770 15.583 38.495 72GTVA Near Conical Surface sediment 18.743 15.585 38.512 BCR-2 USGS rock standa rd 18.767 15.624 38.728 BCR-2 Duplicate 18.765 15.622 38.721 BCR-2 Duplicate 18.767 15.618 38.738 BCR-2 Duplicate 18.763 15.620 38.729 BCR-2 Duplicate 18.762 15.620 38.730 AGV-1 USGS rock standard 18.942 15.653 38.548 AGV-1 Duplicate 18.944 15.655 38.549 AGV-1 Duplicate 18.940 15.652 38.541 AGV-1 Duplicate 18.944 15.655 38.550 AGV-1 Duplicate 18.940 15.652 38.545 *Gn-galena; Sp-sphalerite; Stbn-stibnite; Cp-chalcopyrite; Chd-chalcedony; Rlg-realgar; Jar-jarosite;

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33 from Conical Seamount were carefully microdrilled and the mineral phases (Table 3-1) were determined by XRD at the Geological Su rvey of Canada in Ottawa. Trace element concentrations were determined by ICP-MS at the Geological Survey of Canada (http://www.nrcan.gc.ca/gsc/mr d/labs/chem-e.html). Whole-rock powders (free of xenoliths a nd alteration) were leached in Optima grade 2N HCl at about 70C for several hours, then rinsed several times with 4x distilled H2O to remove leachate residue, and then dissolved in a mixture of HF+HNO3. Microdrilled sample powders from the mineralized zones were dissolved (but not leached) similarly to the whole rock powders. Sr and Pb for isotopic analyses were separated using standard chromatographic methods under clean lab environment (Chapter 4 and Appendix B). Sr isotopic analyses were pe rformed on a Micromass Sector 54 Thermal Ionization Mass Spectrometer equipped with seven Faraday collectors and one Daly detector at the University of Florida. The Sr was loaded on oxidized W single filaments and run in triple collector dynamic mode. Data were acquired at a beam intensity of about 1.5V for 88Sr, with corrections for instrume ntal discrimination made assuming 86Sr/88Sr=0.1194. Errors in measured 87Sr/86Sr are better than +/0.00002 (2 ) based on long-term reproducibility of NBS 987 (87Sr/86Sr=0.71024). Pb isotopic analyses were conducted on a Nu Plasma multi-collector ICP-MS (Nu Instruments, UK), in the Department of Geologi cal Sciences, University of Florida, using the Tl normalization technique (Chapter 1). Sample and standard solutions were aspirated into the plasma source through a Micromist nebulizer with a GE spray chamber. The instrument settings were carefully tuned to maximize the signal intensities on a daily basis. Preamplifier gain calibrations were de termined before each an alytical session. All

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34 analyses reported in this paper were conduc ted in static mode acquiring simultaneously 202Hg on low-1, 203Tl on low-2, 204Pb on Axial, 205Tl on high-1, 206Pb on high-2, 207Pb on high-3, and 208Pb on high-4 Faraday detectors. 42 analyses of NBS 981 conducted with the sample analyses in the period between April and June, 2003 gave the following results 206Pb/204Pb=16.9369 (+/-0.0039 2 ), 207Pb/204Pb=15.4904 (+/-0.0034 2 ), and 208Pb/204Pb=36.6949 (+/-0.0087 2 ). All standard and sample Pb data were normalized with 205Tl/203Tl=3-38750 (for more details see Chapte r 2). Several samples were prepared and analyzed for Pb isotopes as duplicates to evaluate the reproducib ility of the results (Table 3-1). In addition, 10 separate dissolu tions of USGS rock standards (BCR-2 and AGV-1, Woodhead and Hergt, 2000) were prepar ed and analyzed together with the samples to further verify the precision and accu racy of the analytical protocol (Table 3-1). Results Isotopic analyses of rocks, xenoliths, a nd mineralized samples from the study area are presented in Table 3-1. Sr isotopic com positions of fresh lavas from the area show relatively small variations with 87Sr/86Sr, close to 0.704, within the range of 0.7037 to 0.7044 observed in previous studies of TLTF islands and seamounts (Wallace et al. 1983, Kennedy et al. 1990b, Strack e and Hagner 1998). Measured 87Sr/86Sr in sedimentary xenoliths rec overed from Tubaf seamount lavas exhibit much more radiogenic values than the lavas in the area (Fig.3-2). Sr isotopic compositions of the Conical seamount mineralized samples have valu es either similar to or more radiogenic than the host lavas (Fig. 3-2). Lihir mineralized samples also have 87Sr/86Sr values very similar to the host lavas (Table 3-1).

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35 Figure 3-2. (A) 206Pb/204Pb vs Pb concentrations of C onical seamount lavas and highand low-temperature mineralized zones (based on data from Table 3-1). Note that the high-temperature mineralized samples exhibit less variations in their Pb isotopic compositions compared to the lavas and the low-temperature mineralized samples. (B) 87Sr/86Sr vs Pb concentrations of Conical seamount lavas, highand low-temperature mi neralized zones, and sedimentary xenoliths (for discussion s ee the text). Sr data for fresh lavas from this study and Stracke and Hagner (1998).

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36 Fresh volcanic rocks exhibit small, but di stinguishable variat ions in their Pb isotopic compositions, with C onical seamount lavas having the least radiogenic ratios with 206Pb/204Pb=18.721 to 18.746, 207Pb/204Pb=15.539 to 15.551, and 208Pb/204Pb=38.332 to 38.364. Tubaf and Edison seamount lavas are slightly more radiog enic than Conical with 206Pb/204Pb=18.756 to 18.766, 207Pb/204Pb=15.546 to 15.550, and 208Pb/204Pb=38.366 to 38.391 (Fig. 3-3). Most of the analyzed Lihir Island lavas exhibit lead isotopic signatures indistinguishable from Edison and Tubaf lavas with several samples having slightly less radiogenic values, notably the two monzonite drill core samples from the Ladolam mine, with 206Pb/204Pb=18.727 and 18.737, 207Pb/204Pb=15.544 and 15.553, and 208Pb/204Pb=38.342 and 38.369 (Table 3-1). Nine lowand high-temperature mineralized samples from the Conical seamount mineralized zones show slightly more radiogenic ratios than the fresh host lavas, ranging from 206Pb/204Pb=18.742 to 18.768, 207Pb/204Pb=15.543 to 15.554, and 208Pb/204Pb=38.350 to 38.382. The measured lead isotopic compositions in the mineralized C onical seamount sample s exhibit a positive correlation between Pb content and 206Pb/204Pb ratios (Fig. 3-2A). Mineralized samples from Lihir also show slightly different Pb isotopic ratios from the host fresh lavas exposed on the surface with 206Pb/204Pb=18.711 to 18.727, 207Pb/204Pb=15.539 to 15.545, and 208Pb/204Pb=38.331 to 38.340. A surface sediment sample from New Ireland basin and several sedimentary and mafic xenoliths recovered from the Tubaf seamount lavas were also analyzed for Pb isotopes (Table 31). They exhibit larger range in their lead isotopic compositions compared to the fres h volcanic rocks in th e area with values ranging from 206Pb/204Pb=18.627 to 18.901, 207Pb/204Pb=15.509 to 15.585, and 208Pb/204Pb=38.163 to 38.512 (Fig. 3-4).

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37 Figure 3-3. MC-ICP-MS Pb isotopic data for lavas and mineralized samples. Note the similarity between the Tubaf and Edis on lavas and Conical high-temperature mineralization, suggesting magma with similar Pb isotopic composition as a source for the ore metals. Similarly, Lihir mineralized samples plot at the nonradiogenic end of the array, suggesti ng Pb source similar to the least radiogenic Conical lavas or the monzonite intrusion. Note that the ores and lavas from a trend parallel to the NHRL (f or more discussion see the text).

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38 Figure 3-4. Comparison of Pb isotopic co mpositions of TLTF volcanic rocks with sedimentary and mafic xenoliths, and P acific MORB and sediments. Note the homogeneous isotopic character of the TLTF samples compared to the possible sources in the area. TLTF volcan ic rocks field is based on MC-ICPMS data from this study and TIMS data from Stracke and Hagner (1998), Pacific MORB after White et al. (1987) and Pacific sediments after Othman et al. (1989).

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39 Analyses of clinopyroxene phenocryst s from Conical, Tubaf, and Edison seamounts, completed by electron microprobe, provide more constraints on changing magmatic conditions (Table 3-2). T ypically, Conical c linopyroxenes are larger compared to the clinopyroxenes in the other two seamounts and show concentric zoning with oscillatory chemical variati ons. The analyzed clinopyroxenes from Tubaf and Edison seamounts are smaller, homogeneous and do not exhibit significant chemical variations. Discussion Physical Constraints on Magmatism and Pb Isotopic Variations in the Volcanic Rocks Seamount volcanism is younger than volcanism associated with TLTF island formation. Tubaf seamount proba bly records the youngest volcan ic event in the area (222 +/-34 Ka, one biotite Ar-Ar age determinati on). Conical seamount yi elded slightly older age: 287 (+/-20) Ka (M. Hannington, personal communication). Edis on seamount may be older than the other two seamounts, based on field observations (H erzig et al. 1998). Lihir Island is a Pliocene to Holocene volcan ic complex built from several coalescent volcanoes (Wallace et al., 1983). Volcanism and hydrothermal activity in the area of the Ladolam deposit have been active during the la st 1 Ma with biotit e samples yielding K-Ar ages from 900 (+/-100) to 340 (+/-40) Ka in altered intrusions and 610 (+/-250) to 150 (+/-20) Ka for hydrothermal adularia and alunite samples (Carman 2003 and references therein). The northernmost pa rt of Lihir (Londolovit block) was formed during the oldest volcanic event on the island and shows slightly elevated 206Pb/204Pb ratios compared to the younger Luise volca no, which hosts the Ladolam deposit (Table 3-1).

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40 Conical seamount has Pb isotopic rati os similar to, but extending to lower 206Pb/204Pb ratios compared to Luise vol cano (Fig. 3-3, Table 3-1). Table 3-2. Microprobe analyses on seamount s clinopyroxene phe nocrysts. Conical seamount data are for the CPx show n on figure 3-6, numbers correspond to the analysis points. Conical Conical Conical Conical Coni cal Conical Tubaf Tubaf Edison Edison Smpl# 50DR1 C.1 50DR1C. 2 50DR1C. 3 50DR1C. 4 50DR1C. 5 50DR1C. 6 21DR1 A 21DR1 B 11GTVA2 A 11GTV A2B SiO2 51.20 48.30 49.91 52.02 48.20 49.90 52.01 51.9 53.32 53-1 TiO2 0.25 0.53 0.38 0.2 0.44 0.52 0.15 0.19 0.15 0.14 Al2O3 3-76 5.74 4.14 3-13 4.06 3.89 1.99 3-1 1.91 3-1 Cr2O3 0.25 0.03 0.14 0.36 0.04 0.18 0.2 0.29 0.27 FeO 5.16 9.10 8.80 5.51 9.51 9.37 5.44 5.21 4.12 5.8 MnO 0.25 0.32 0.29 0.10 0.26 0.44 0.25 0.25 0.20 0.2 MgO 15.66 13.85 13.88 15.57 14.37 14.31 15.91 15.85 17.59 16.8 CaO 23.31 21.89 23.48 23.92 21.91 23.15 23.11 23.0 23.78 23.9 Na2O 0.23 0.57 0.32 0.19 0.30 0.38 0.32 0.33 0.25 0.25 K2O 0.05 0.03 0.01 0.02 0.01 0.03 Total 99.12 100.3 100.3 100.0 99.05 101.0 99.3 99.1 100.6 100.5 The two other seamounts, Tubaf and Edison have elevated lead isotopic ratios compared to Conical, but similar to the nor thernmost Londolovit block (Fig. 3-3, Table 3-1). Overall, there appears to be no correl ation between the rela tive ages and the Pb isotopic compositions of the volca nic rocks. The variations can be either a result of small Pb isotopic variations in the magma sources or a result of magma modification processes during ascent. The presence of a large number of sedimentary, mafic, and ultramafic xenoliths in Tubaf and Edison seamount lavas a llows me to address this possibility. The common occurrence of xenoliths in alkaline rocks is attributed to the high ascent velocities of the magmas, on th e order of several kilometers per hour (Spera 1984, Morin and Corriveau 1996). Calculated minimum ascen t velocity (after Spera 1984) for Tubaf magma required to lift an u ltramafic xenolith (3.3 g/cm3, 15 cm diameter) to the surface is on the order of 2 to 3 km/h. McInnes et al., (2001) argue on the basis of the observed

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41 mineral assemblages that some of the ultr amafic xenoliths come from about 60 to 70 km depth. Assuming an ascent velocity of 3 km/h, the Tubaf and Edison magmas must have ascended to the surface in about 20 to 24 h from their mantle source region. Rapid ascent rates calculated for the two seamounts magmas suggest that existence of a magma chamber and/or upper crustal modification of the magmas close to the surface is highly unlikely. In addition, sharp contacts between the sedimentary and mafic xenoliths and host lavas provide no evidence for melting or resorption of the xenol iths, suggesting that no significant assimilation of sedimentary or basaltic/gabbr o material occurred. This suggests that the lead isotopic compositions of the two seamounts are probably inherited from their source region. The mantle wedge beneath the TLTF is land chain has been metasomatically modified by fluids released during earlier subduction of the Pacific plate beneath the Indo-Australian plate (McInne s et al. 2001, Grgoire et al. 2001) and geochemical studies of fresh lavas suggest multiple pare ntal melts (Kennedy et al. 1990a,b). Lead isotopic compositions of volcanic rocks from TLTF islands show relatively uniform 206Pb/204Pb ratios, ranging from 18.7 to 18.8 and slightly elevated 207Pb/204Pb ratios relative to Pacific MORB (Fig. 3-4) and were interpreted to be a result of a small contribution (ca. 1 to 2%) of subducted sediment to the mantle source region (Stracke and Hagner 1998). The fresh lavas shown on Figure 3-4 fall within the field defined by the sediment and oceanic crust samples, but ove rall are confined to the Pacific MORB field, which suggests that the Pb budget of the lava s is probably controlled mainly by oceanic crustal lead (Chapter 4).

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42 Phase Chemical Constraints on Petrogenesis The paucity of xenoliths in the lavas fr om Lihir Island and Conical seamount and the more fractionated chemistry of the lavas (Chapter 4) are cons istent with slower magma ascent and evolution in a magma cham ber close to the surface. Mller et al. Figure 3-5. Representative compositional ch anges from core to rim in Conical clinopyroxene shown in Figure 3-6. Bars represent the observed variations in Tubaf and Conical clinopyroxenes.

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43 (2001) conducted an extensive study of Lihir volcanic rocks and minerals exposed in the vicinity of the Ladolam gold deposit and the P-T conditions inferred from mineral analyses suggested that their main evolution stage occurred at a shallow crustal level. Mller et al. (2003) also inve stigated P-T conditions of crystallization of Conical seamount lavas and calculated crystallization pressu res of 10-14 kbar based on Al content in hornblende cores and 0-4.5 kbar (+/2 kbar) crystallization pressure for clinopyroxenes. This data suggests magma evol ution in 2 stages: one deeper, controlled by early hornblende crystal lization, and a second stage, shallower (crustal levels) associated with clinopyroxene and plagioclas e crystallization (Mller et al., 2003). The presence of plagioclase in Conical lavas also suggests relatively shallower crystallization compared to the Tubaf and Edison magmas. Pl agioclase crystallization is suppressed in magmas with high concentration of water (Carmichael et al., 1996), therefore, its presence is consistent with the model that Conical seamount lavas evolved at a shallower level under relatively low water fugacity, probably in a degassing magma chamber close to the surface. Microp robe elemental analyses of Coni cal clinopyroxenes reveal chemical zoning (Fig. 3-5), whereas Tubaf and Ed ison clinopyroxenes do not show zoning and their compositions are similar to the central parts of the Conical clinopyroxenes (Table 3-2 and Fig. 3-5). Stracke and Hagner (1998) argue that clinopyroxene was the major fractionating phase during the magmatic evol ution of the seamounts and TLTF islands. As a result, Mg and Ca will behave compatibly and decrease in the more evolved lavas. Furthermore, other elements, such as Al and Fe will not be incorporated preferentially in the clinopyroxenes and their concentrations will increase in the evolving liquid. The observed chemical variations in the inte riors of the analyzed clinopyroxenes are

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44 consistent with such an evolutionary tre nd. The cores of the C onical clinopyroxenes are higher in Mg and Ca and lower in Al, Fe, and Ti, similar to the compositions of Tubaf and Edison clinopyroxenes (Fig. 3-5). The da rker colored zones surrounding the lighter zones (Fig. 3-6) show lower Ca and Mg and elevated Al, Fe, and Ti reflecting changes likely occurring in the evolving liquid. Light co lored areas toward rims of the crystals, however, have elevated Mg and Ca and lower Fe, Al, and Ti contents and this reverse zoning is consistent with a mafic magma recharge episode in the evolving magma chamber beneath Conical seamount. In addi tion, careful observati on reveals that the darker zones (also along the bor ders between the lighter a nd darker zones) contain a number of small inclusions (Fig. 3-6). The in clusions are composed of a glass and fluid phase, and often contain clusters of needle-s haped apatite crystals. The later observation suggests that during the growth of the dark zones the clinopyroxenes were in contact with highly evolved magma. Apatite usually forms toward the end of the crystallizing Figure 3-6. Photomicrograph of Conical seamount cli nopyroxene. Numbers correspond to the analysis numbers presented in Table 3-2 and shown in Figure 3-5.

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45 sequences and is observed in the Conical lavas groundmass (Mller et al. 2003). The presence of a fluid phase in the inclusions along the zones provides evidence for episodes of volatile saturation and ma gmatic fluid exsolution, processes considered to play important roles in the ore formation. Pb and Sr Isotopic Variations in th e Conical and Lihir Mineralizations Petersen et al. (2002) obse rved that the early, high-t emperature mineralization stages at Conical seamount are overprinted by a low temperature stage that mainly produced amorphous silica and arsenic sulfides. Sr isotopic compositions of high and low temperature mineralized samples compared with Conical seamount lavas and sediments from the area are shown in Figures 3-2 and 3-7. The high-temperature ore-rich samples with high Cu, Ag, Au, and Pb contents show overall similar 87Sr/86Sr to Conical, Lihir, and the other seamount lavas, suggesting that the Sr is mainly derived from the alkaline lavas in the area. One of the high-temperature mineralizatio n samples exhibits elevated 87Sr/86Sr, which indicates incorpor ation of more-radiogenic Sr Two of the analyzed 3 Lihir mineralization samples have Sr isotopi c compositions that are slightly elevated compared to the analyzed monzonite sample from Luise volcano (Table 3-1), which is also suggestive of minor incorporation of more-radiogenic Sr. Conical low-temperature mineralized samples with relatively lower P b, Zn, Cu, and Au contents show either similar or elevated 87Sr/86Sr compared to the fresh lavas (Table 3-1, Fig. 3-7). Seawater and/or sediments are the two possible source s for more radiogenic Sr in the area, however, simple mixing can not explain the obs erved variations (Fi g. 3-7, see caption for more discussion). Observations at MOR black smoker systems show only minor increase in Sr concentrations in th e emanating fluids compared to the ambient seawater (Teagle et al. 1998). Experiments by S eewald and Seyfried (1990) indicate KD Sr values

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46 between 0.017 and 0.027 during high-temperature (300 to 500C) fluid/ba salt interactions, which also suggests that Sr will not be str ongly enriched in the hydrothermal fluids. Therefore, ore-forming fluids probably will no t contain very high Sr content, then, even small amounts of seawater incorporated in the Conical or Ladolam ore-forming hydrothermal fluids will result in elevated 87Sr/86Sr in the mineralized zones compared to the host lavas. In addition to seawater, the thick Miocene-Recent sedimentary sequences beneath the seamounts also can be a possible so urce of more radiogenic Sr. Due to their Figure 3-7. 87Sr/86Sr vs Sr concentrations fo r mineralized samples in comparison with Conical fresh lavas and sediments. Sy mbols are the same as in figure 2. Arrows show possible mixing trends be tween the possible end-members in the region: A exsolution and/or dissolu tion of Sr from magma/lava without significant seawater or sediment Sr incorporation; B mixing between magmatic Sr and seawater Sr (note that Sr has low fluid-rock Kd (less than 0.1) and that is why the fluid Sr concentr ation is expected to be lower that the lavas (for more discussion see the text); C simple Sr mixing between lava and sediment; D mixing between lava, sediment, and seawater Sr. Sr data from Table 3-1 and from Stracke and Hagner (1998).

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47 calcareous character, the se diments in the area have 87Sr/86Sr values either close to or lower than the present day seawater (Fig. 3-7) and this fact prevents unequivocal distinction based solely on Sr isotopic data whether the more radiogenic end-member in the mineralized zones is seawat er or sediment derived Sr. Seawater contains negligible amounts of Pb compared to the lavas and sediments in the area and, although it may affect the Sr budget, it will not affect the Pb isotopic compositions of the altered and mineralized zones. Overall, the mineralized samples exhibit Pb isotopic compositions similar to the volcanic rocks in the area (Fig. 3-3), suggesting that Pb and possibly other ore meta ls with similar geochemical behavior were derived from the latter. An complicating issue, however, fo r the isotopic composition of the lead in the mineralized samples is that their suggested source (i .e., the volcanic rocks in the area) originated from a mantle th at has been metaso matically altered by components released from subducted sediment s and oceanic crust (Stracke and Hagner 1998, McInnes et al. 2001, Gregoire et al 2001). These components most probably had Pb isotopic compositions varying between the Pacific sediments (including the sedimentary xenoliths), and the mafic xenoliths (Fig. 3-4), and so make it very difficult to resolve if the isotopic signatures observed in the lavas and mineralizations were acquired in the mantle or in the crust. Althoug the above arguments suggest that the Pb isotopic signature of the lavas is inhe rited directly from their man tle source, the same arguments can not be applied to the origin of the ore me tals in the Conical and Lihir mineralizations. Therefore, two possible scenario s for the origin of the Pb in the mineralized zones must be considered: (1) lead in the mineralized samples can be derived from the host lavas (either hydrothermally leached or exsolved fr om solidifying magmas at depth), and (2) it

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48 can be hydrothermally scavenged from the thick layer of sediments and underlying oceanic crust. An Os isotopic study conducted on Au ores from the Ladolam deposit indicates that the primary source of the ore metals is the mantle underlying the TLTF island chain (McInnes et al. 1999), providi ng evidence that material derived from the MioceneRecent sediments in the area has not contribut ed significantly to the metal budget of the ore deposits. Overall, the sediments exhibit much higher 207Pb/204Pb and 208Pb/204Pb compared to the TLTF lavas (Fig. 3-4), therefor e, incorporation of se dimentary Pb in the hydrothermal system will shift the isotopic compositions of the mineralized samples to higher 207Pb/204Pb and 208Pb/204Pb at relatively constant 206Pb/204Pb, which is not the case (Fig. 3-3). Pb isotopic compositions of th e lavas and mineralized samples do show a minor increase in 207Pb/204Pb and 208Pb/204Pb, however, accompanied with an increase in 206Pb/204Pb and form a trend parallel to the NHR L (Fig. 3-3, note that only high-precision MC-ICP-MS data are shown). This trend, and the fact that the mineralized samples fall within the field of the Pacific MORB (Fig. 34, note that at the scale of the figure the mineralized samples can not be distinguished from the lavas), suggest mainly oceanic crust (or mantle) control on their Pb isotopi c compositions. In addi tion, the relatively non-radigenic Pb isotopic composition of th e Ladolam deposit further argues against scavenging of sedimentary Pb within the hydrothermal system (Fig. 3-3). Hydrothermal scavenging of Pb from th e oceanic crust underl ying the thick layer of sediments also must be considered as a po ssible process for incorporation of ore metals in the mineralizations. The mafic xenoliths recovered from Tubaf lavas are lessradiogenic than the lava and mineralized samp les (Table 3-1, Fig. 3-4), but mixing of Pb

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49 from the sediments and the underlying mafic crust within the hydrothermal system may generate Pb isotopic values similar to the observed in the mineralized samples and fresh lavas. However, a Pb isotopic study on Escan aba Trough sulfide deposits indicates that the source for the ore metals is the thick, up to 1 km layer of sediments, without any significant contribution from the underlying Gorda Ridge MORB (German et al. 1995). It is hard to believe, theref ore, that the mafic crust that underlies the sediments in the TLTF area, with its low Pb content will ha ve significant impact on the Pb isotopic composition of hydrothermal fluids that mu st pass through 5 km of sediments and/or volcanic rocks with much higher Pb content (Table 3-1). The most probable sources for the ore metals in the giant Ladolam depos it and the Conical seam ount mineralization, therefore, are the ma gmas in the area. Intriguingly, mineralized z ones bracket the host lavas (Fig. 3-3), with Lihir mineralization plotting at the low-radiogenic end, and Conical mine ralization plotting at the high-radiogenic end of the array, suggesti ng distinct sources for each mineralization. In addition, both mineralizations exhibit less variation in their Pb isotopic compositions than the Conical and Lihir host lavas, sugge sting a relatively hom ogeneous source and/or homogenizing process (Fig. 33). The Tubaf and Edison magmas, which rapidly ascended from their source region, also show relative ly homogeneous Pb compared to Conical and Lihir lavas, indicating that in some instances single magm atic pulses in the area were composed of relatively homogeneous Pb. The Conical high-temperature mineralized samples have Pb isotopic compositions similar to the nearby Tubaf and Edison seamount lavas (Fig. 3-3), suggesting that a single pulse of magma with similar isotopic composition could be the metal source fo r the high-temperature mineralization.

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50 Conical low-temperature mineralized samples containing relatively low Pb concentrations and elevated 87Sr/86Sr, have Pb isotopic compositions scattered between those of the host lavas and th e high-temperature, ore-rich samples (Fig. 3-2). This suggests that Pb was remobilized from both th e host lavas and from the high-temperature ore zones during the waning stages of the hydrot hermal system in order to form the latestage low-temperature zones. Mechanism for ore Formation Sillitoe (1997) suggest ed that the gold deposits associ ated with alkaline rocks may be related to formation of highly oxidized magma generated by quenching of a volatilerich mafic melt during injection into a shal low magma chamber. The resultant volatile phase will be charged with ore-forming metals such as Au, Ag, Cu, Pb, Zn, and Fe. In addition, alkaline rocks us ually contain elevated amount s of sulfur and chlorine, important ligands for hydrothermal gold a nd base metal transport (Candela and Piccoli 1995). Sulfur isotopic ratios in sulfide minerals from mineralized Conical seamount samples are consistent with minera lization from a magmatic fluid (Herzig et al. 1999, Petersen et al. 2002). Chemical compositions of clinopyroxene and petrological observations desc ribed above, provide evidence for the presence of a relatively shallow magma chamber beneath C onical seamount and an episode of magma recharge involving a mafic magma similar in composition to Tubaf and Edison lavas. Conical and Lihir lavas are more evolved comp ared to Tubaf lavas, however, the latter are more volatile-rich (Fig. 3-8). In addition to being highly enriched in chlorine, Tubaf lavas also show about two times higher Pb c ontent than Conical lavas (Table 3-1). The

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51 Figure 3-8. MgO vs Cl content of seamounts and Lihir island. Arrows show general direction of changes dur ing magma degassing, fractionation, and fractionation accompanied with degassing. Note the hi ghest Cl content in Tubaf lavas. presence of chlorine greatly increases the fluid/magma partitioning coefficients of base metals, and fluids with 103-104 ppm base-metal concentrations can be rapidly generated from magmas with tens of ppm base-m etal concentrations (Candela 1989). Mafic, volatile-rich magmas travel upward through subvertical cracks in brittley fractured rocks as dikes, and in extensional tectonic settings over-pressured magmas may ascend through such self-generated cracks for tens of kilometers, even through less dense rocks (Best and Christensen 2001). Once such rapidly ascending magma encounters magma stored in a shallow magma chamber, th en its upward motion will be retarded due

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52 to lack of significant density contrast and b ecause the mechanism of brittle fracturing will not operate in the fluid medium. Quenching of this volatile-rich magma that is recharging the system will cause rapid fluid saturation a nd metal-bearing solutions will exsolve (note the higher Cl and Pb content in Tubaf compar ed to the other lavas (Table 3-1, Fig. 3-8) from the magma body at depth (Fig. 3-9). The ore-bearing fluids, therefore, will be primarily derived from the mafic magmas (s imilar to Tubaf) recharging the system, not from the lavas that host the mineraliza tion close to the surface (Fig. 3-9). A similar ore-forming mechanism can be proposed for the Ladoloam gold deposit. Sulfur isotopic data, simila r to the sulfur data for th e Conical seamount hydrothermal system, indicate mineralization from a magm atic fluid (Carman 2003). Similar to the observed relationships between Conical ores and lavas, a small offset between the Pb isotopic compositions of the Ladol am ores and surficial Lihir la vas is apparent (Fig. 3-3), suggesting that the ore metals were also probably not hydrothermally leached from the host lavas, but possibly exsolved from a r echarging volatile-rich magma at depth (Fig. 3-9). Conclusions High precision lead isotopic measuremen ts conducted with MC-ICP-MS on ores, lavas, and sedimentary and mafic xenoliths in the area of Lihir Island provide a clear picture of the sources of metals in the Ladol am gold deposit and in mineralized zones on Conical seamount. Although Sr isotopic data suggest some involvement of seawater, particularly during the waning stages of the hydrothermal system, Pb isotopes suggest that neither seawater nor the sediments in th e area have significan tly contributed to the metal budget of the ore mineralizations di scovered on the island and the seamount. The ore Pb in the hydrothermal systems and pres umably other ore metals with similar

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53 Figure 3-9. Cartoon depicting th e inferred ore formation mechanism. Evidences suggest that a magma chamber under Conical seamount was recharged by a mafic magma and the exsolved fluids formed the ore deposit. Pb isotopes in the ore mineralization will inherit their signature from the recharging magma. Note that the Tubaf magmas have the highest Pb content (Table 3-1). Barren seamounts magmas (Tubaf and Edison) ascended rapidly to the surface from their mantle source region and no ore mineralization was formed. However, during their ascent a number of xe noliths were trapped, thus providing representative samples from the possi ble lithosphere sources beneath the TLTF volcanoes. geochemical behavior were derived from al kaline magmas in the area. The extent of mineralization during a magmatic event can be related to the volcano-magmatic evolution of the volcanic complex. I propose that the magmas forming the two barren Tubaf and Edison seamounts ascended rapidly to the surf ace and preserved their volatile content close to the moment of eruption and thus precluded significant exsolution of metalbearing fluids and consequent potential for ore formation. On the other hand, Conical seamount, composed of similar lavas to Tuba f and Edison, shows evidences for magmatic

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54 evolution at shallower levels, and thus e nhanced ore-forming pot ential. The triggering ore-mineralization event, however appears to be related to a mafic, volatile-rich magma recharging event in the evolving magma ch amber beneath the seamount. The latter hypothesis is supported by petrologic observatio ns indicative of a complex magmatic history including magma recharge and fl uid saturation and exsolution at Conical seamount. In addition, small but distinguishab le differences between the Pb isotopic compositions of ores and host lavas were obs erved, suggesting that the ore metals were not hydrothermally leached out from the hos t lavas but most probably exsolved from quenched mafic magmas injected into magm a chambers existing beneath the Conical seamount and Luise volcano.

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55 CHAPTER 4 DECIPHERING MANTLE AND CRUSTA L CONTROLS IN AN ISLAND ARC ENVIRONMENT: A SR, ND, AND PB ISOT OPIC STUDY OF SW PACIFIC ISLAND ARCS AND SUB-ARC XENOLITHS. Introduction In the classical model for magma generation in continental and island arc settings it is believed that magmatism results from lo wering the melting point of the mantle wedge peridotite as a result of fluid and/or melt introduction from the subducted slab (Poli and Schmidt 2002, Stern 2002). It can be c onsidered, then, that complex interaction between components derived from the subducted slab, the crust underlyi ng the island arc, and the subarc mantle wedge play major role in arc petrogenesis. Samples from these sources in the form of man tle and crustal xenoliths brought to the surface by arc lavas, therefore, can provide impor tant constrains on the composition and evolution of the mantle wedge and the contribution of the cr ust to the chemistry of arc lavas. Such samples, however, are rare and limited to only a few localities: Japan, Cascades, Philippines, Mariana, and Kamchatk a arcs (McInnes et al. 2001). The Tabar-Lihir-Tanga-Feni (TLTF) ar c, Papua New Guinea provides an outstanding opportunity to look in detail at the components directly involved in the petrogenesis of the island arc because numer ous sedimentary, mafic, and ultramafic xenoliths were recovered from TUBAF seamoun t, located on the flank of Lihir island (Chapter 3). Detailed studies on the minera logy and trace element geochemistry of the xenoliths indicated that the mantle wedge and crust underlying the TLTF island chain resembles oceanic lithosphere possibly formed at mid-ocean ridge (MOR) settings (Franz

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56 and Wirth 2000, McInnes et al. 2001, Gre goire et al. 2001, Franz et al. 2002). Many of the ultramafic xenoliths exhibit ev idence for metasomatic alteration by hydrous fluids, possibly released duri ng the subduction of the Pacifi c plate underneath the IndoAustralian plate (McInnes et al. 2001, Gregoire et al 2001, Franz et al. 2002). TLTF lavas are characterized mainly by s ilica undersaturated, alkaline rocks and two mechanisms can be invoked to explain thei r origin: 1) the lava s can be a product of high pressure, small degree melting of a MORB-lik e mantle source, or 2) the lavas can be derived from partial melting of a metasomati zed mantle wedge. Elemental and isotopic data from the xenoliths and the lavas from the region will allow us to determine whether the ultramafic xenoliths are the source or a component for the alkaline magmas. In addition, the mantle xenoliths will provide in formation for the isotopic composition of the mantle in the region, which will be used to constrain the origin of the isotopic signatures in New Britain and Solomon isla nd arcs and the Manus and Woodlark back-arc basins. Geological Settings The oceanic region to the north and east of northeastern Australia includes the island arcs of New Britain, New Ireland, and the Solomon Islands and numerous small basins and seas (Fig. 4-1). The Tabar, Lihir, Tanga, and Feni (TLTF) island chain is located in the former fore-arc region of New Ireland, extending for about 250 km parallel to the presently inactive Ma nus-Kilinailau trench. Some studies suggest that the Solomons, New Britain, and New Ireland arcs rifted away from the eastern Australian margin around 40 Ma ago, in contrast to other in terpretations that indi cate that the arcs originated as intra-oceanic arcs within the Pacific plate (H all 2001 and references therein). The formation of the arcs led to the formation of the Solomon Sea, and recovered

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57 Figure 4-1. Regional map of the study area, m odified after Hall (2001 ). See chapter 3 for more detailed map of the TLTF area. seafloor basalts show geochemical similar ity to evolved MORB from the East Pacific Rise (Davies and Price 1987). Solom on Islands, New Britain, and New Ireland volcanism began during the Eocene as a result of south-westerly subduction of the Pacific plate under the Indo-Austra lian plate along the Manus-Kil inailau trench (Johnson 1979). During the Oligocene New Ireland and New Brita in islands formed continuous arc with the Solomon Islands (Hall 2001). Subduction ce ased about 20-10 Ma ago, when the thick and relatively buoyant Ontong Java oceanic pl ateau collided with the subduction zone (Coleman and Kroenke 1981, Petterson et al 1997). After the collision, regional relocation of stress caused reversal of the subduc tion direction and caused Solomon plate subduction beneath New Brita in to the north, and beneath Solomon Islands to the east, along the newly formed New Britain-San Cristobal trench (Petterson

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58 et al. 1997). New Britain began to migrate southwestward and as a result, Manus backarc basin opened at about 3.5 Ma (Taylor 1979). The floor of Manus basin is composed of diverse magmatic rocks, ranging from nor mal MORB to BAB and lavas with arc-type compositions (Sinton et al. 2002). Woodlark Basin spreading began about 5 to 6 Ma ago and at present, typical MORB erupt at the spreading center, although some of the lavas exhibit back-arc characte ristics (Perfit et al. 198 7, Trull et al. 1990). The main part of the New Britain arc is composed of Eocene-Oligocene calcalkaline rocks, generated dur ing the previous subduction of the Pacific plate along the currently inactive Manus-Kilinailau trench (Woodhead and Johnson 1993). The arc migrated southward after the opening of the Manus back-arc basin as the subduction began along the New Britain tr ench. A number of Quaterna ry volcanic centers exposed along the north rim of the New Britain are related to the present Solomon Sea plate subduction and Woodhead et al. (1998) divided the volcanic centers into zones with specific chemical characteristics related to the depth of the subducted slabs. The zones closer to the trench show a Solomon plate sl ab contribution dominate d by altered oceanic crust, and the zones further aw ay from the trench exhibit decreasing slab influence and an increase in the Manus basin mantle component (Woodhead and Johnson 1993, Woodhead et al. 1998). The TLTF island groups, located about 30 to 70 km off-shore New Ireland, are equally spaced at about 75 km apart. Volcanism in the TLTF area began on Simberi island (part of Tabar island group) about 3.7 Ma ago (Johnson et al., 1976), coeval with the initiation of back-arc spreading in the Manus Basin, and migrated southward to Feni island (2300 y), as the island of New Brita in was transported to the sout heast. Taylor (1979) suggests

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59 that the local extension and volcanism are re lated to the opening of the Manus back-arc spreading center. Pliocene-Plei stocene TLTF islands are char acterized mainly by silica undersaturated, alkaline rocks, ranging from basanites to trachyandesites with rare occurrences of more evolved tephriph onolites and phonolites (Johnson et al. 1976; Wallace et al. 1983; Kennedy et al. 1990a,b; McInnes and Cameron 1994; Stracke and Hegner 1998). More detailed description of the TLTF geological settings is available in Chapter 3. Samples and Analytical Methods Fresh volcanic rocks used in this study were recovered from the seamounts during the SONNE-133 cruise in 1998. Conical seamount trachybasalts are composed of about 20 to 25% clinopyroxene, 5% plag ioclase, and rare olivin e and phlogopite phenocrysts embedded in a slightly vesicular, fine-gra ined groundmass which consists of glass, plagioclase, clinopyroxene, spinel, and apatit e microcrysts ( Mller et al. 2003). The clinopyroxene phenocrysts are large (up to about 6 mm), euhedral to subhedral, and exhibit distinct zoning. Plagioclase phenocryst s are much smaller, up to about 1mm long. Tubaf and Edison seamounts lavas are also trachybasalts and contain phenocrysts of clinopyroxene, as well as phlogopite a nd amphibole embedded in a groundmass containing about 20% vesicles and glass (M cInnes et al. 2001). Clinopyroxene crystals in these samples are smaller (up to 1 mm) and more homogeneous than those in Conical lavas. Sedimentary (limestones and carbonaceous mudstones), mafic (gabbroic and basaltic), and ultramafic (l herzolite, harzburgite) xenoliths were recovered from Tubaf seamount lavas and have been described in de tail in several recent publications (Franz and Wirth 2000, McInnes et al. 2001, Gre goire et al. 2001, Franz et al. 2002).

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60 Sedimentary xenoliths are mainly composed of coralline and coralgal limestones, interbeded with layered carbonaceous and volca noclastic sediments, similar to uplifted sedimentary sequences expos ed on New Ireland, and some pelagic deep-sea sediments (McInnes et al. 2001). The mafic xenoliths are composed mainly of gabbros and few fine-grained basalts. Representative gabbro samples 543E and 563I co ntained uralitized pyroxene and altered plagioclase common low-temperature alterati on products that probably formed as the oceanic lithosphere was transported away from the ridge axis (McInnes et al. 2001). Sample 553E shows little to no alteration of the plagioclase and pristine pyroxene, suggesting that the oceani c crust beneath the arc is not entirely altered. The ultramafic xenoliths are course-grained spinel lherzolites composed mainly of olivine and lesser amount of orthopyroxene (Opx) crystals, with minor clinopyroxene (Cpx) and spinel. In some cases deformed exsolution lamellae in pyroxenes and kinkbanded olivines are present, suggesting a pe riod of ductile deformation (McInnes et al. 2001). Peridotite samples 542D, 562B, 562P, 542S and 542K contain numerous vein and veinlets composed mainly of fibrous Opx, some amphibole, phlogopite, spinel, and Fe-Ni sulfides. These vein components are secondary in origin and we re formed as a result of a release of metasomatic agents from the s ubducted Pacific oceanic crust underneath the TLTF chain (McInnes et al. 2001, Gregoire et al. 2001, Fr anz et al. 2002). Samples 562F, 542G, and 552H are spinel harzburgites that do not contain vein assemblages, although 542G and 552H show some areas of irregular metasomatic alteration. Clinopyroxene separates 611B Cpx, 611C Cpx, and 671 Cpx are from ultramafic xenoliths collected from Tubaf seamount during the 1994 SONNE cruise (B. McInnes,

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61 pers. comm.). The clinopyroxens were separated from lherzolites, discussed in detail by McInnes et al. (2001) and Gregoire et al. (2001). For this study, xenolith samples were separated from the host lavas using a microdrill being particularly careful to avoid any contamination by the host lavas (R. Chopra, pers. comm.). Before isotope analyses fresh lava, mafic and ultramafic xenolith samples were leached for several hours in warm 2N HCl. Then, the residues were rinsed several times with 4xH2O and were dissolved in sealed Te flon vials for several days at 100 C in a HF-HNO3 mixture (see also Chapter 3). Major and trace element concentrations Table 1) were determined by ICP-AES and ICP-MS at the Geological Survey of Canada (http://www.nrcan.gc .ca/gsc/mrd/labs/chem-e.html). Radiogenic isotopic analyses were perfor med at the Department of Geological Sciences at the University of Florida. Sr Pb, and Nd were separated using standard chromatographic methods used in our lab (H eatherington and Muelle r 1999). Procedural blanks determined several times during samp le preparation were maximum 70 ppt for Sr, 80 ppt for Pb, and 30 ppt for Nd. Sr isotope measurements were collected using a Micromass Sector 54 Thermal Ionization Mass Spectrometer equipped with seven Faraday collectors and one Daly collector. Sr samples were loaded on oxidized W single filaments and run in dynamic collection mode. Data were acquired at a beam intensity of 1.5V for 88Sr, with corrections for instrume ntal discrimination made assuming 86Sr/88Sr=0.1194. Errors in measured 87Sr/86Sr are better than +/0.00002 (2 ) based on long-term reproducibility of NBS 987 (87Sr/86Sr=0.71024). Nd isotopic analyses were performed on a Nu Plasma multiple-collect or magnetic-sector inductively coupled mass spectrometer (MC-ICP-MS). Samples and stan dard solutions were aspirated into the

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62 plasma source either via a Micromist nebulizer with GE spray chamber (wet plasma) or through DSN-100 desolvating nebuliser (dry pl asma). The instrument settings were carefully tuned to maximize the signal intens ities on a daily basis. Preamplifier gain calibration was performed before each analy tical session. Nd isotope measurements were conducted for 60 ratios in static mode simultaneously acquiring 142Nd on low-2, 143Nd on low-1, 144Nd on Axial, 145Nd on high-1, 146Nd on high-2, 147Sm on high-3, 148Nd on high4, and 150Nd on high-5 Faraday detectors. The measured 144Nd, 148Nd, and 150Nd beams were corrected for isobaric interference from Sm using 147Sm/144Sm = 4.88, 147Sm/148Sm = 1.33, and 147Sm/150Sm = 2.03. All measured ratios were normalized to 146Nd/144Nd = 0.7219 using an exponential law for mass-bi as correction. The mean value of 143Nd/144Nd for our Ames Nd in-house standard based on 23 repeat analyses during the samples analyses was 0.512140 (2 = 0.000012). Three repeated analyses of the JNdi-1 and LaJolla Nd standards during the same time interval produced mean values of 0.512106 (2 = 0.000013) and 0.511856 (2 = 0.000013), respectively. Thr ee separate dissolutions of USGS SRM BCR-1 were prepared and anal yzed for Nd isotopes together with the samples in order to further evaluate the analytical protocol. The mean value of 143Nd/144Nd for the analyses of BCR-1 was 0.512645 (2 = 0.000011), which is indistinguishable from the published TI MS value of 0.51264 (Gladney et al. 1990). Pb isotopic analyses were conducted on a Nu Plasma multi collector ICP-MS using the Tl normalization technique on fresh mixtures to prevent oxidation of thallium to Tl3+ (for more details see chapter 2 and 3). An alyses of NBS 981 conducted in wet plasma mode during the period of time the samples were analyzed gave the following results (n=42): 206Pb/204Pb=16.937 (+/-0.004 2 ), 207Pb/204Pb=15.490 (+/-0.003 2 ), and

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63 208Pb/204Pb=36.695 (+/-0.009 2 ). Due to their low Pb content the ultramafic xenoliths were analyzed in dry plasma mode and the reported data are relative to the following NBS 981 values (n=29): 206Pb/204Pb=16.937 (+/-0.001, 2 ), 207Pb/204Pb=15.491 (+/-0.001, 2 ), and 208Pb/204Pb=36.694 (+/-0.004, 2 ) Results Major and trace element concentrations of lava, xenolith, and sediment samples from the study area are shown in Table 41. Lihir and the nearby seamount lavas vary from subalkaline basalts to trachyandesites, with the majority of the samples plotting in the field of trachybasalts on an alkali-silica diagram (Fig. 4-2), similar to the lavas found on the other TLTF islands (Wallace et al 1983; Kennedy et al. 1990a,b; McInnes and Cameron 1994; Stracke and Hegner 1998). Tubaf and Edison lavas are characteristically more primitive, whereas Lihir lavas vary from subalkaline basalts to trachyandesites and include a few phono-tephrites. The seamount la vas have more restricted MgO content, ranging between about 8 and 4 wt % in comparison to Lihir lavas, which have quite variable MgO content, ranging from as high as about 8 wt% to as low as about 1 wt% (Table 4-1, Fig. 4-3). Both, the seamounts and the island exhibit decreasing CaO and Fe2O3 and increasing SiO2, Al2O3, Na2O and K2O with the decrease in the MgO (Fig. 4-3). TiO2 data, on the other hand, exhibit large sc atter and do not show clear correlation with MgO variation. Compatible trace elements, such as V, Ni, and Cr have relatively moderate concentrations and show an overall decrease with decreasing MgO concentration (Fig. 4-4). Inco mpatible trace elements, such as Ba, Rb, Cs, Nb, and La show increase with decreasing MgO (Fig. 44), although most of the elements exhibit large scatter when plotte d on correlation diagrams.

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64 Table 4-1. Major and trace element data for lavas and xenoliths Location: Conical Conical Conical Conical Conical Conical Conical Sample: 7DR 12DR 2A 13DR 4 36GT VA 1 42GTVA-1 42GTVA-3 50DR 1C Type: lava Lava lava lava lava lava lava SiO2 47.2 48.5 47.6 50.2 51.6 48.7 49 TiO2 0.69 0.75 0.72 0.77 0.84 0.77 0.74 Al2O3 19.3 15.4 18.4 16.5 16.6 16.1 15.4 Fe2O3T 9.6 10.1 9.8 10.2 8.45 10.3 10.5 MnO 0.18 0.19 0.2 0.19 0.14 1.08 0.19 MgO 5.99 6.36 5.84 5.68 4.2 3.83 6.49 CaO 10.6 11.4 10.3 11 10.3 9.5 11.7 Na2O 2.35 2.66 2.94 3.03 3.67 3.41 2.64 K2O 2.89 2.71 3.02 2.94 2.45 2.81 2.84 P2O5 0.31 0.32 0.42 0.36 0.52 0.52 0.36 Total 99.3 98.6 99.5 101.1 99.2 97.4 100.1 LOI 1.3 1.2 1.1 1.3 3.3 3.6 0.9 Ba 202 221 234 245 294 279 210 Be 1 0.9 1.2 1.1 0.9 1.3 1.1 Co 28 30 27 30 27 73 31 Cr 141 124 97 90 98 89 157 Cs 0.77 0.77 0.84 0.85 1.2 0.89 0.82 Cu 84 117 116 107 148 246 102 Hf 1.6 1.7 2.4 1.7 1.5 1.5 1.6 Nb 1.3 1.4 1.6 1.6 1.8 1.8 1.3 Ni 29 30 30 25 22 35 31 Pb 4 4 6 5 10 10 4 Rb 57 56 57 63 32 56 63 Sc 38 40 33 36 30 27 40 Sr 869 936 1195 1011 1180 1100 915 Ta 0.08 0.09 0.1 0.11 0.09 0.09 0.08 Th 0.82 0.75 1.2 0.91 1 0.97 0.84 U 0.52 0.51 0.85 0.64 1.7 1.5 0.58 V 279 302 297 307 247 239 290 Zn 77 82 85 86 75 111 82 Zr 54 57 92 63 55 55 60 Ce 19 20 34 23 23 21 20 Dy 2.8 2.9 3.8 3.2 3 2.9 3.1 Er 1.5 1.5 1.7 1.6 1.6 1.5 1.6 Eu 1.2 1.2 1.6 1.2 1.3 1.2 1.2 Gd 3.6 3.7 4.7 3.8 3.8 3.5 3.9 Ho 0.57 0.58 0.67 0.63 0.6 0.56 0.62 La 8.7 8.6 14 9.8 11 10 8.9 Lu 0.25 0.24 0.29 0.26 0.26 0.25 0.25 Nd 13 14 20 15 14 14 14 Pr 2.8 2.9 4.3 3.2 3.2 3.1 2.9 Sm 3.5 3.6 4.9 3.9 3.7 3.4 3.7 Tb 0.5 0.54 0.66 0.56 0.52 0.49 0.54 Tm 0.22 0.23 0.26 0.25 0.24 0.22 0.23 Y 17 17 20 18 17 17 17 Yb 1.5 1.6 1.8 1.7 1.6 1.6 1.6

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65 Table 4-1. Continued Location: Conical Edison Edison Edison Edison Edison Edison Sample: 52GTVA 1 10GTVA 5A2 10GTVA 6B1 10GTVA 6B2 11GTVA 2A1 11GTVA 2A2 33GTVA 2J2 Type: lava lava lava lava lava lava lava SiO2 48.4 45.7 46.7 47 47.1 47.1 46.5 TiO2 0.77 0.79 0.78 0.78 0.8 0.82 0.79 Al2O3 16.4 14.2 16.9 14 14 14 15 Fe2O3T 8.98 11.2 11 11.3 11.3 11.3 11.3 MnO 0.16 0.19 0.19 0.19 0.18 0.19 0.19 MgO 5.14 7.5 7.41 7.09 6.83 7.36 7.3 CaO 11.3 11.9 11.5 11.7 12 12.1 12 Na2O 2.37 2.22 2.28 2.3 2.13 2.25 2.46 K2O 2.88 2.31 3.38 2.01 2.17 1.98 2.39 P2O5 0.31 0.44 0.44 0.44 0.43 0.43 0.47 Total 96.9 96.7 100.8 97 97.2 97.8 98.6 LOI 3.1 2.2 1.5 2.2 2.6 2.5 1.1 Ba 230 242 250 280 246 252 245 Be 1 0.9 1.1 1 0.9 0.9 1.1 Co 24 35 34 34 32 33 34 Cr 89 98 99 115 110 127 107 Cs 0.64 0.61 0.68 0.65 0.62 0.64 0.74 Cu 108 118 139 127 107 111 141 Hf 1.7 1.9 1.7 1.9 1.9 1.8 1.9 Nb 1.5 1.4 1.5 1.4 1.5 1.4 1.5 Ni 21 57 52 56 46 51 53 Pb 5 5 5 6 4 5 5 Rb 41 26 42 30 28 28 43 Sc 38 43 41 43 45 44 41 Sr 1073 1160 1116 1161 1179 1172 1173 Ta 0.09 0.09 0.09 0.12 0.1 0.1 0.09 Th 0.82 1 1 1.1 1.1 1.1 1 U 0.58 0.82 0.72 0.71 0.78 0.66 0.79 V 317 317 311 317 321 319 316 Zn 80 86 85 85 85 85 87 Zr 56 63 59 65 64 62 65 Ce 20 26 26 26 26 25 27 Dy 3.2 3.4 3.4 3.4 3.6 3.5 3.6 Er 1.6 1.7 1.7 1.8 1.7 1.8 1.8 Eu 1.2 1.5 1.4 1.5 1.4 1.5 1.5 Gd 3.7 4.6 4.7 4.7 4.7 4.7 4.7 Ho 0.63 0.67 0.68 0.67 0.69 0.67 0.69 La 9.3 11 11 11 11 11 12 Lu 0.26 0.26 0.27 0.27 0.28 0.27 0.29 Nd 14 18 18 18 18 17 18 Pr 3 3.7 3.8 3.8 3.8 3.7 3.9 Sm 3.8 4.7 4.5 4.7 4.6 4.6 4.9 Tb 0.58 0.66 0.64 0.65 0.66 0.66 0.67 Tm 0.24 0.25 0.26 0.26 0.26 0.27 0.27 Y 17 19 20 19 20 19 20 Yb 1.7 1.7 1.7 1.7 1.8 1.7 1.8

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66 Table 4-1. Continued Location: Edison Edison Tuba f Tubaf Tubaf Tubaf Tubaf Sample: 34GTVA 3.1 34GTVA 3.2 54GTVA 2R1 54GTVA 2R2 54GTVA 4.1 54GTVA 4.2 55GTVA 5F1 Type: lava lava lava lava lava lava lava SiO2 45.9 45.6 47.1 47.8 47.1 46.4 47.1 TiO2 0.78 0.78 0.75 0.8 0.76 0.75 0.76 Al2O3 14.1 14.6 16.2 16.2 16.9 17.7 16.3 Fe2O3T 11 10.8 10.1 10.6 10.1 10.2 10.4 MnO 0.19 0.18 0.21 0.2 0.21 0.2 0.2 MgO 7.16 7.11 7.24 7.64 7.24 7.16 7.8 CaO 11.7 12.2 9.22 9.19 9.07 9.04 9.01 Na2O 2.29 2.38 3.31 3.34 3.3 3.19 3.3 K2O 2.35 2.06 2 2.16 2.18 2.1 2.05 P2O5 0.44 0.48 0.54 0.34 0.46 0.43 0.47 Total 96.1 96.4 97 98.6 97.6 97.5 97.7 LOI 2.9 3.1 2.2 2 1.6 1.8 2 Ba 245 239 233 238 245 230 240 Be 1 0.9 1.9 1.9 1.9 2 1.9 Co 32 31 31 32 30 32 31 Cr 125 107 140 154 159 180 145 Cs 0.67 0.66 0.53 0.51 0.56 0.55 0.37 Cu 126 124 69 66 82 86 89 Hf 1.8 1.9 2.9 3 2.8 3 2.9 Nb 1.5 1.5 2.8 2.9 2.8 2.8 2.8 Ni 48 45 87 99 89 105 84 Pb 5 5 11 10 10 10 10 Rb 30 28 20 21 23 22 23 Sc 42 43 29 30 29 30 29 Sr 1161 1169 1920 1942 1901 1927 1961 Ta 0.09 0.08 0.16 0.15 0.16 0.16 0.16 Th 1.1 1.1 2.1 1.9 2.1 2.1 2 U 0.75 0.69 1.4 1.4 1.4 1.5 1.3 V 307 299 297 307 297 302 294 Zn 83 80 89 90 89 91 87 Zr 63 65 103 101 100 100 100 Ce 26 27 42 39 39 42 39 Dy 3.5 3.5 4.4 4.2 4.2 4.3 4.1 Er 1.7 1.7 2.3 2.1 2.1 2.1 2.1 Eu 1.5 1.5 1.9 1.9 1.9 1.9 1.9 Gd 4.8 4.6 6.1 5.5 5.8 5.7 5.8 Ho 0.69 0.68 0.86 0.83 0.82 0.81 0.83 La 11 11 20 18 19 19 18 Lu 0.28 0.28 0.35 0.34 0.36 0.37 0.34 Nd 18 18 26 24 25 25 25 Pr 3.8 3.9 5.7 5.2 5.5 5.8 5.6 Sm 4.7 4.7 6.3 5.8 6.1 6.3 6.3 Tb 0.67 0.63 0.83 0.79 0.79 0.81 0.82 Tm 0.26 0.25 0.32 0.33 0.33 0.32 0.3 Y 19 19 24 24 23 24 23 Yb 1.8 1.7 2.3 2.2 2.1 2.2 2.2

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67 Table 4-1. Continued Location: Tubaf Tubaf Tuba f Tubaf Tubaf Tubaf Tubaf Sample: 55GTVA 5F2 56GTVA 2C 56GTVA 3F1 56GTVA 3F2 54GTVA2D 54GTVA2G 54GTVA2J Type: lava lava lava lava lh erzolite harzburgite lherzolite SiO2 47.9 49.5 47.7 47 41.40 37.30 38.60 TiO2 0.78 0.76 0.79 0.74 0.03 bdl 0.02 Al2O3 16.4 16.3 16 16 0.90 2.30 1.70 Fe2O3T 10.5 10.4 10.5 10.1 11.80 9.50 11.10 MnO 0.2 0.21 0.2 0.19 0.17 0.13 0.16 MgO 7.37 7.77 7.35 7.63 39.00 48.70 47.20 CaO 9.42 9.2 9.23 9.13 5.20 0.38 2.10 Na2O 3.37 3.5 3.42 3.33 0.10 bdl 0.04 K2O 2.13 2.6 2.04 2.08 0.05 bdl bdl P2O5 0.49 0.45 0.43 0.43 bdl bdl bdl Total 98.9 101 98 97 98.1 98.2 100.8 LOI 1.7 1.5 1.8 2.3 0.0 bdl bdl Ba 240 247 229 232 399 bdl bdl Be 2 2 2 2 bdl bdl bdl Co 31 31 30 32 131 145 170 Cr 135 166 147 175 3890 4400 7410 Cs 0.49 0.68 0.66 0.63 0.04 bdl bdl Cu 92 79 125 142 20 11 16 Hf 3.2 3.2 3.2 3.2 0.17 0.55 0.18 Nb 2.9 2.8 2.8 2.9 0.08 0.10 0.10 Ni 85 109 92 103 2050 2940 2120 Pb 11 10 10 11 19 2 3 Rb 20 29 24 22 0.52 0.13 0.21 Sc 29 29 30 30 18.0 4.4 12.0 Sr 1962 1963 1874 1959 bdl bdl 10 Ta 0.18 0.15 0.18 0.18 bdl 0.03 0.04 Th 2.2 2.2 2.2 2.3 0.02 0.03 0.03 U 1.5 1.5 1.5 1.6 0.03 0.02 0.04 V 300 297 301 298 69 39 58 Zn 89 93 92 93 68 51 57 Zr 107 103 103 102 6.8 22.0 6.6 Ce 43 43 42 43 0.4 1.7 0.4 Dy 4.5 4.4 4.4 4.5 0.15 0.15 0.07 Er 2.2 2.3 2.3 2.2 0.09 0.03 0.05 Eu 2 2 2 2 0.03 bdl 0.02 Gd 6.2 6 6 6.1 0.10 bdl 0.07 Ho 0.86 0.85 0.87 0.84 0.03 bdl bdl La 20 20 20 20 bdl bdl 0.2 Lu 0.37 0.35 0.38 0.36 bdl bdl bdl Nd 26 25 26 26 0.2 bdl 0.2 Pr 5.9 5.8 5.8 6 0.03 0.02 0.06 Sm 6.4 6.2 6.2 6.3 0.06 0.02 0.06 Tb 0.84 0.83 0.83 0.82 bdl bdl bdl Tm 0.34 0.32 0.33 0.34 bdl bdl bdl Y 24 24 26 25 0.83 0.38 0.49 Yb 2.3 2.2 2.3 2.3 0.09 0.05 0.06

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68 Table 4-1. Continued Location: Tubaf Tubaf Tuba f Tubaf Tubaf Tubaf Tubaf Sample: 54GTVA2L 55GTVA2A 55GTVA2D 55GTVA2F 55GTVA2H 56GTVA2A 56GTVA2B Type: lherzolite lherzolite lherzolite lherzolite harzbur gite lherzolite lherzolite SiO2 42.00 42.60 42.70 41.50 42.60 43.80 39.50 TiO2 bdl bdl bdl 0.02 bdl bdl 0.02 Al2O3 2.10 1.20 1.90 4.90 3.50 1.00 1.50 Fe2O3T 9.14 9.10 8.45 8.16 9.12 8.83 12.00 MnO 0.13 0.13 0.13 0.12 0.14 0.13 0.17 MgO 46.40 45.70 44.40 42.80 43.60 46.20 46.00 CaO 0.75 0.72 0.72 0.97 0.79 1.02 1.59 Na2O bdl 0.03 bdl 0.09 bdl 0.11 0.06 K2O bdl bdl bdl 0.09 bdl bdl 0.05 P2O5 0.01 bdl bdl bdl bdl bdl bdl Total 100.2 99.4 98.1 98.4 99.4 100.9 100.4 LOI bdl bdl 0.1 bdl bdl bdl bdl Ba bdl 60 23 bdl bdl bdl 159 Be bdl bdl bdl bdl bdl bdl bdl Co 130 124 116 114 120 125 167 Cr 2210 2870 3000 3100 2730 2850 4970 Cs bdl bdl bdl 0.04 bdl 0.06 0.07 Cu 14 23 14 12 10 15 17 Hf 0.46 0.21 0.37 0.71 0.57 0.14 0.94 Nb 0.07 bdl bdl 0.17 0.07 0.07 0.12 Ni 2710 3930 2510 2380 2480 3210 2040 Pb 1 bdl bdl bdl bdl bdl bdl Rb 0.14 0.07 0.10 1.10 0.25 0.34 1.00 Sc 9.1 9.0 10.0 10.0 10.0 12.0 8.4 Sr bdl bdl bdl 48 bdl bdl bdl Ta bdl bdl bdl 0.03 bdl 0.04 bdl Th 0.03 bdl 0.02 0.08 0.03 bdl 0.17 U 0.02 bdl 0.02 0.06 0.03 bdl 0.05 V 45 45 50 52 43 54 52 Zn 47 43 43 45 41 40 66 Zr 19.0 9.2 16.0 30.0 25.0 5.4 43.0 Ce 0.1 0.7 1.1 3.9 2.3 bdl 5.0 Dy 0.03 0.08 0.10 0.35 0.19 bdl 0.38 Er 0.03 0.02 0.02 0.08 0.02 bdl 0.05 Eu bdl bdl bdl 0.06 bdl bdl bdl Gd bdl bdl bdl 0.16 0.02 bdl 0.08 Ho bdl bdl bdl 0.04 bdl bdl 0.02 La bdl bdl bdl 0.6 0.1 bdl 0.5 Lu bdl bdl bdl 0.02 bdl bdl bdl Nd bdl bdl bdl 0.7 0.1 bdl 0.4 Pr 0.02 0.03 bdl 0.18 0.04 bdl 0.12 Sm bdl 0.03 bdl 0.19 0.04 bdl 0.06 Tb bdl bdl bdl 0.03 bdl bdl bdl Tm bdl bdl bdl bdl bdl bdl bdl Y 0.22 0.27 0.24 1.10 0.48 0.12 0.96 Yb 0.05 0.04 0.04 0.10 0.05 0.03 0.06

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69 Table 4-1. Continued Location: Tubaf Tubaf Tuba f Tubaf Tubaf Tubaf Tubaf Sample: 56GTVA2T 56GTVA2P 56GTVA2G 56GTVA2H 56GTVA2X 56GTVA2F 54GTVA3A Type: lherzolite lherzolite dun ite dunite dunite dunite gabbro SiO2 47.10 41.40 40.30 43.40 40.60 38.80 57.70 TiO2 bdl bdl bdl bdl 0.03 bdl 1.76 Al2O3 3.10 2.40 1.70 1.10 2.30 1.60 15.50 Fe2O3T 7.94 8.84 11.70 8.72 7.53 10.10 8.89 MnO 0.14 0.13 0.17 0.13 0.11 0.14 0.10 MgO 41.90 47.20 45.90 45.70 44.80 48.00 2.75 CaO 0.87 0.79 1.00 0.84 2.25 0.32 6.70 Na2O 0.05 0.03 0.07 bdl 0.19 bdl 5.17 K2O bdl 0.05 0.06 bdl 0.38 bdl 0.30 P2O5 bdl bdl bdl bdl bdl bdl 0.13 Total 101.0 100.6 100.0 99.7 99.1 98.6 98.5 LOI 0.1 bdl bdl 0.2 0.0 bdl 0.2 Ba bdl bdl bdl bdl 90 bdl bdl Be bdl bdl bdl bdl bdl bdl 1.0 Co 101 129 165 128 120 153 17 Cr 4350 3030 682 3200 4250 3460 bdl Cs 0.03 0.06 0.08 bdl 0.35 0.02 0.10 Cu 14 19 15 bdl 71 bdl bdl Hf 0.47 0.58 0.70 0.18 0.23 0.17 6.50 Nb 0.08 0.06 bdl bdl 0.08 0.06 3.00 Ni 1430 3090 1870 2650 4060 2510 bdl Pb bdl 3 bdl bdl 1 8 bdl Rb 0.34 0.50 0.88 0.15 11.00 0.14 3.20 Sc 10.0 9.5 7.9 11.0 11.0 5.0 28.0 Sr bdl bdl bdl bdl 24 bdl 142 Ta bdl bdl bdl bdl bdl bdl 0.21 Th 0.03 0.05 0.04 bdl 0.10 bdl 0.32 U bdl 0.02 0.02 bdl 0.03 bdl 0.17 V 46 45 29 53 50 32 161 Zn 44 42 55 45 38 46 73 Zr 21.0 27.0 32.0 7.3 9.0 7.3 242.0 Ce 2.1 0.1 3.7 bdl 0.9 bdl 23.0 Dy 0.19 0.03 0.33 bdl 0.24 bdl 12.00 Er 0.04 0.03 0.05 0.02 0.12 bdl 6.60 Eu bdl bdl bdl bdl 0.07 bdl 2.10 Gd 0.04 0.02 0.05 bdl 0.21 bdl 9.60 Ho bdl bdl 0.02 bdl 0.05 bdl 2.40 La bdl bdl 0.1 bdl 0.3 bdl 6.5 Lu bdl bdl bdl bdl 0.02 bdl 1.10 Nd bdl bdl 0.1 bdl 0.4 bdl 21.0 Pr 0.03 bdl 0.03 bdl 0.11 bdl 3.90 Sm 0.03 bdl 0.04 bdl 0.18 bdl 7.20 Tb bdl bdl bdl bdl 0.04 bdl 1.70 Tm bdl bdl bdl bdl bdl bdl 1.10 Y 0.55 0.23 0.82 0.14 1.30 0.12 72.00 Yb 0.06 0.05 0.06 0.04 0.14 0.03 7.20

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70 Table 4-1. Continued Location: Tubaf Tubaf Tubaf Tubaf Tubaf Tubaf Tubaf Sample: 54GTVA3D 54GTVA3E 54GTVA3I 55GTVA3E 55GTVA3F 56GTVA3I 56GTVA3H Type: gabbro gabbro gabbro gabbro gabbro gabbro gabbro SiO2 57.30 50.50 52.60 42.60 42.50 50.10 47.00 TiO2 1.98 1.44 2.20 2.38 2.14 0.86 0.43 Al2O3 16.00 15.70 13.50 15.80 15.30 16.60 16.40 Fe2O3T 9.90 10.80 15.70 17.40 17.30 11.20 8.12 MnO 0.14 0.17 0.16 0.19 0.20 0.21 0.14 MgO 2.69 6.39 5.26 7.57 8.83 8.00 9.29 CaO 5.55 10.80 4.85 12.60 11.80 11.80 14.40 Na2O 4.73 2.93 4.06 1.58 1.65 2.18 1.91 K2O 0.69 0.30 0.36 0.11 0.30 0.09 1.56 P2O5 0.07 bdl 0.09 bdl bdl bdl 0.01 Total 98.4 98.2 98.0 99.4 99.1 100.2 98.8 LOI 0.9 0.3 1.2 bdl 0.4 0.1 0.3 Ba 29 36 93 bdl bdl 21 38 Be 1.1 0.5 0.9 bdl bdl bdl bdl Co 24 38 40 56 66 40 42 Cr bdl 112 11 12 200 253 135 Cs 0.16 0.07 1.40 0.04 0.35 0.02 0.75 Cu 11 29 bdl 80 82 36 66 Hf 5.50 2.50 4.70 0.93 0.62 0.98 0.32 Nb 3.00 1.90 2.30 0.40 0.21 0.31 0.13 Ni 40 51 24 68 129 61 85 Pb bdl bdl bdl 5 4 bdl bdl Rb 6.60 2.80 13.00 1.00 8.50 0.57 30.00 Sc 29.0 41.0 35.0 50.0 52.0 49.0 52.0 Sr 148 120 102 161 106 130 316 Ta 0.25 0.15 0.19 0.04 0.04 0.04 0.02 Th 0.30 0.12 0.20 0.07 0.10 0.03 0.05 U 0.18 0.10 0.14 0.06 0.07 0.03 0.06 V 169 330 382 1030 1120 301 299 Zn 60 105 60 128 121 123 122 Zr 207.0 81.0 169.0 31.0 22.0 35.0 9.2 Ce 17.0 14.0 21.0 4.6 3.9 5.7 1.7 Dy 11.00 7.70 9.50 2.80 1.80 3.70 1.50 Er 6.50 4.30 5.30 1.50 0.99 2.20 0.90 Eu 2.20 1.40 1.90 0.67 0.42 0.97 0.43 Gd 9.20 6.10 8.00 2.30 1.30 3.00 1.20 Ho 2.40 1.60 2.00 0.56 0.36 0.79 0.33 La 4.3 3.7 6.3 1.2 1.2 1.1 0.7 Lu 1.10 0.70 0.87 0.23 0.16 0.36 0.13 Nd 19.0 14.0 18.0 3.8 1.9 4.8 1.7 Pr 3.30 2.50 3.50 0.67 0.41 0.76 0.28 Sm 6.60 4.50 5.90 1.40 0.78 1.90 0.77 Tb 1.60 1.10 1.40 0.40 0.25 0.56 0.24 Tm 1.00 0.71 0.85 0.23 0.16 0.34 0.12 Y 67.00 49.00 57.00 16.00 11.00 22.00 8.40 Yb 7.00 4.70 5.70 1.50 1.00 2.20 0.80

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71 Table 4-1. Continued Location: Tubaf Tubaf Tubaf Tubaf Tubaf Tubaf Tubaf Sample: 56GTVA3O 56GTVA4D 56GTVA.4E 56GTVA.4F 54GTVA.6F 54GTVA.6B 54GTVA.6E Type: basaltic sedimentary sedimentary sedime ntary sedimentary sedimentary sedimentary SiO2 51.00 42.20 46.50 15.30 48.40 41.30 32.70 TiO2 2.56 0.19 0.51 0.12 0.68 0.43 0.34 Al2O3 14.10 10.90 13.90 4.20 16.00 9.80 6.70 Fe2O3T 15.00 2.34 6.29 1.21 5.80 4.31 3.31 MnO 0.28 0.07 0.32 0.04 0.12 0.12 0.11 MgO 4.89 2.14 2.89 0.76 2.25 1.60 1.15 CaO 9.14 22.30 17.40 43.10 13.10 23.50 29.70 Na2O 3.52 2.34 2.21 0.67 2.82 1.77 1.26 K2O 0.12 1.02 0.64 0.33 2.27 0.78 0.74 P2O5 0.20 0.12 0.15 0.12 0.08 0.16 0.11 Total 100.9 83.8 90.9 66.0 91.6 83.9 76.3 LOI bdl 16.1 9.9 32.3 7.5 15.9 22.7 Ba bdl 163 92 64 212 70 201 Be 0.5 0.5 0.5 bdl 0.5 bdl bdl Co 34 bdl 11 bdl 12 6 7 Cr 37 16 19 12 24 30 22 Cs 0.04 0.69 10.00 0.52 4.00 0.57 0.44 Cu 47 96 223 25 123 38 49 Hf 2.20 1.70 1.40 0.70 1.30 1.50 1.10 Nb 3.40 2.50 1.40 0.99 1.30 1.50 1.20 Ni 15 13 23 bdl 18 12 11 Pb bdl 5 3 2 4 2 2 Rb 1.00 15.00 42.00 7.70 45.00 15.00 13.00 Sc 36.0 3.9 18.0 2.1 24.0 12.0 9.0 Sr 144 1221 518 911 537 892 818 Ta 0.25 0.16 0.07 0.06 0.09 0.10 0.07 Th 0.05 1.20 0.59 0.48 0.37 0.52 0.40 U 0.04 0.66 0.68 0.65 0.79 1.00 1.40 V 454 28 114 23 165 83 74 Zn 134 65 81 68 80 67 68 Zr 81.0 64.0 51.0 28.0 46.0 61.0 42.0 Ce 14.0 24.0 13.0 7.5 7.5 17.0 8.3 Dy 9.30 1.50 3.00 1.10 2.60 2.50 2.00 Er 5.40 0.86 1.90 0.69 1.60 1.40 1.20 Eu 2.40 0.54 0.80 0.28 0.63 0.72 0.41 Gd 8.20 1.80 2.90 1.10 2.30 2.60 1.80 Ho 2.00 0.31 0.65 0.24 0.56 0.52 0.44 La 3.3 13.0 7.2 5.9 4.0 9.2 5.2 Lu 0.88 0.18 0.32 0.12 0.25 0.25 0.22 Nd 15.0 12.0 9.6 5.4 5.9 11.0 5.9 Pr 2.40 3.30 2.00 1.30 1.20 2.50 1.30 Sm 5.90 2.00 2.40 1.00 1.80 2.50 1.40 Tb 1.50 0.26 0.47 0.16 0.41 0.43 0.30 Tm 0.81 0.14 0.30 0.11 0.26 0.22 0.20 Y 53.00 12.00 22.00 11.00 16.00 17.00 14.00 Yb 5.40 0.96 1.90 0.72 1.60 1.50 1.30

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72 Figure 4-2. Classification of th e Conical, Edison, Tubaf, a nd Lihir lavas based on their SiO2 vs Na2O+K2O concentrations (after LeMaitre, 1989). Data from this study and from Kennedy et al. (1990a ), Stracke and Hegner (1998), and Mller et al. (2001). The field for the mafic xenoliths is based on the several analyses presented in Table 4-1. Note that although the mafic xenoliths show relatively wide range in their SiO2 content, they plot at lower alkali contents relative to the TLTF lavas.

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73 Figure 4-3. Comparison of MgO vs CaO, Fe 2O3, Na2O, and Al2O3 between Lihir and seamount lavas. Note that CaO and Fe2O3 decrease, and Na2O, and Al2O3 increase with the decrease in the MgO, suggesting that lavas were affected by fractionation processes (for more discu ssion see the text). Data sources and symbols are the same as in Figure 4-2.

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74 Figure 4-4. Comparison of MgO vs Ni, V, Ba, and Rb between Lihir and seamount lavas. Ni and V behave compatible and decrease with the decrease in MgO, and Ba and Rb behave incompatible and incr ease with the decrease in MgO, both trends indicating fractination (see the te xt for more discussion). Data sources and symbols are the same as in Figure 4-2.

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75 Figure 4-5. Normalized REE patterns of TLTF lavas and mafic and ultramafic xenoliths. TLTF data from this study and from Stracke and Hegner (1998). Ultramafic xenoliths data from Gregoire et al. (2001) and Franz et al. (2002). Primitive mantle-normalized rare-earth el ements (REE) patterns for the seamounts and TLTF islands are subparallel and show enrichments in light rare-earth elements (LREE) relative to heavy rare-earth elements (HREE) (Fig. 4-5). Similarly, the primitive mantle-normalized trace element variations (spider diagram) for the seamount lavas follow the same pattern as the Lihir and the other TLTF island lavas (Fig. 4-6). They are characterized by negative Nb and Ti anomalies, and positive U, K, Pb, and Sr anomalies, typical for island arc volcan ics (Perfit et al. 1980). 0.001 0.01 0.1 1 10 100 LaCePrNdSmEuGdTbDyHoErTmYbLuTLTF lavasSample/Primitive Mantle Mafic xenoliths Ultramafic xenoliths 542D 562B

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76 Figure 4-6. Primitive Mantle normalized trace element patterns of TLTF lavas and mafic xenoliths. Primitive Mantle values from Sun and McDonough (1989).

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77 The mafic xenoliths cont ain 42.5 to 57.7 wt % SiO2 (i.e., from gabbro to diorite, Table 4-1, Fig. 4-2). A few plagiogranite xenol iths also have previously been recovered (McInnes et al., 2001). Overall, the mafic xenolit hs are subalkaline with low total alkalis relative to the Lihir and seamount lavas (Fig. 4-2). They exhibit depleted LREE patterns (Fig. 4-5) and enrichments in some fluid-mobile elements, such as Ba, K, U, and Pb (Fig. 4-6). Ultramafic xenoliths have high MgO cont ent, ranging from 26% to 48.7% and low contents of CaO, Na2O, Al2O3 (Table 4-1). They are also enriched in compatible elements, such as Ni and Cr, and depleted in incompatible elements, most below detection limits (Table 4-1). Isotopic analyses of lavas, xenolith, and sediment samples from the study area are presented in Table 4-2 and also in Kame nov et al. (2004b) (Chapter 3). Sr isotopic compositions of fresh lavas from the region, including TLTF, New Britain, and Solomon Islands and Manus and Woodlark Basins, show relatively small 87Sr/86Sr variations with values close to 0.704 (Table 4-2), similar to previously published data (Wallace et al. 1983; Johnson et al. 1988; Trul l et al. 1990; Kennedy et al. 1990b; Stracke and Hegner 1998; Woodhead et al. 1993, 1998). Measured 87Sr/86Sr in sedimentary xenoliths recovered from Tubaf seamount lavas have more radiogenic 87Sr/86Sr values, ranging from 0.7067 to 0.7083, compared to the lava s in the area (Table 4-2). Sr isotopic compositions of the analyzed ultramafic xe noliths exhibit a wider range than the TLTF lavas, ranging from 0.7037 to 0.7059 (Table 4-2).

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78 Table 4-2. Pb, Sr, and Nd isotopic anal yses of lava and xenolith samples Location Type sample 206Pb/204Pb 207Pb/204Pb 208Pb/204Pb 87Sr/86Sr 143Nd/144Nd Tabar lava TAB 3 18.773 15.553 38.377 0.70355 0.513037 Tabar lava TAB 4 18.714 15.547 38.345 Tanga lava DR11.1/2A 18.735 15.542 38.342 0.704195A 0.51301 Feni lava DR12.1/8A 18.679 15.555 38.370 0.704005A Rabaul lava RAB 2 18.813 15.543 38.423 0.70367 0.513006 Rabaul lava RAB 981 18.815 15.545 38.423 Bouganville lava 69B 18. 573 15.533 38.283 0.70370 0.512988 Solomon lava 24-12 18.622 15.580 38.448 0.703916B Solomon lava 31-1 18.443 15.509 38.252 0.703503B Savo lava SAVO 2 18.424 15.515 38.225 0.70408 0.512971 Kavachi lava 35-2 18.394 15.518 38.203 0.703724B Woodlark lava 4 29-H 18.273 15.484 37.927 0.702826B Woodlark lava 4 26-5 18.288C 15.522 38.005 0.70258B 0.513095 Woodlark lava 4 26-14 17.947C 15.492 37.629 0.70270 Woodlark lava 4 29-8 17.890C 15.511 37.591 0.70282 0.513078 Woodlark lava 4 32-10 18.117C 15.494 37.758 0.70267 0.513151 Woodlark lava 4 32-16 18.116C 15.530 37.910 0.703311B 0.513063 Woodlark lava 4 32-17 0.70351 0.513028 Tubaf lava 54 4-2 18.759 15.554 38.391 0.70397 0.5130017 Tubaf lava 56 2C 18.756 15.550 38.377 0.70397 0.513003 Tubaf lava 56 2M 18.767 15.550 38.366 Xenoliths: Tubaf sedimentary 54 6G 18.640 15.568 38.255 0.70828 0.5129289 Tubaf sedimentary 54 6B 18.627 15.557 38.302 0.70751 0.5129586 Tubaf sedimentary 56 4F 18.770 15.583 38.495 0.70669 0.5129378 Tubaf sedimentary 54 6E 18.901 15.560 38.494 0.70759 0.5128994 Tubaf sedimentary 54 6F 18.771 15.567 38.441 0.5129111 Tubaf sedimentary 56 4D 18.712 15.528 38.307 0.70791 Tubaf veined lherzolite 54 2D 19.120 15.652 38.633 0.70374 0.513010 Tubaf harzburgite 562F 18.694 15.557 38.321 0.70398 0.512910 Tubaf veined lherzolite 56 2B 18.340 15.567 38.263 0.70437 Tubaf harzburgite 54 2G 18.499 15.641 38.523 0.70592 0.512920 Tubaf veined lherzolite 56 2P 18.800 15.577 38.414 0.70453 Tubaf harzburgite 55 2H 18.514 15.524 38.192 0.70404 Tubaf veined lherzolite 54 2S 18.853 15.672 38.652 Tubaf veined lherzolite 54 2K 19.099 15.759 38.781 Tubaf lherzolite 55 2F 0.70398 Tubaf CPx separate 61 1C CP x 17.859 15.567 37.672 0.70446 0.51298 Tubaf CPx separate 67 1 CP x 17.591 15.561 37.429 0.70411 0.51292 Tubaf CPx separate 61 1B CP x 17.892 15.567 37.716 0.70457 0.512963 Tubaf basaltic 56 3O 18. 692 15.509 38.163 0.70297 0.513077 Tubaf gabbro 55 3E 18.722 15.533 38.280 0.70333 0.513030 Tubaf gabbro 56 3I 18.647 15.529 38.191 0.70293 Tubaf gabbro 54 3E 18.747 15.509 38.196 0.70302 0.513056 Tubaf gabbro 56 3H 0.70366 Tubaf gabbro 55 3F 0.70343 A-data from Johnson et al. 1988; B-data from Trull et al.1990; C-TIMS Pb analyses;

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79 Sr isotopic compositions in th e analyzed 3 clinopyroxene se parates from the ultramafic xenoliths range from 0.7041 to 0.7046. The mafic xenoliths have slightly less radiogenic 87Sr/86Sr values, ranging from 0.7029 to 0.7036 (Table 4-2). Fresh volcanic rocks from Lihir a nd the seamounts exhibit small, but distinguishable variations in their Pb isot opic compositions, with Conical seamount lavas having slightly less, and Tubaf and Edis on seamount lavas havi ng slightly more radiogenic ratios (Kamenov et al. 2004b). Lavas from Tabar and Tanga show overall similar Pb isotopic compositions to Lihir and the seamounts (Table 4-2). Feni lavas have slightly less radiogenic, whereas Rabaul lavas have slightly more radiogenic Pb isotope values compared to the other TLTF lavas (Table 4-2). Solomon Islands and Woodlark Basin samples show less radiogenic Pb isotopi c ratios than the TLTF islands (Table 4-2). Sedimentary xenoliths recovered from the Tuba f seamount lavas exhibit a larger range in their lead isotope compositions compared to th e fresh volcanic rocks in the area (Table 4-2, Chapter 3) with values ranging from 206Pb/204Pb=18.627 to 18.901, 207Pb/204Pb=15.509 to 15.585, and 208Pb/204Pb=38.163 to 38.512. Surprisingly, th e ultramafic xenoliths exhibit a very wide range in their Pb isotopic compositions, ranging from 206Pb/204Pb=18.340 to 19.120, 207Pb/204Pb=15.524 to 15.759, and 208Pb/204Pb=38.192 to 38.781. The 3 clinopyroxene separates, however, show distinct Pb isotopic compositions from the ultramafic xenoliths, with 206Pb/204Pb=17.591 to 17.892, 207Pb/204Pb=15.561 to 15.567, and 208Pb/204Pb=37.429 to 37.716. The mafic xenoliths, on the other hand, show Pb isotopic compositions from similar to the lavas to slightly le ss-radiogenic, ranging from 206Pb/204Pb=18.647 to 18.747, 207Pb/204Pb=15.509 to 15.533, and 208Pb/204Pb=38.163 to 38.280 (Table 4-2).

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80 Nd isotopic compositions of the lavas fr om the region show small variations, ranging from 0.51297 to 0.51315, similar to prev iously published data (Stracke and Hegner 1998). The mafic and ultramafic xenol iths overall exhibit similar Nd isotopic compositions to the lavas (Table 2). Sedi mentary xenoliths have slightly lower 143Nd/144Nd compared to the lavas and igneous xenoliths (Table 4-2). Discussion Major and Trace Element Variations in Seamount and Lihir Lavas Although the lavas in the TLTF area are relatively alkaline their overall major and trace element abundances are typical for potas ic magmatic rocks recovered in island arc settings (Mller et al. 2001 ) Elemental variations in the seamount and Lihir samples suggest that most of the lavas were modified to some degree by fr actional crystallization. The observed decrease in CaO and increase in Al2O3 with decreasing MgO (Fig. 4-3) suggests that plagioclase fracti onation did not play a major role in magmatic evolution. Although a common phenocryst in the Conical seamount and Li hir lavas, plagioclase is not common in Edison and is rare in T ubaf lavas. The Cpx is the most abundant phenocryst observed in the lavas from the ar ea, and Lihir island samples form a positive correlation between the MgO and CaO/Al2O3 ratio (Fig. 4-7), indicating Cpx as a dominant phase in the crystallization sequen ce, in agreement with previous observations (Stracke and Hegner 1998). An experime ntal study on Lihir lavas also provided evidences that the magma tic evolution was mainly controlled by clinopyroxene, accompanied by amphibole, magnetite, and minor olivine fractionation (Kennedy et al. 1990a).

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81 Figure 4-7. Plot CaO/Al2O3 vs MgO indicating clinopyrox ene as a major fractionation phase. Symbols and data sources ar e the same as in Figure 4-2. Mller et al. (2001 and 2003) conducted exte nsive P-T studies on mineral phases in Lihir and Conical seamount lavas and suggested that ma gma evolution occurred in two stages: one deeper, controlled by early hornbl ende crystallization, and a second shallower stage, associated with clinopyroxene and plag ioclase crystallization. Plagioclase is not a stable phase at higher pressures and in a ddition, the plagioclase stability field is suppressed in magmas with high H2O concentrations (Carmichael et al. 1996). The absence of plagioclase in the more primitive Tubaf lavas is consistent with the Tubaf lavas containing the highest volatile contents of the TLTF suite (Chapter 3). Furthermore, the abundance of ultramafic xenoliths in Tuba f suggest that this magma rose rapidly to the surface, without much time for low-pressu re evolution close to the surface (Chapter 3), another potential explanation for ab sence of plagioclase from the phenocryst assemblage. Decreasing Fe2O3 with decreasing MgO indica tes removal of a Fe-rich

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82 phase, most probably magnetite, during fracti onal crystallization. Magnetite is abundant in the lavas and also was suggested to pl ay a role in the magma evolution by the experimental study of Kennedy et al. (1990a). It is a common feature for arc magmas to show a decrease in Fe2O3 with evolution indicating high fO2, which stabilizes magnetite early in the crystallization sequence. Indee d, several previous studies confirmed the high fO2 state of the magmas in the area (Ke nnedy et al. 1990a, McInnes and Cameron 1994, McInnes et al. 2001). In addition, the positive correlation between MgO and Ni (Fig. 4-4) also indicates that olivine was present in the early fractionation sequence, even though it is not observed as a phenocryst in the lavas. The major element data for Lihir Isla nd and the seamounts show considerable scatter on Harker diagrams, which suggests th at the magmas did not evolve along a single liquid line of descent from a common parental magma. Incompatible trace elements, such as Rb, Ba, La, Ce, U, Th, and Pb plotted ve rsus MgO also show considerable scatter indicating multiple parental magmas (Fig. 4-4) similar to the observations derived from the major element variations. This is fu rther supported by the observed small, but significant differences in the lavas isotopic compositions (Stracke and Hegner 1998, Kamenov et al. 2004b). Variations in parental melt compositions, on the other hand, are probably a result of small differences in the so urce materials, as indicated by the isotope data, and as well as a result of different degrees of partial melting. For example, Sr is higher in the relatively more primitive lava s from Tubaf seamount, and the higher Sr content correlates with higher La/Yb, suggesting smaller degrees of partial melting created parental Tubaf magmas (Fig. 4-8) Alternatively, these differences can be

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83 explained by magma derivation from s ources with different Sr, La, and Yb concentrations. Figure 4-8. La/Yb vs Sr for Lihir island and seamount lavas. For more discussion see the text, data symbols same as Fig. 4-2. Overall, the TLTF lavas exhi bit trace element signatur es typical for subductionrelated volcanism, in particular exhibiti ng high LILE/HFSE and el evated LREE relative to HREE ratios (Fig. 4-5 and 4-6). Although currently not associated with active subduction, this observation indi cates that the TLTF volcanism is related to a previous episode of subduction, as suggested previously by a number of studies (Kennedy et al. 1990b, McInnes et al. 1994; Stracke and Hegner 1998; Franz et al. 2002).

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84 Figure 4-9. Primitive Mantle normalized trace element patterns of mafic xenoliths compar ed with MORB, altered MORB, and oceanic gabbros. Primitive Mantle values from Sun and McDonough ( 1989), MORB data from Kelley et al. (2003), Indian Ocean gabbros from Coogan et al. (2001), and Cayman Rise gabbros from Perez-Suarez (2001).

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85 Major and Trace Element Variations in Mafic and Ultramafic Xenoliths The wealth of xenoliths recovered from T ubaf seamount allow us to look more closely at the processes and s ources in the crust and mantle and/or subducted slab and their role in the formation of the TLTF magmas. The gabbroic xenoliths have major and trace element abundances similar to gabbros recovered from oceanic settings. This observation is in agreement with the McInnes et al. (1999) study that suggested that the mafic crust underlying the TLTF islands originat ed at an oceanic spreading system. The mafic xenoliths exhibit depleti on in the LREE (Fig. 4-5) indica ting origin from a depleted mantle source. Their extended trace element patterns exhibit overall lower abundances than typical MORB (Fig. 4-9) in dicating that the gabbros are cumulates, similar to other mid-ocean ridge gabbros (Coogan et al. 200 1). The elevated concentration of Ti in some samples (Fig. 4-9) is due to Ti-magnetite accumulation, a common accessory mineral observed in the mafic xenoliths (McI nnes et al. 2001). The mafic xenoliths traceelement patterns exhibit enrichments in fluidmobile elements, such as Ba, U, K, and Pb (Fig. 4-9) suggesting that part of the mafic crust was affect ed by alteration probably after its formation at an oceanic spreading system Indeed, a majority of the studies on ODP cores penetrating into the basa ltic and gabbro layer of the oc eanic crust provide evidences for chemical exchange between the oceanic lithosphere and seawat er (Bach et al. 2001, Kelley et al. 2003). The overall high MgO and low CaO, Na2O, Al2O3 content observed in the ultramafic xenoliths (Table 4-1) indicates that they are residues after partial melting event (Gregoire et al. 2001, McInnes et al. 2001, Franz et al. 2002, for detailed discussion). Almost all of th e xenoliths exhibit metasomatic alteration features, such as veins and irregular alteration ar eas (Gregoire et al. 2001, Fran z et al. 2002). Overall, the

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86 Figure 4-10. Primitive Mantle normalized trace element patterns of ultramafic xenoliths compared to TLTF lavas. Primitive Mantl e values from Sun and McDonough (1989), ultramafic xenol iths data from Gre goire et al. (2001).

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87 ultramafic xenoliths have strongly depleted trace element abundances compared to the TLTF lavas and mafic xenoliths. The REE patte rns observed in the ultramafic xenoliths range from LREE and HREE enriched, so calle d spoon-shape pattern, to almost flat, slightly LREE enriched pattern (Fig. 4-5). The LREE enrichment can be explained by a metasomatic enrichment from a hydrous fluid phase (Gregoire et al. 2001, Franz et al. 2002). In addition, the metasomatized xe noliths exhibit high LILE/HFSE and LREE/HFSE ratios (Fig. 4-10), al so indicative of enrichment by hydrous fluids (Gregoire et al. 2001). Sr and Nd Isotopic Variations in Xenolith and Lava Samples. The measured Sr and Nd isotopic compositions in the mafic xenoliths are similar to the Pacific-Indian MORB (Fig. 4-11), indi cating derivation from a depleted mantle source which is consistent with their trac e element and REE patterns. Some of the ultramafic xenoliths, on the other hand, exhibit elevated 87Sr/86Sr at relatively high 143Nd/144Nd (Table 4-2, Fig. 4-11) and their trac e element patterns provide evidence for metasomatic alteration by hydrous fluid releas ed most probably from the subducted slab. Experiments show that Sr is partitioned in hydrous fluid phase while the REE are relatively immobile and do not partition into the fluid (Brenan et al. 1995). Therefore, fluids released during dehydration of subducte d sediments and/or a ltered oceanic crust are expected to have relatively high Sr and low Nd contents. Metasomatism of the TLTF mantle wedge by such fluids will shift the Sr isotopic compositions of the ultramaifc xenoliths to more radiogenic ratios, but at th e same time will not have strong effect on the Nd isotopic ratios, which appears to be the case (Fig. 4-11). 143Nd/144Nd measured in the lavas from the re gion are generally similar to Indian and Pacific MORB, indicating th at Nd reflects deri vation from a depleted mantle source.

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88 Figure 4-11. Nd vs Sr isotopic compositions of the mafic and ultramafic xenoliths relative to possible source in the area. Data fr om this study; and White (1987), Othman et al. (1989), Ewart and Hawkeswo rth (1987) Vroon et al. (1993), and Stracke and Hegner (1998). Although the TLTF lavas have a MORB-type Nd isotopic signature, some of them are displaced to slightly elevated 87Sr/86Sr values compared to the Indian and Pacific MORB. The elevated Sr isotopic ratios suggest invol vement of sedimentary and/or altered oceanic crust material. Altered oceanic crus t is expected to acquire higher 87Sr/86Sr as a result of interaction with seawater, but at the same time it will preserve its 143Nd/144Nd due to the relative immobility of the REE during hydrothermal altera tion. Incorporation of material in the TLTF magmas from altered oceanic crus t, therefore, may produce the trend exhibit by the TLTF volcanoes (Fig. 4-11). In addi tion, incorporation of small amount of

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89 sediment or material from the metasomatized mantle wedge in the TLTF magma also can produce the observed elevated 87Sr/86Sr ratios coupled with elevated 143Nd/144Nd and this mechanism will be discussed in greater details later below. Pb Isotopic Variations in the Mafic and Ultramafic Xenoliths The mafic xenoliths show an overall small range in their Pb isotopic compositions with two of the analyzed samples having co mpositions similar to the TLTF lavas, and two exhibiting slightly lower 207Pb/204Pb compared to the TLTF lavas (Fig. 4-12). The mafic xenoliths plot entirely within the field of the Pacific MORB, in agreement with their trace element and Nd isotopic characteri stics indicating derivation from a depleted mantle source. The similarity between the Pb isotopic compositions of the mafic xenoliths with the Pacific MORB indicates that the oceanic crust beneath the TLTF islands most probably originated at a spreading center in the Pacific Ocean. The ultramafic xenoliths trace element patterns and petrogr aphic observations indicate that the New Ireland forearc mantle wedge (i.e., at present, the area beneath the TLTF island chain) has been metasomatically modified by fluids released during earlier subduction of the Pacific plate beneath the Indo-Australian pl ate (McInnes et al. 2001, Grgoire et al. 2001, Franz et al. 2002). It is difficult to put a time frame on the metasomatic events, but it is plausible to s uggest that the forearc region was continuously flushed by hydrous fluids more or less as l ong as the Pacific plat e subduction was active. TLTF volcanism, however, postdates the activ e subduction by several million years (McInnes et al. 2001), indicating that it is not directly caus ed by the metasomatic event.

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90 Figure 4-12. Pb isotopic compositions of mafi c and ultramafic xenoliths relative to possible sources in the area. Data sources form this study and same sources as in Figures 4-11.

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91 During active subduction, the cold oceanic lithosphere will maintain a relatively cold thermal regime in the forearc regi on, unless very young oceanic lithosphere is subducted (Harry and Green 1999). It is difficu lt to determine the age of the subducted slab beneath New Ireland, but we can speculate that it was relatively old, Jurassic or Cretaceous in age, formed probably sometime before the Ontong Java Plateau emplacement (Cretaceous). The thermal regime therefore, was probably not suitable to induce partial melting in the fore-arc mantle wedge, and during that time fluids released mainly from compaction and dehydration of the sedimentary layer of the subducted oceanic slab flushed the fore-arc mantle wedge and caused the widespread metasomatism observed in the ultramafic xenoliths. The Pb isotopic compositions of most of the ultramafic samples show elevated 207Pb/204Pb relative to the Paci fic MORB and plot close to the Pacific sediment field and Banda ar c volcanics (Fig. 4-12). Previous studies showed that the isotopic compositions of T onga and Banda arc volcanics are result of subduction of continental material (Vroon et al. 1993) and, therefore, the Pb isotopic compositions of the ultramafic xenoliths with elevated 206Pb/204Pb and 207Pb/204Pb can be best explained by addition of Pb by de hydration and compaction of the subducted sediments during the earlier period of subduc tion. Two of the analyzed ultramafic xenoliths (562F and 552H), however, plot within the field of the Pacific MORB indicating that they may repres ent samples from the Pacific oceanic mantle more or less uncontaminated by sedimentary material (Fi g. 4-12, Table 4-2). Sample 562F have Pb isotopic composition similar to the TLTF lava s and its Pb isotopic composition can be generated also by contamination of the xe nolith with Tubaf magm a during the eruption. On the other hand, sample 562B, and particul arly the 3 clinopyroxene separates from

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92 ultramafic xenolith samples show higher 207Pb/204Pb at relatively lower 206Pb/204Pb and plot away from the fields defined by th e Pacific MORB and sediments (Fig. 4-12). Several authors have suggested involvement of Indian-type mantle in the SW Pacific arcs (Hickey et al. 1995, Woodhead et al. 1998 ) and in the Eastern Australia Cenozoic basalts (Zhang et al. 2001). The clinopyroxe nes and sample 562B show Pb isotopic compositions similar to the Indian type mantle indicating that there may be remnants of old lithosphere in the TLTF mantle wedge. A lternatively, their isot opic compositions can be a result of involvement of Australian s ubcontinental mantle in the mantle wedge beneath the TLTF, as will be discussed below. Re gardless of the origin of the Pb isotopic signature in the clinopyroxene se parates, the ultramafic samples show a very wide range and point toward 3 different Pb sources i nvolved in the mantle wedge beneath TLTF, which can be identified as Pacific-type man tle, Pacific sediments, and either Indian-type mantle or Australian subcontinental mantle. Indian Ocean-Type Mantle Versus Austra lian Subcontinental Mantle Incorporation in the Mantle Wedge Beneath TLTF Two possibilities must be considered for the origin of the Indian Ocean-type Pb end-member identified in the mantle wedge beneath the TLTF islands. It can be either a result of involvement of Indian-type mantle or Australian subcontinental mantle in the TLTF mantle wedge, or alternatively, it could be created by local proces ses. The origin of the Indian-type mantle signature has been at tributed to a contamination of a depleted mantle source, isotopically similar to Pacifi c-Atlantic MORB, with either deep plume material or with Gondwana subcontinental li thosphere (Hickey-Vargas et al. 1995 for extensive discussion). In the case of the plume hypothesis, it is considered that one or a number of plumes that presumably caused the Gondwana break-up, contaminated the

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93 depleted mantle in the regi on. In the case of the sub-Gondw ana lithosphere contamination hypothesis, the isotopic signa ture was acquired as a resu lt of a long-term subduction along the Gondwana margins that supplied sedi mentary material to the deep mantle beneath the region. The Pb isotopic signature in both scenarios requires involvement of old (continental) material that has evolved initially at high (238U/204Pb) and then has experienced U loss (or Pb gain) and has evolve d after that at low for a long period of time. Subcontinental li thosphere, Earths core, and subducte d slabs have been considered as candidates for the low reservoir (Murphy et al. 2002). Locally, contamination of the TLTF mantle wedge with ancient low material can explain the origin of the isotopic signat ure of the clinopyroxenes and sample 562B. Contamination as a result of the most recent subduction or by deep mantle plumes, however, can be ruled out because the Pacific subducted sediments do not have the required Pb isotopic composition, and the isot opic signature of the TLTF magmas does not support deep plume material involvement. Therefore, it is highly unlikely that the observed isotopic compositions can be created as a result of local processes, suggesting that Indian-type mantle materi al or Australian subcontinenta l mantle was incorporated in the TLTF mantle wedge. On Figure 4-13, the xenoliths are shown in comparison with Cenozoic alkali basalts from Eastern and North Eastern Australia. The basalts tap different parts of the Australian subcontinenta l mantle (Zhang et al. 2001) and it can be seen that overall they plot within the fiel d of the Indian Oceantype MORB. Basically, it appears that whether we will call the elevated 207Pb/204Pb end-member Indian Oceantype mantle or Australian sublithospheric ma ntle is more or a less circular argument, because Australia was part of the Gondw ana before the super-continent breakup.

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94 Figure 4-13. Pb isotopic compositions of xeno liths relative to Indian, Pacific MORB, and Eastern and North Eastern Au stralia basalts. Data sour ces same as in Figure 4-11. Australian basalts data fr om Zhang et al. (2001).

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95 The Australian sub-lithospheric mantle, therefor e, is expected to have more or less similar Pb isotopic signature to the Indian-type mantle. It is not clear, however, how this component was incorporated beneath the TLTF island arc. Some studies demonstrate that clinopyroxene can be very refrac tory and preserve its chemical signatures even if the rest of the rock is altered (Machado et al 1986). Therefore, it is possible that the clinopyroxenes are surviving fragments from the old sub-Gondwana lithosphere, trapped in the area after the super-c ontinent breakup. Alternatively, it is possible that the Australian sublithospheric mantle is disp laced east and northeast with the northward movement of the Australian continent (R. Russo, personal communication) and so becomes incorporated in the source of the magmas in the region. Further support for the latter possibility comes from isotopic com positions of lavas from Woodlark and Manus back-arc basins, as will be discussed below. Relationships between Compositions of Xenoliths and Regional Volcanism TLTF lavas show a very restricted ra nge in their Pb isotopic compositions, compared to the possible sources in the area a nd they plot within the field of the Pacific MORB (Fig. 4-12). In addition, if only the hi gh-precision Pb isotopic compositions of Lihir and seamount lavas (and ores) are consid ered, they form a trend parallel to the NHRL and plot in the Pacific MORB field (Fig. 3-3 and 3-4, Chapter 3), indicating that the Pb is mainly contributed by Paci fic MORB-type source. There are, however, small, but distinct isotopic variations betw een the volcanic centers, indicating of some variations in the source components. The Sr isotopic compositions in the lavas indicate involvement of material from either altere d oceanic crust or sediments or from the metasomatized ultramafic assemblage ben eath the volcanoes (Fig. 4-14). The TLTF lavas plot at the higher 207Pb/204Pb end of the Pacific MORB array, which may also indicate

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96 Figure 4-14. Nd vs Sr isotopic compositions for island arc and back-arc basins lavas in the region with possible mixing relati onships between Indian-Pacific MORB and Pacific sediments or mafic and sedi mentary xenoliths. Note that some of the TLTF lavas overlap with the Pacifi c-Indain MORB field, and some show elevated 87Sr/86Sr that can be explained by small amount of sedimentary component contribution, for more disc ussion see the text. Data from this study, White (1987), Othman et al. ( 1989), Hickey-Vargas et al. (1995), Woodhead et al. (1998), and Kamenetsky et al. (2001). minor incorporation of more radiogenic Pb, ei ther from subducted sediments or from the metasomatized xenoliths. Overall, the Pacific sediments show much higher 207Pb/204Pb than the lavas in the area (Fig. 4-12), and mixing calculations indi cate that only minor (ca. from 0% to maximum around 5-6%) incorp oration of Pacific se diments can explain the isotopic compositions of the TLTF lavas (Appendix C). Using se dimentary and mafic xenoliths as end-members in the mixing calcu lations indicates very high (more than 50%) sedimentary contribution (Appe ndix C), which is not supporte d by the lavas Sr and Nd

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97 isotopic compositions. Sr and Nd isotopic values indicates maximum of about 5 to 6% sedimentary material incorporat ion in the lavas, more or le ss independent on whether the Pacific MORB-Sediment or mafic-sediment ary xenolith components are used as endmembers in the mixing calculations (Fig. 4-14). The mixing calculations involving sedimentary xenoliths (and also near-surf ace sediments) from the region must be interpreted with care because some of the analyzed sediments, notably sediments from the Solomon plate (Woodhead et al. 1998) s how isotopic similarity with the TLTF and New Britain volcanics. These Late Cenozoic sediments, how ever, contain large amount of pyroclastic material delivered to the basi n from the surrounding arcs (Lackschewitz et al. 2003). Therefore, it is plau sible to suggest that the Pb and Nd (note for example the elevated 143Nd/144Nd end of the Pacific sediment fiel d on Figure 4-11) isotopic similarity in this case is simply due th e fact that a significant amount of material was provided to the local sediments from the surrounding vol canoes. An argument in favor of this suggestion is the fact that the metasomatic signature in the ultramafic xenoliths suggests that fluids released from the subducted sediments had quite variable Pb isotopic compositions compared to the Cenozoic So lomon plate sediments. The Cretaceous Ontong Java Plateau must had been emplaced in Cretaceous or older oceanic crust, which is now subducted underneath TLTF area. We can thus argue that the sediments subducted beneath the TLTF islands were quite variab le, in age and composition, consisting of young and Cretaceous or older deep marine sediments, th erefore, spanning the whole range of the Pacific sediment field shown of Figure 4-12. Lavas from the New Britain arc front a nd Eastern Manus Basin spreading center (Fig. 4-1) show very similar Pb isotopic compositions to the TLTF volcanoes (Fig. 4-15).

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98 Figure 4-15. Pb isotopic compositions of lavas from TLTF, New Britain, Solomon Islands, Manus and Woodlark back-arc ba sins. Data sources form this study and same sources as in Figure 4-11.

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99 The current volcanism on New Britain is rela ted to the Solomon plate subduction, which is composed of basalts with geochemical and isotopic similarity to Pacific MORB (Woodhead et al. 1998). The Pb isotopic sign ature in New Britain frontal arc volcanoes was interpreted to reflect de rivation from subducted altered oceanic crust with Pacific affinity (Woodhead et al. 1998). The eastern part of the Manus spreading center (Fig. 4-1) with its Pb isotopic simila rities to TLTF and New Britain arc lavas, suggest that the magma composition is either i nherited from the previous Pa cific plate subduction, or is related to the presen t-day Solomon plate subduction. New Britain, E. Manus, and the TLTF lavas plot within the field of the Pacifi c MORB indicating that their Pb is primarily controlled by the subducted oceanic slab (Fig. 4-15). Once, however, the active volcanism in the region is located relatively far away from a subduction zone, it appears that an isotopic signal from the local mantle begins to be identifia ble in the lavas. For example, the New Britain lavas erupti ng behind the volcanic front show lower 206Pb/204Pb and not so low 207Pb/204Pb and 208Pb/204Pb, a trend interpreted to be a result of mixing between Indian and Pacific type mantle, as the subducting slab influence decreases with the increase in the distance from the trench (Woodhead et al. 1998). A similar trend is identifiable in the lavas from the Manus basi n. The eastern part of the basin is in close proximity to active subducti on and the Pb isotopes indi cate derivation from subducted slab with Pacific affinity, similar to the TLTF and New Britain lavas. The Pb isotopic composition of the lavas in the NW part of the basin, however, is different (Fig. 4-15). There is probably no subducting slab undernea th the NW part of the basin and appears that the Indian-type mantle signal begins to show in the local upwelling mantle as evidenced by the lavas plotting at slightly higher 207Pb/204P and 208Pb/204Pb relative to

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100 Pacific MORB (Fig. 4-15). Solomon Islands, located south-south east of TLTF and New Britain, show less radiogenic 206Pb/204Pb compared to TLTF and New Britain, and also form a trend that can be explained by mixi ng of components between Pacific and Indian type mantle components (Fig. 4-15). The older volcanism in the Solomons was related to the Pacific plate subduction, however, after th e collision of the Ontong Java plateau, the Pacific plate subduction ceased and subduction of the Woodlark Basin slab underneath the Solomon Islands began in an eastward direction (Mann and Taira 2004). The origin of the islands isotopic signature can be explained by mixi ng between Pacific and Woodlark material because the available data from young eruptive centers in the Solomon Islands form a trend toward the W oodlark basin samples (Fig. 4-15). The basin basalts exhibit higher 207Pb/204Pb than the Pacific MORB, and plot entirely within the Indian mantle field (i.e. apparently the local mantle Pb isotopic composition), identifiable also in the New Britain behindarc volcanoes, NW Manus spreading center, and in the Tubaf ultramafic xenoliths. Th e Woodlark Basin samples have Pb isotopic compositions similar to young Eastern Australia basaltic centers (compare Fig. 4-13 and Fig. 4-15), indicating that the Woodlark lavas ma y be related to material derived from the Australian subcontinental mantle. Recent studies indicate the importance of lateral flow in the mantle, and in this cas e, the northward movement of Australia may cause lateral displacement of the mantle in the region (R. Russo 2004 personal comm.). Therefore, it may be that the Woodlark basin lavas are tapping Australian sublithospheric mantle laterally squeezed out by the northward mo tion of the continent. Alternatively, fragments from the subaustralian mantle can be entrapped in the upwelling mantle

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101 underneath Woodlark basin and so can influe nce the isotopic compositions of the ridge lavas. Model for the Alkaline M agmatism in the TLTF Area It is generally accepted that island arc calc-alkaline magmatism results from lowering the melting point of the mantle wedge peridotite as a result of fluid introduction from the dehydration of subducted slab (Ta tsumi 1989). Some models also indicate that the subducted slab may undergo partia l melting and produce ad akites (Martin 1999), or the resultant melts ma y metasomatize the depleted mantle wedge peridotites, which subsequently melt to produce the island-arc magmas (McInnes and Cameron 1994, Schiano et al. 1995). Some recent experiments, however, indicate that there is a complete miscibility between silicate melts and hydrous fluids at high temperatures and pressures, suggesting that it may be difficult or even irrelevant to attempt to distinguish fluids and melts in subducti on zones, unless we are con cerned with relatively low temperature and pressure zones, such as the forearc region, where hydrous fluids are expected to dominate (Bureau and Kepler 1999). So, in the TLTF island-arc system, therefore, we can suggest that part of the fluids, mainly from the subducted sedimentary assemblages, is released in the forearc area a nd the fluid reacts with the mantle wedge to create the observed metasomatic features in the ultramafic xenoliths (Fig. 4-16A). No significant mantle melting occurs at this stag e because the temperature is relatively low due to the subducting cold oceanic slab. It is quite possible that most of the metasomatized ultramafic xenoliths r ecovered from Tubaf lavas are samples representative of this hydrated forearc ma ntle wedge. They contain hydrous minerals such as amphibole and phlogopite (Franz et al 2002) and my results indicate that their

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102 Figure 4-16. Cartoon summarizing the suggested origin of the TLTF volcanism. Mantle isotherms after Tatsumi (1989). A. Normal Pacific plate subduction under the Indo-Australian plate and formati on of New Ireland, New Britain, and Solomon Islands island arcs as a resu lt of dehydration of the subducting slab and melting in the mantle wedge (zone 2). Dehydration of the sedimentary layer of the subducting slab without melting due to cold thermal regime causing widespread hydrous metasomatism in zone 1. B. Ontong Java plateau collision and termination of the New Ireland volcanism due to the cease in the active subduction. Mantle isotherms gra dually recover and eventually cause release of melt-fluid phase from the stalled slab in zone 3, which leads to the TLTF volcanism (see text for more discussion).

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103 Pb and Sr isotopic compositions are quite variable, a result of incorporation of material from at least 3 sources, including cont ribution from the Pacific sediments. The TLTF lavas are alkaline, and although not uncommon in island arc settings, they are not the dominant rock series asso ciated with subduction zones, nor are they common in the island arcs around Paupua New Guinea. In general, such alkaline magmas is believed to be generated either by low de grees of partial melting at high pressures (e.g. alkaline basalts) or by low degrees of me lting of metasomatized mantle source (e.g. lamproites, kimberlites) (Winter 2001). Experiments show that low degrees of partial melting of metasomatized mantle xenol iths generate highly alkaline magmas, such as lamproites and kimberlites (Lloyd et al. 1985), not observed in the TLTF area. Although the ultramafic xenoliths are metasoma tized, their major and trace element data indicate that they are overall very depleted (Table 4-1, Fig. 4-5 and 4-10). Unrealistically small degrees of partial melti ng (ca. 0.1% or less) of thes e mantle wedge samples are required to produce the enrichment of trace elements observed in the TLTF lavas. Isotopically, the TLTF lavas are very different from the ultramafic xenoliths (Fig. 4-12), also indicating that the magmas can not be a direct result of partial melting of this hydrated part of the mantle wedge. The isotopic compositions of the TLTF lava s suggest that a major portion of their incompatible elements was probably derive d from the subducted oceanic slab. However, there are no samples directly available from the subducted oceanic slab in the region, therefore, we can either estimate it from available data for fresh Pacific MORB or alternatively, we can use the data for the mafic xenoliths, obtained in this study as a proxy for the composition of the subducted oceanic crust. Tectonic considerations

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104 suggest that the oceanic crus tal portion of the slab subducte d beneath the TLTF arc could be similar to the mafic xenoliths. If the Ne w Ireland-New Britain arc originated as an intra-oceanic arc, then we can assume that the Pacific oceanic crust began to subduct underneath its own detached fragment, pr obably sometime during Oligocene-Eocene time, based on the oldest subduction-related volcanic rocks on New Britain (Woodhead et al. 1998). Therefore, at some point at the pa st, the subducted slab and the oceanic crust underlying the TLTF chain, probably were part s of the same oceanic slab. The traceelement compostitions of the mafic xenoliths (F ig. 4-9) indicate that the oceanic crust beneath the arc is altered; in agreement with many studies th at show oceanic crust slab alteration shortly after its formation (Bach et al. 2001, Kelley et al. 2003). Therefore, the use of altered oceanic crust is likely a better proxy for the composition of the subducted slab compared to fresh MORB. The question remains as to how material from the subducted slab was in corporated in TLTF magmas. Components can be released from the s ubducted lithosphere either as hydrous fluids or melts, or as a homogeneous mi xture of both (Bureau and Kepler 1999). Extended trace element patterns (Fig. 46) and elemental ratios, such as K2O/Ba vs Ba/La (Fig. 4-17) for the TLTF lavas indicate enri chment by hydrous fluids. K and Ba are fluid mobile and highly incompatible elements, La is also highly incompatible but is fluid immobile element. Elevated Ba/La ratios indica te fluid-generated enri chment or sediment contribution, if the ratios are greater than 70 (Patino et al. 2000). The TLTF lavas show relatively low Ba/La, notably the Tubaf lavas, indicating very minor sediment contribution, in agreement with the isotope data. The K2O/Ba ratios of the TLTF lavas are close to MORB, especially altered MORB, and overlap w ith the data for the mafic

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105 xenoliths (Fig. 4-17), further indicating that the TLTF magmas are probably related to components from the subducted oceanic slab. The increase in the alkalinity (mainly K2O) in island arc lavas is generally attributed to either a decrease in the amount of melting as a result of a decrease in the slab H2O content with depth or to K-rich fluid/melt release as a result of phlogopite or phengite breakdow n with increasing depth of subduction. Figure 4-17. Comparison of K2O/Ba vs Ba/La between lavas and mafic xenoliths and pelagic and carbonate sediments. Increase in Ba/La at relative ly constant K/Ba is expected if the elements were deri ved from the subducted slab via aqueous fluids, which is not the case. Data fo r sediments from Pa tino et al. (2000). Schmidt (1996) demonstrated that during lo w-T dehydration of a subducting slab most of the K is contained in phengite, which is stab le to about 300 km dept h. Fluids and/or melts released during the phengite breakdown woul d be strongly enriched in K and as a

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106 consequence, arc magmas erupting behind the volcanic front, over d eeper Benioff zone, will be enriched in K (Schmidt 1996). The high volatile contents in TLTF magmas, notably the Tubaf mafic lavas (Chapter 3, Fig. 3-8), suggests abundant fluid presence in the magma source, and thus a decrease in H2O content with depth along the slab was probably not responsible for the alkali ne character of the TLTF magmas. It is generally considered that melting of the slab does not occurs in subduction zones because the geothermal gradient al ong the Benioff zone is low and subducted oceanic lithosphere dehydrates before it re aches the wet solidus temperature. Only in some extreme situations, such as subduction of very young lithosphere or cessation of the subduction, is the geotherm high enough to potentia lly melt the oceanic crust. In the case of subduction termination, as is the TLTF situation, rising temperature in the mantle wedge will transfer its heat to the once cold slab and the solidus front will move to the forearc region (Fig. 4-16B). The resultant me lts are expected to have adakitic, sodicalkaline, or potassic-ultrapot assic affinity (Martin, 1999, Defant and Kepezhinskas 2001, Hermann and Green 2001, Mungall 2002). Adakitic compositions, (andesitic to dacitic lavas with relativ ely high alkali, MgO, and Sr contents, and very low Y and HREE), are indicative of melting of a mafic source where garnet and/or amphibole are residual phases (Martin 1999). Although the TLTF magmas exhibit some features similar to adakites, such as high Sr and alkali contents, their SiO2 is much lower than typical adakites and they do not show the characteristic Y and HREE depletions of slab melts. Some experimental studies, however, indicate that alkalic melts are generated during reaction between slab-der ived partial melt with man tle wedge peridotite (Yaxley 2000). Intriguingly, McInnes and Cameron (1994) observed hydrous, alkali-rich

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107 alumosilicate melt inclusions in xenoliths and xenocrysts from Simberi Island, part of TLTF chain. The melt in these inclusions named SCHARM (sulfate-carbonate-H2Oalkali-rich-alumosilicate melt), is similar to phonolite and is enriched in Sr, Ba, Cl, F, S, and CO2. Partial melting of the stalled hydrated altered oceanic crus t beneath TLTF with a feldspathic or phengite phase contributi ng to the initial melt probably produced the alkali-rich alumosilicate melts observed in the inclusions (McInnes and Cameron 1994). Sr content in the SCHARM is very high, ra nging from 2000 to 4800 ppm, and Sr isotopic compositions in the host xenoliths range from 0.7036 to 0.7042, similar to the range observed in the TLTF lavas (McInnes a nd Cameron 1994). These melt inclusions provide direct evidence for partial melting of the oceanic crust beneath TLTF islands. Dilution and mixing of this hydrous slab melt with peridotite melts from the hot mantle wedge could possibly produce magmas with major and trace element compositions similar to the TLTF alkaline lavas. For ex ample, on Figure 4-18 (A and B) the SCHARM composition is shown relative to the ultramafic xenoliths fr om the area. TLTF lava major element characteristics can be modeled by dilu ting the slab melt with ultramafic material from the mantle wedge. Simple mixing betw een SCHRAM and ultramafic xenoliths, and subsequent crystal fractionation may produce magmas with chemical characteristics similar to the lavas in the re gion. In such scenario, the inco mpatible elements will be mainly derived from the slab component b ecause even the metasomatized ultramafic xenoliths are very depleted in incompatible elements (Table 4-1, Fi g. 4-5 and 4-10). It is difficult to produce a definitive quantitative mode l due to the lack of trace element data for the slab component, however, the Pb isotop ic compositions of the lavas support this

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108 Figure 4-18. Interaction of slab melt (SCHARM) with ultramaf ic mantle and formation of the TLTF magmas. Note that simple mixing accompanied with crystal fractionation may account for the chem istry of the TLTF lavas. SCHARM data from McInnes et al. (1994).

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109 model, because they suggest involvement of su bducted oceanic crust. Sr and Nd isotopic data are also consistent with deri vation from altered oceanic crust. The mantle-isotherm rebound mechanis m summarized on Figure 4-16 may provide explanation for the origin of the TLTF magmatism, however, the inactive ManusKilinailau trench appears to extend further to the north and northwest (Fig. 4-1), without any evidences for active volcanism. If the ma ntle isotherm rebound mechanism is correct, then it will be expected that the TLTF-type volcanism will continue further to the north, parallel to the inactive trench, which apparen tly is not the case. An alternative model for the development of the TLTF arc, based on extensional tectonics was proposed by McInnes (1992). In this m odel, the back-arc spreadi ng in the Manus basin caused formation of pull-apart basins in the TLTF area which lead to decompressional meting in the mantle wedge underneath the islands. Th e isotope data for the mantle xenolith samples presented here do not support origin of the magmas only by partial meting of the metasomatized mantle wedge. However, the extensional tectonics in the area probably played important role for the generation of conduits for the magma as cend to the surface. In addition, large-scale extension in the area will lead to mantle upw elling underneath the TLTF islands. Then, if there is no stalled subducted slab, th ere wont be a barrier to prevent Pacific mantle influx in the re gion. Such scenario for Pacific mantle incorporation is consistent w ith the Pacific isotopic sign ature in the lavas, however, decompressional melting involving Pacific mantle is not consistent with the island-arc signature in the TLTF lavas. The isotope and trace-element data for the TLTF lavas, in combination with tectonic considerations can probably be best explained by a hybrid model involving mantle isotherm rebound, whic h generated the required thermal regime

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110 to induce partial slab melting, accompanied by ex tensional tectonics that created suitable conditions for ascent of the magmas that fo rmed the volcanic islands. Then, the absence of extensional tectonic regime north of the TLTF islands may explain the lack of volcanic activity further north along the inactive Manus-Kilinailau trench. It is difficult to conclude if the slab component was solely melt or fluid derived, based on the existing data. Fluid-mobile elements will be readily released during dehydration of the slab and so will easily swamp the isotopic and fluid-mobile traceelement characteristics of the mantle we dge. Observed enrichments in fluid-mobile elements,uch as Ba, K, U, and Pb in the TLTF lavas (Fig. 4-6) are in good agreement with slab dehydration. Other in compatible, but fluid immob ile element concentrations, however, can not be solely derived from the mantle wedge, due to its very depleted nature (Fig. 4-5 and Fig. 4-10) Therefore, a hybrid melt+fl uid component release from the slab will probably best e xplain the chemical features of the TLTF lavas. Experiments by Bureau and Keppler (1999) show that any proportion of melt and fluid can exist as a homogeneous mixture at subduction P-T regi me. This homogeneous mixture separates into a hydrous melt and fluid phase with decreasing in temperature and pressure, conditions that will occur during magma ascen t to the surface. Therefore, erupted lavas (even the ones with highest volatile conten t, e.g. Tubaf lavas) will not preserve the starting volatile c ontent of such a fluid:melt mixture. The fate of the fluid component, however, may be important for the ore formation mechanism. If, for example, a homogeneous 50:50 fluid-melt mix is formed at the slab-mantle interface and then this mixture ascends quickly and encounters a st alled magma body clos e the surface, the ascent will stop and rapid exsolution of great amount of volatile components will occur.

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111 This component will be highly charged with ore-forming metals and may form large ore deposits, as described in Chapter 3. In addition to this study, it has been note d before that Au and Cu deposits are associated with slab melts (Defant and Ke pezhinskas 2001, and references therein). Mungall (2002) also suggested that arc magmas with high potential to generate Au and Cu deposits will be either adakitic, sodic-al kaline, or potassic-ultrapotassic. Favorable tectonic settings for generation of such ma gmas include regions of subduction of very young lithosphere or very slow or oblique convergence, flat subduction, and the cessation of subduction and subsequent release of slab -derived melt:fluid component, as is the TLTF case. Indeed, our Pb isotope study on the Au deposit on Lihir Island, and Au mineralization on Conical seamount show that the ore formation is intimately related to the magmatism in the area.

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112 CHAPTER 5 SUMMARY AND CONCLUSIONS The wealth of isotope and chemical com position data, including precise Pb isotope measurements, obtained in this study allowed me to look in details in the relationships between magmatism and ore deposit formation in SW Pacific island arcs. Important step was the development of the method for prec ise Pb isotope measurements by MC-ICP-MS (Nu Plasma). During the initial experiments, Pb and Tl from single-element, concentrated stock solutions were mixed in 2% HNO3 prior to isotope ratio measurements. The results revealed relatively poor precision and accuracy of the Pb isotope measurements, large variations in 205Tl in the same standard (rang ing from –3.9 to +30.1), and large variations in the observed Pb/Tl intensity ratios. When analyses were restricted to freshly mixed (<1 hour) Pb-Tl solutions, however, hi ghly precise isotopic ra tios were obtained for lead (206Pb/204Pb=16.9373 (+/-0.0011, 2 ), 207Pb/204Pb=15.4907 (+/-0.0012, 2 ), and 208Pb/204Pb=36.6935 (+/-0.0039, 2 )) and for thallium ( 205Tl=1.5 (+/-0.8, 2 )). In addition, Pb/Tl intensity ratios were constant and corresponded to the mixing ratios of the prepared solutions. A series of experiments re vealed that the poor precision and accuracy observed for the initial set of isotope ra tio measurements resulted from variable photoxidation of Tl+ to Tl3+ that occurs in the presence of Pb and solar UV radiation. This reversible reaction generates Tl3+ that behaves distinct from Tl+ during desolvation and leads to consistently higher measured Pb/Tl and 205Tl/203Tl ratios. The extent of the interaction between Pb and Tl and the subseque nt effect on isotope ratio measurements is sensitive to a combination of factors, incl uding differences in the acid matrix and

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113 molarity, desolvation conditi ons, and UV light exposure. It appears that the observed 205Tl variations in the Pb-Tl3+-bearing solutions dominantly result from mass-dependant differential diffusion of Tl during desolvation. Utilizing high-precision Pb isotope measurements allowed me to look in unpreced ented details in the connection between magmatism and Lihir and Conical seamount gold deposit formation. Lihir Island, part of the Tabar-Lihir-Tanga-Feni island chain in the SW Pacific, hosts the giant Ladolam gold deposit. Po lymetallic gold mineralization also was discovered offshore at Conical Seamount in 1998 during RV SONNE cruise “SO-133”. The seamount and the island are composed mainly of trachybasalts and basaltic trachyandesites, although some monzonites are found on Lihir. Sr isotopic analyses of mineralized samples from the island and the seamount suggest that most of the Sr is derived from the local al kaline lavas. Elevated 87Sr/86Sr ratios in some of the samples suggest that during the waning stages of the hydrothermal system, part of the Sr was contributed either from seaw ater or the thick sequence of marine sediments underlying the island. High-precision Pb isotopic analyses c onducted with MC-ICP-MS show that the ores and volcanic rocks share similar Pb isot opic compositions, suggesting that the Pb in the mineralized zones was ultimately derive d from the local magmatic sources. The Pb isotopic data, however, reveal small, but signi ficant, differences be tween the mineralized zones and the associated host lavas. Minerali zed samples from Lihir have slightly less radiogenic lead isotopic ratios than their host lavas. These le ad isotopic compositions are similar, however, to some of the fresh lava s recovered from Conical seamount and to a monzonite intrusion underlying the Ladolam de posit. Lead isotopic ratios in mineralized

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114 samples from Conical seamount, however, are slightly more radiogenic than their host lavas and similar to those of fresh lava s recovered from nearby Tubaf and Edison seamounts. Petrographic observations and mi croprobe elemental analyses reveal a complex magmatic history for the magma ch amber inferred beneath Conical seamount. Based on zoning patterns in the Conical cli nopyroxenes, it appears that a sub-seamount magma chamber was recharged with a mafi c magma similar to the most primitive, volatile-rich, and xenolith-bear ing lavas recovered from Tubaf and Edison seamounts. Rapid cooling of this mafic magma accompanie d by exsolution of metal-bearing fluids in the relatively shallow magma chamber is proposed as the mechanism that ultimately resulted in the observed gold mineralization. Similarity of the lead isotopic ratios between the ores and the mafi c magmas in the area suggest that the ore metals were primarily derived from the recharging magm a, not by hydrothermal leaching of the host lavas by seawater-deriv ed hydrothermal fluid. Numerous xenoliths recovered from TU BAF seamount, located in Tabar-LihirTanga-Feni (TLTF) island chain (Papua New Guinea) provide a unique view of the elemental and isotopic com position of the mantle wedge beneath the arc and its relationship to the magmatism in the region. The ultramafic xenoliths exhibit a wide range in their Pb isotopic compositions, sugge sting involvement of at least 3 components. Two of the components appear to be the Paci fic oceanic mantle and Pacific sediments. Clinopyroxene separates from some of the ultramafic xenoliths exhibit low 206Pb/204Pb and high 207Pb/204Pb, suggesting that the third component was Indian Ocean-type mantle or Australian subcontinental lithospheric man tle. The Sr and Nd isotopic compositions of

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115 the xenoliths are also consistent with involvement of Pacifi c and/or Indian mantle and sediment components in the mantle wedge beneath TLTF islands. Compared to the mantle wedge samples, TLTF lavas have very restricted Pb isotopic compositions within the Pacific MORB range, suggesting that the metasomatized part of the mantle did not play a major role in determining the isotopic characteristics of the magmas. Comparison of the volcanic rocks from the TLTF arc to lavas from New Britain arc and Eastern Manus basin suggests that the isotopic compositions of the lavas in the area are mainly controlled by subducte d oceanic crust with Pacific MORB affinity. Intriguingly, once the subducted slab influence decreases, the Indian Ocean-type mantle signal begins to appear in the isotopic signa ture of the lavas in the region, as is demonstrated in lavas from the Solomon Islands, NW Manus and Woodlark back-arc basins. The fact that TLTF lavas have isotopic compositions mainly inherited from the subducted oceanic crust suggests that the volcan ism in the area is intimately related to the stalled oceanic crust beneath the islands. During ac tive subduction of the Pacific plate the thermal regime in the fore-arc region wa s not suitable to indu ce partial melting. Once, however, active subduction ceased, the mantle isotherms gradually recovered and warmed up the subducted oceanic crust beneat h the fore-arc region. The triggering event that caused the TLTF volcanism is probably related to hydrous melts released from the subducted oceanic crust accompanied by extensi onal tectonics caused by the spreading in the Manus back-arc basin. Dilution of the slab melt with depleted peridotitic component during melting of the mantle wedge resulted in magmas with similar chemical characteristics to the TLTF lavas. In such s cenario the incompatible elements will be

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116 completely controlled by the slab melts, which is evident in the Pacific isotopic signature observed in the TLTF lavas. In addition, the formation of highly hydrous melt or homogeneous fluid-melt mixture has important implications for the formation of ore deposits. If such fluid-melt mixture ascends quickly through the mantle and encounters a stalled magma body close the surface, the ascen t will be terminated and rapid separation into hydrous melt and fluid component will occu r. The exsolved fluid component will be highly charged with ore-forming metals and ma y contribute to the formation of large ore deposit close to the surface.

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117 CHAPTER 6 FUTURE RESEARCH A number of new questions that can be investigated furthe r arose as a result of this study. The proposed ore deposit formation mechanism can be further addressed by detailed study of the fluid-melt inclusi ons discovered in the Conical Seamount clinopyroxenes. It is difficult to analyze thes e samples due to the f act that they contain fluid phase that will be lost if an attempt is made to open the inclusions. However, it may be possible to open the inclusions with small diameter laser beam (5-10 microns) while scanning on-line for certain ore-forming meta ls such as Cu, Zn, Au, Ag, and Pb on the high-resolution Element 2 massspectrometer. By utilizing time-resolved analysis software it may be possible to obtain quantit ative information for the concentration of these elements in the inclusions and so provi de evidences if they really trapped samples of the ore-forming solutions exsolving fr om a recharging magma. In addition, the developed high-precision Pb isotope measurem ent technique must be utilized in other studies of ore deposits associated with volcan ic complexes. Such studies will confirm or rule out if the proposed ore-fo rmation mechanism is a valid ex planation for the origin of other ore deposits, or is applicable on ly for the Lihir and Conical seamount mineralizations. This will contribute to our understanding of the ore-formation mechanisms, and so will enhance our cap abilities for finding new ore deposits. More detailed study of young volcanic ro cks from the arcs in the region, specifically if new samples from New Irela nd and the Solomon Islands can be obtained, will provide further constrains on the magmat o-tectonic evolution in the region. Useful

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118 information about the changes through time in the subduction settings also can be obtained from studies of older magmatic rocks th at at present built the cores of the arcs in the region. Important tectonic question, that remained unanswered, is why there is no volcanism along the inactive trench north of the TLTF chain. Topographic mapping and geophysical survey in the region of the inac tive trench may reveal presence of subducted slab, thermal anomalies in the mantle wedge, and young seamounts. One lava sample from Manus Island ha s very unusual Pb isotopic composition (206Pb/204Pb=16.240, 207Pb204Pb=15.404, and 208Pb/204Pb=35.899) that may represent important end-member in the mantle in the re gion. These results were not included in the dissertation because more samples are needed fr om the Manus Island to asses if this is the real isotopic composition or is a result of contamination. In addition, further useful information can be derived from new isotopic analyses of the availa ble lava and xenolith samples. For example, Hf isotope measuremen ts can provide invaluab le constrains on the origin of HFSE signature in the arcs. An atte mpt can be made to separate vein material from the xenolith samples. In addition, sepa ration of the xenolith samples on their main mineral constituents (i.e., Ol, Opx, Cpx) and subsequent elemental and isotopic analyses of the separated phases can provide useful in formation about the or iginal composition of the mantle and the metasomatic agents (i.e., the vein material). This will contribute to our quest for deciphering of the composition of the mantle wedge in subduction zones.

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APPENDIX A MC-ICP-MS ANALYSES OF NBS 981 LE AD ISOTOPIC STANDARD MIXED WITH QCD ICP-MS THALLIUM STANDARD. TABLE INCLUDES WET PLASMA AND DSN FRESH AND AGED, EXPOSED TO LIGHT MIXED PB-TL SOLUTIONS ANALYSES.

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120Wet plasma analyses (fresh and aged mixtures) date time Pb(ppb) Tl(ppb) Pb(V) Tl(V) Pb/Tl 205Tl/203Tl 206Pb/204Pb error 207Pb/204Pb error 208Pb/204Pb error 4/9/2003 15:47 100 25 2.33 0.62 3.8 2.428083 16.93839 0.0013 15.49278 0.0012 36.7019 0.0033 4/9/2003 17:35 100 25 2.41 0.64 3.8 2.42749 16.93873 0.0015 15.49294 0.0013 36.70266 0.0036 4/10/2003 16:35 100 25 3.22 0.85 3.8 2.422347 16.93712 0.0012 15.49134 0.0012 36.69602 0.0028 4/10/2003 16:45 100 25 3.16 0.83 3.8 2.422695 16.93406 0.0012 15.48756 0.0014 36.69166 0.0034 4/10/2003 17:14 100 25 3.09 0.81 3.8 2.422703 16.93411 0.0009 15.48776 0.0011 36.68809 0.0031 4/10/2003 17:27 100 33 3.12 1.1 2.8 2.422574 16.93626 0.0011 15.49027 0.0009 36.69268 0.0024 4/10/2003 17:40 100 50 3.05 1.62 1.9 2.422236 16.93344 0.0011 15.48801 0.001 36.6878 0.0024 4/15/2003 15:08 100 25 2.87 0.75 3.8 2.424311 16.93694 0.0012 15.49196 0.0013 36.69877 0.0032 4/15/2003 15:36 100 25 2.76 0.73 3.8 2.424916 16.93693 0.0013 15.49151 0.0014 36.69651 0.0036 4/16/2003 11:42 100 33 3.07 1.06 2.9 2.423438 16.93728 0.0012 15.49082 0.0011 36.69555 0.0028 4/16/2003 13:46 100 25 3.04 0.8 3.8 2.419924 16.93752 0.0015 15.49203 0.0013 36.69769 0.0034 4/16/2003 15:18 100 33 3.09 1.07 2.9 2.420987 16.93562 0.0012 15.48968 0.0012 36.69069 0.0028 4/16/2003 16:08 100 33 2.49 0.85 2.9 2.426636 16.93395 0.0017 15.48901 0.0014 36.69096 0.0035 4/16/2003 16:19 100 25 2.41 0.62 3.9 2.426396 16.93787 0.0011 15.49222 0.0012 36.698 0.0029 4/16/2003 17:34 100 25 2.38 0.61 3.9 2.426063 16.93501 0.0012 15.49009 0.0011 36.69259 0.003 4/18/2003 14:50 50 12.5 1.79 0.47 3.8 2.413887 16.93834 0.0018 15.49158 0.0017 36.69704 0.0041 4/18/2003 16:14 50 12.5 1.7 0.45 3.8 2.416825 16.93423 0.0019 15.48863 0.0018 36.68889 0.0045 4/18/2003 17:24 ~30 ~9 1.26 0.37 3.4 2.41719 16.93765 0.0026 15.49079 0.0025 36.69517 0.0059 4/18/2003 18:40 ~30 ~11 1.33 0.45 3.0 2.417309 16.93446 0.0021 15.48931 0.0023 36.69089 0.0053 4/18/2003 20:00 100 25 3.27 0.86 3.8 2.416442 16.93934 0.0009 15.49234 0.0012 36.69809 0.0025 4/24/2003 11:46 100 10 2.55 0.27 9.4 2.426466 16.93575 0.0016 15.48898 0.0019 36.6941 0.005 4/24/2003 12:19 100 10 2.47 0.27 9.1 2.426784 16.93876 0.0016 15.49272 0.0016 36.70267 0.0042 4/24/2003 12:35 100 33 2.59 0.89 2.9 2.427411 16.93843 0.0011 15.49191 0.0011 36.70167 0.0029 4/30/2003 12:46 100 25 3.01 0.79 3.8 2.4236 16.93893 0.001 15.4926 0.0009 36.70008 0.0021 4/30/2003 14:03 100 25 3.11 0.82 3.8 2.421732 16.93965 0.0011 15.49267 0.001 36.70033 0.0027 4/30/2003 16:02 60 15 2.25 0.59 3.8 2.417424 16.93736 0.0012 15.49025 0.0012 36.69381 0.0028 4/30/2003 17:50 60 15 2.12 0.56 3.8 2.417969 16.93196 0.0015 15.48568 0.0015 36.68265 0.0033

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121 4/30/2003 18:01 60 15 2.13 0.56 3.8 2.418011 16.93634 0.0015 15.48963 0.0014 36.69259 0.0034 5/19/2003 16:02 100 25 3.04 0.801 3.8 2.422256 16.93312 0.0013 15.48761 0.0013 36.69085 0.0032 5/19/2003 17:50 100 25 2.97 0.79 3.8 2.422927 16.93621 0.0013 15.48972 0.0012 36.69285 0.0032 5/19/2003 18:50 100 25 2.94 0.78 3.8 2.423181 16.93726 0.0013 15.49064 0.0013 36.69631 0.0029 6/17/2003 13:53 150 37.5 4.43 1.01 4.4 2.425222 16.9382 0.00064 15.49128 0.00064 36.69752 0.0017 6/17/2003 15:00 150 37.5 3.64 0.83 4.4 2.427048 16.93839 0.0009 15.49084 0.00099 36.69604 0.0025 6/17/2003 16:33 150 37.5 3.46 0.81 4.3 2.427193 16.93987 0.0009 15.49159 0.0009 36.69683 0.0021 6/17/2003 16:44 150 37.5 3.42 0.79 4.3 2.427209 16.93718 0.00092 15.49078 0.0011 36.69592 0.0027 6/18/2003 15:19 150 37.5 5.11 1.17 4.4 2.423866 16.93729 0.00072 15.48866 0.0007 36.69168 0.0017 6/26/2003 10:45 150 37.5 4.69 1.06 4.4 2.425101 16.93913 0.0009 15.49174 0.0009 36.69796 0.0024 6/26/2003 12:39 150 37.5 4.16 0.95 4.4 2.426988 16.93753 0.0013 15.49046 0.0014 36.69438 0.0029 6/26/2003 14:24 150 37.5 4.27 0.98 4.4 2.426716 16.93709 0.0008 15.49002 0.0008 36.69328 0.0021 6/29/2003 15:47 150 37.5 3.98 0.97 4.1 2.427079 16.93874 0.0009 15.49019 0.0009 36.69629 0.0024 6/29/2003 18:02 150 37.5 4.21 0.99 4.3 2.427373 16.93749 0.0009 15.48933 0.00085 36.69143 0.0021 8/4/2004 10:05 150 37.5 5.28 1.18 4.5 2.421467 16.93667 0.0007 15.4905 0.0007 36.69382 0.0017 8/4/2004 11:15 150 37.5 4.76 1.06 4.5 2.422327 16.93683 0.0007 15.49096 0.0009 36.69546 0.0022 8/4/2004 12:39 150 37.5 4.92 1.1 4.5 2.422691 16.93729 0.0007 15.49104 0.0008 36.69461 0.0019 5/24/2004 12:20 150 37.5 5.05 1.13 4.5 2.423905 16.93849 0.0006 15.49276 0.0006 36.70134 0.0016 5/24/2004 13:52 150 37.5 5.12 1.1 4.7 2.423473 16.93766 0.0009 15.49254 0.001 36.69913 0.0031 5/24/2004 17:07 150 37.5 5.14 1.15 4.5 2.42434 16.9364 0.0007 15.49059 0.0007 36.69534 0.0019 DSN analyses (only fresh mixtures) 6/27/2003 11:33 30 15 6.33 2.93 2.16 2.43142 16.9372 0.0007 15.49037 0.0006 36.69368 0.0014 6/27/2003 13:21 30 15 5.94 2.81 2.11 2.429287 16.93817 0.0011 15.49172 0.001 36.69416 0.0024 6/27/2003 18:26 30 15 5.8 2.9 2.00 2.428392 16.93736 0.0008 15.49052 0.0007 36.69332 0.0018 6/29/2003 12:10 30 15 6.58 3.24 2.03 2.429962 16.93753 0.0008 15.49166 0.0006 36.69706 0.0018 6/29/2003 13:27 30 15 6.72 3.38 1.99 2.426896 16.93721 0.0004 15.49079 0.0004 36.69423 0.001 6/29/2003 15:13 30 15 6.09 2.98 2.04 2.428361 16.93697 0.0007 15.48969 0.0006 36.6905 0.0016 7/7/2003 13:58 30 7.5 6.74 1.6 4.21 2.428993 16.93792 0.0008 15.49195 0.0006 36.69843 0.0018

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122 7/7/2003 14:30 30 7.5 6.54 1.67 3.92 2.429184 16.93787 0.0005 15.49113 0.00047 36.69477 0.0011 7/17/2003 12:09 30 5 6.16 0.99 6.22 2.430151 16.93796 0.0007 15.49089 0.00075 36.69515 0.0018 7/17/2003 12:28 30 10 6.05 1.94 3.12 2.430076 16.93666 0.00055 15.49024 0.00054 36.69326 0.0012 7/17/2003 12:44 30 5 6.14 0.99 6.20 2.430005 16.93751 0.00075 15.49067 0.0007 36.69286 0.0019 7/17/2003 13:41 30 5 5.87 0.93 6.31 2.430216 16.93639 0.0005 15.48991 0.00046 36.69044 0.0013 7/25/2003 12:31 35 6 6.52 1.11 5.87 2.427294 16.9365 0.00077 15.49013 0.00078 36.69213 0.0019 7/25/2003 13:04 35 7 6.58 1.38 4.77 2.427246 16.93718 0.0006 15.49025 0.00062 36.69288 0.0018 7/28/2003 10:37 35 8.75 7.32 1.72 4.26 2.427816 16.93728 0.0006 15.49128 0.0007 36.69338 0.0017 7/28/2003 12:24 35 7 8.39 1.94 4.32 2.427203 16.93746 0.00047 15.49042 0.00038 36.69278 0.0011 7/28/2003 12:45 25 6.25 5.24 1.29 4.06 2.427322 16.93744 0.00083 15.4905 0.00083 36.69278 0.0022 8/4/2003 11:10 35 8 9.46 2.14 4.42 2.429772 16.93789 0.0004 15.4909 0.0004 36.69409 0.0009 8/4/2003 12:46 30 10 7.95 2.43 3.27 2.427151 16.93671 0.00045 15.4897 0.00044 36.68978 0.0014 8/11/2003 11:09 35 11 7.8 2.33 3.35 2.42999 16.93803 0.00039 15.49155 0.00038 36.69588 0.0009 8/11/2003 14:16 35 7 8.02 1.65 4.86 2.428814 16.93651 0.0005 15.49026 0.0006 36.69296 0.0015 8/11/2003 14:27 35 7 7.92 1.61 4.92 2.42895 16.93695 0.00057 15.48995 0.00058 36.69175 0.0017 8/30/2003 11:27 30 7.5 7.65 1.87 4.09 2.429561 16.93809 0.0006 15.4912 0.0006 36.6946 0.0016 8/30/2003 12:35 30 5 7.14 1.23 5.80 2.429206 16.93745 0.0006 15.49028 0.0008 36.69353 0.0014 9/9/2003 11:30 30 6 9.2 1.65 5.58 2.428018 16.93726 0.0004 15.4902 0.0004 36.69323 0.001 1/16/2004 11:45 30 5 14.3 1.9 7.53 2.43584 16.93775 0.00024 15.49001 0.0003 36.69566 0.0007 1/16/2004 12:15 7 1.2 5.83 2.435932 16.9373 0.0007 15.4892 0.0007 36.69238 0.0025 1/16/2004 12:30 5.92 1.7 3.48 2.436826 16.93821 0.0007 15.49174 0.0007 36.69983 0.0015 1/16/2004 12:45 6.1 1 6.10 2.435133 16.93665 0.0006 15.48869 0.0006 36.6912 0.0016 1/16/2004 13:00 5.8 0.8 7.25 2.436877 16.93666 0.0025 15.48961 0.0025 36.69389 0.0062 1/28/2004 11:22 30 1.5 10.15 0.46 22.07 2.434523 16.93753 0.0009 15.491 0.0009 36.69798 0.0026 3/20/2004 11:50 30 6 6.08 1.11 5.48 2.426919 16.93671 0.0007 15.49018 0.0007 36.69198 0.0019 3/20/2004 10:28 30 7 6.2 1.45 4.28 2.428351 16.93667 0.0007 15.4888 0.0008 36.69082 0.0019

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123 DSN analyses (only aged mixtures) 1/31/2003 14:38 25 6.25 3.91 0.81 4.83 2.425995 16.92747 0.0007 15.47911 0.0007 36.66228 0.0017 3/7/2003 14:57 25 6.25 3.88 0.79 4.91 2.42585 16.92768 0.0007 15.47957 0.0007 36.66337 0.0034 3/7/2003 12:29 25 6.25 3 0.4 7.50 2.431511 16.92131 0.0013 15.47028 0.0016 36.63652 0.0037 3/7/2003 13:45 25 6.25 2.82 0.35 8.06 2.430128 16.92479 0.0014 15.47405 0.0015 36.64273 0.0043 3/7/2003 14:32 25 6.25 2.75 0.33 8.33 2.430539 16.92527 0.0015 15.47463 0.0015 36.64484 0.0039 4/14/2003 15:55 25 6.25 5.61 0.4 14.03 2.436885 16.90209 0.0009 15.44429 0.001 36.55217 0.0028 4/14/2003 16:29 25 12.5 4.59 0.43 10.67 2.439107 16.88809 0.0011 15.4265 0.0011 36.49594 0.0027 4/14/2003 16:43 25 6.25 4.71 0.34 13.85 2.437605 16.89876 0.0009 15.44126 0.0009 36.54276 0.0027 4/14/2003 16:53 25 6.25 4.52 0.33 13.70 2.437095 16.90074 0.0009 15.44337 0.0014 36.55172 0.0033 4/14/2003 17:06 25 12.5 4.12 0.39 10.56 2.438322 16.89115 0.0009 15.43005 0.0011 36.50629 0.0028 5/1/2003 9:54 25 12.5 4.61 0.6 7.68 2.432966 16.90446 0.001 15.44745 0.001 36.55822 0.0029 5/1/2003 10:05 25 12.5 4.57 0.62 7.37 2.433105 16.90472 0.0008 15.44633 0.0007 36.55523 0.0033 6/17/2003 10:27 30 15 9.07 2.19 4.14 2.42782 16.94429 0.00049 15.50005 0.00056 36.72487 0.0018 6/17/2003 11:58 30 15 7.87 1.78 4.42 2.429406 16.94533 0.00054 15.50076 0.00076 36.72605 0.0012 6/17/2003 11:04 20 10 4.08 1.88 2.17 2.430352 16.93843 0.00097 15.49086 0.00097 36.69435 0.00218 6/17/2003 11:27 20 10 4.99 2.26 2.21 2.430697 16.93709 0.00084 15.4891 0.00085 36.68974 0.0019 6/17/2003 11:39 20 10 4.89 2.23 2.19 2.43063 16.93606 0.00062 15.4885 0.00057 36.68808 0.0015 6/18/2003 16:33 30 15 10.6 2.44 4.34 2.424665 16.94188 0.00045 15.49872 0.00045 36.723 0.0012 6/18/2003 16:43 30 15 9.7 2.23 4.35 2.425039 16.9459 0.0004 15.50152 0.0005 36.72995 0.0014 6/18/2003 17:02 20 10 4.54 2.07 2.19 2.42668 16.93618 0.0009 15.48832 0.0009 36.68654 0.0021 6/27/2003 11:05 30 15 6.94 1.3 5.34 2.430499 16.94407 0.0007 15.50022 0.0006 36.72609 0.0014 6/27/2003 13:00 30 15 6.2 2.87 2.16 2.428866 16.93925 0.0009 15.49179 0.0009 36.69594 0.002 7/7/2003 15:14 30 15 7.4 0.93 7.96 2.429591 16.93539 0.00052 15.48757 0.00065 36.68606 0.0017 7/7/2003 16:12 30 15 7.13 0.91 7.84 2.429926 16.93383 0.00055 15.48591 0.0007 36.67956 0.0019 7/9/2003 11:21 30 7.5 9.13 0.37 24.68 2.432247 16.91608 0.0013 15.46207 0.0017 36.60579 0.0053 7/9/2003 11:34 30 7.5 8.46 0.23 36.78 2.43694 16.88894 0.0009 15.42336 0.0011 36.48609 0.0035 7/9/2003 11:56 20 10 4.31 1.34 3.22 2.431438 16.92969 0.0009 15.478 0.0011 36.65302 0.0028 7/18/2003 11:12 30 5 6.97 1.06 6.58 2.431888 16.93518 0.0006 15.48854 0.0006 36.68767 0.0016

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124 7/18/2003 11:28 30 3.3 6.46 0.65 9.94 2.431786 16.93477 0.0006 15.48829 0.0006 36.68743 0.0016 7/18/2003 11:44 30 5 6.51 0.95 6.85 2.431789 16.93478 0.0009 15.48808 0.0009 36.68764 0.002 7/25/2003 11:45 30 3.3 5.33 0.52 10.25 2.427554 16.93397 0.0007 15.48726 0.0008 36.68437 0.0022 7/25/2003 12:00 30 5 5.48 0.73 7.51 2.427259 16.93548 0.0007 15.48988 0.0008 36.69147 0.0019 7/28/2003 11:34 30 5 6.2 0.916 6.77 2.427692 16.93424 0.0011 15.48771 0.00062 36.68274 0.0027 7/28/2003 11:51 30 3.3 5.82 0.54 10.78 2.427588 16.9348 0.0007 15.48855 0.0009 36.68694 0.0022 7/28/2003 12:06 30 5 6.03 0.61 9.89 2.427538 16.93341 0.0008 15.48644 0.0009 36.6812 0.0023 7/28/2003 13:00 30 5 6.16 0.37 16.65 2.42872 16.92823 0.001 15.4793 0.001 36.65755 0.0033 7/28/2003 13:41 30 5 6.02 0.72 8.36 2.428334 16.93579 0.0007 15.48782 0.0009 36.68416 0.0023 7/28/2003 14:15 30 10 6.06 1.17 5.18 2.42813 16.93683 0.0006 15.4901 0.0006 36.69353 0.0012 7/28/2003 14:35 30 5 6.31 0.89 7.09 2.428283 16.93605 0.0007 15.48947 0.00076 36.6901 0.0019 7/28/2003 14:50 30 3.3 6.04 0.55 10.98 2.428343 16.93248 0.0006 15.48494 0.0007 36.67786 0.002 7/28/2003 15:06 35 1.75 7.44 0.31 24.00 2.428129 16.93212 0.00077 15.48503 0.0009 36.67919 0.0026 8/4/2003 11:31 30 5 7.97 0.84 9.49 2.430205 16.93298 0.0006 15.48504 0.0006 36.67574 0.0018 8/4/2003 11:59 30 10 7.89 1.28 6.16 2.428692 16.94049 0.0005 15.49543 0.0005 36.70888 0.0012 8/4/2003 13:04 30 5 7.8 0.7 11.14 2.427786 16.93514 0.0006 15.48794 0.0008 36.68531 0.0021 8/4/2003 13:31 30 3.3 7.5 0.52 14.42 2.428532 16.92716 0.0005 15.4786 0.0006 36.65861 0.0022 8/4/2003 13:54 30 5 7.54 0.29 26.00 2.428887 16.92593 0.0007 15.47625 0.0009 36.65009 0.0027 8/4/2003 14:16 30 5 7.02 1.1 6.38 2.427826 16.93439 0.00045 15.48732 0.0005 36.68501 0.0011 8/11/2003 11:49 30 5 7.55 0.748 10.09 2.430022 16.93321 0.0006 15.48512 0.0008 36.67716 0.0022 8/11/2003 12:29 30 10 7.37 1.08 6.82 2.428507 16.94217 0.0006 15.49706 0.0006 36.71342 0.0017 8/11/2003 12:52 30 5 7.46 0.49 15.22 2.429095 16.93461 0.0008 15.48749 0.0009 36.68453 0.0025 8/11/2003 13:10 30 3.3 7.15 0.39 18.33 2.430377 16.92745 0.0009 15.47646 0.001 36.65055 0.0027 8/11/2003 13:26 30 5 7.23 0.24 30.13 2.430564 16.92407 0.0013 15.47411 0.0013 36.64125 0.0039 9/9/2003 11:11 30 5 8.81 1.31 6.73 2.427693 16.93839 0.0004 15.49219 0.0004 36.69981 0.0012 9/9/2003 11:41 30 5 8.76 1.37 6.39 2.427364 16.94009 0.0005 15.49383 0.0006 36.7053 0.0012 9/9/2003 13:36 30 3.3 8.36 0.54 15.48 2.429 16.92854 0.0008 15.47801 0.001 36.65691 0.0027 9/9/2003 14:23 30 5 8.13 0.31 26.23 2.429496 16.92261 0.0009 15.47144 0.0012 36.63511 0.0039 9/9/2003 14:37 30 5 7.47 0.72 10.38 2.429389 16.92407 0.0009 15.47384 0.0009 36.64265 0.0025

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125 1/16/2004 12:15 30 5 7 1.2 5.83 2.435932 16.9373 0.0007 15.4892 0.0007 36.69238 0.0025 1/16/2004 12:30 30 10 5.92 1.7 3.48 2.436826 16.93821 0.0007 15.49174 0.0007 36.69983 0.0015 1/16/2004 12:45 30 5 6.1 1 6.10 2.435133 16.93665 0.0006 15.48869 0.0006 36.6912 0.0016 1/16/2004 13:00 30 3.3 5.8 0.8 7.25 2.436877 16.93666 0.0025 15.48961 0.0025 36.69389 0.0062 1/16/2004 13:15 30 5 5.41 0.41 13.20 2.440755 16.91161 0.0011 15.45493 0.0013 36.58905 0.0046 1/20/2004 11:15 14.1 0.9 15.67 2.435648 16.91741 0.0006 15.46184 0.0006 36.60861 0.0024 1/20/2004 11:30 30 5 7.3 0.74 9.86 2.43271 16.93091 0.0006 15.48179 0.0007 36.6681 0.002 1/20/2004 11:57 30 5 6.45 0.33 19.55 2.434024 16.92081 0.0008 15.46764 0.0008 36.62215 0.0023 1/20/2004 12:12 30 5 6.25 0.4 15.63 2.435168 16.91234 0.0008 15.45594 0.0008 36.58676 0.0022 1/20/2004 12:47 14.66 1.04 14.10 2.43811 16.92143 0.0009 15.46541 0.0005 36.61826 0.0024 1/20/2004 13:04 30 5 7.39 0.58 12.74 2.438278 16.92494 0.0006 15.47282 0.0009 36.63845 0.0025 1/20/2004 13:26 30 5 6.6 0.4 16.50 2.437774 16.91029 0.0009 15.45343 0.0012 36.58043 0.0034 1/20/2004 13:54 30 3.3 6.21 0.2 31.05 2.440531 16.91092 0.0018 15.45447 0.0023 36.57848 0.006 1/26/2004 7.96 0.99 8.04 2.435563 16.93419 0.0006 15.48671 0.0006 36.68498 0.0017 1/26/2004 14.8 1.14 12.98 2.438093 16.92407 0.0005 15.4686 0.0005 36.62939 0.0014 1/26/2004 30 5 7.44 0.59 12.61 2.43752 16.91946 0.0012 15.4667 0.0013 36.62141 0.0036 1/26/2004 30 5 6.57 0.44 14.93 2.439954 16.91026 0.0009 15.45536 0.0014 36.58461 0.0039 1/26/2004 30 3.3 6.78 0.31 21.87 2.437076 16.92539 0.0016 15.47331 0.0016 36.64231 0.0048 2/5/2004 14:06 30 5 13.97 1.1 12.70 2.437465 16.92709 0.0007 15.4717 0.0008 36.63827 0.0019 2/5/2004 14:23 30 5 12.17 1.03 11.82 2.438611 16.92756 0.0003 15.47285 0.0003 36.64295 0.0009 2/10/2004 10:10 30 5 7.41 0.68 10.90 2.437107 16.92688 0.0005 15.47728 0.0007 36.65289 0.0019 2/10/2004 10:50 5.36 0.53 10.11 2.434553 16.92505 0.0007 15.47549 0.0006 36.64613 0.0019 2/10/2004 15:39 5.56 0.31 17.94 2.439537 16.91111 0.0011 15.45522 0.0013 36.58326 0.0039

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126 APPENDIX B LEAD EXTRACTION PROCEDURE (MODI FICATION OF THE TIMS HBR PB SEPARATION OF MANHES ET AL. (1978), SIMPLIFIED AND ADAPTED FOR MC-ICP-MS MEASUREMENTS OF PB ISOTOPES BY TL SPIKING. 1. Pack columns with a stem (resin bed) volume about 100 l with 100-200 mesh Dowex 1X-8 resin; 2. Wash resin with 2 ml 6N HCl (optima grade); 3. Load sample dissolved in 100 l -200 l 1N HBr (seastar); 4. Wash 1 ml 1N HBr (seastar); 5. Wash 1 ml 1N HBr (seastar); 6. Wash 1 ml 1N HBr (seastar); 7. Collect Pb fraction in 1ml 20% HNO3 (optima grade); 8. Evaporate the Pb solution to dryness on the hot plate; 9. Dissolve the Pb solution residue in 2% HNO3 optima and spike it with Tl just before the analysis; A fraction of the NBS 981 Pb isotopic standard was dried down and passed through the above column procedure to test for yield and for mass-dependant fractionation during the ion exchange chemis try. The yield was about 95%-97% and the measured isotopic values in NBS 981 after the column chemistry were: 206Pb/204Pb=16.9367, 207Pb/204Pb=15.4897, and 208Pb/204Pb=36.6924. The ratios are indistinguishable from the wet and fresh dry plasma NBS 981 an alyses (Appendix A), indicating that no detectable isotopic fractionation occurs during the ion exchange chromatography.

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127 APPENDIX C MIXING CALCULATIONS WITH 207PB/ 204PB BETWEEN SEDIMENTARY AND MAFIC SOURCES Average values: Othman et al. (1989) Pacific sediments Pb(ppm) 206/204 207/204 208/204 8 18.707 15.62638.702 White et al. (1987) Pacific MORB 18.586 15.51138.0851 Mafic Xenoliths 18.702 15.52038.2081 Sedimentary xenoliths 18.738 15.56438.4013 Results: Pacific MORB Sedimentary and Mafic Sediment mix xenoliths mix f 207/204 207/204 0 15.63015.564 0.05 15.62915.563 0.1 15.62815.562 0.2 15.62615.561 0.3 15.62415.559 0.4 15.62115.556 0.5 15.61715.553 0.6 15.61115.549 0.7 15.60315.545 0.8 15.59015.539 0.9 15.56715.531 0.95 15.54615.526 1 15.51115.520 Mixing equation after Faure (1986): (207/204A*PbA*f+207/204B*PbB*(1-f)/(PA*f+PbB*(1-f)

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139 BIOGRAPHICAL SKETCH George Kamenov was born on October 6, 1970, in Sofia, Bulgaria. He graduated from high school in 1988 and then served for 2 years in the Bulgarian Army as a border patrol. From 1991 to 1996 he completed a bach elor’s/master’s degree in geology from the Sofia University, Bulgaria, where he c onducted research on metalliferous sediment samples from the Mid-Atlantic Ocean ridge. He worked as a research geologist in the Bulgarian Academy of Sciences from 1997 to 1998. In 1998, George arrived in Miami, Florida, where he completed a Master of Science degree in geology from the Florida International University in the fall of 2000. His master’s research was focused on ore deposits in the Central Andes. In the fall of 2000, he began his Ph.D. at the University of Florida where his research is focused on d eciphering the relationshi ps between volcanism and ore deposit formation in SW Pacific island arcs. He will complete his Ph.D. at the University of Florida in the fall of 2004.


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MAGMATISM AND ORE DEPOSIT FORMATION IN SW PACIFIC ISLAND ARCS


By

GEORGE DIMITROV KAMENOV
















A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY

UNIVERSITY OF FLORIDA


2004

































Copyright 2004

by

George Dimitrov Kamenov















ACKNOWLEDGMENTS

I wish to thank the members of my committee for their support, patience and

guidance during the project. This dissertation would not be possible without the

insights, expert guidance, and support of Drs. Michael Perfit and Paul Mueller. Dr.

Ann Heatherington and Howard Scher provided invaluable help during the clean lab

sample preparation and TIMS analyses. I am grateful to Drs. David Foster, Joseph

Delfino, and Ian Jonasson for providing insights and reviewing this work.

I am grateful to Drs. Michael Perfit, Brent McInnes, Mark Hannington, and Peter

Herzig, and the crew of the RV SONNE for obtaining the samples from the TLTF

area. Dr. David Foster provided the biotite Ar-Ar data on Tubaf seamount. The

accomplishment of this research would not be possible without the major and trace

element analyses at the Canadian Geological Survey under the supervision of Dr. Ian

Jonasson. I am grateful to Rahul Chopra for separating the xenolith samples from the

Tubaf lavas. I am thankful to Tom Bisley for providing technical assistance with the

microprobe analyses at the Florida International University. Field and laboratory

investigations were supported by an NSF OCE-9403773 grant awarded to Dr.

Michael Perfit and a GSA 7430-03 student grant. This study would not be possible

without the NSF ARI grant 96-01872 for the purchase of the "Nu-plasma" MC-ICP-

MS. Many thanks go to Dr. Jamie Williams for his invaluable assistance during the

set up of the MC-ICP-MS lab. I am grateful for the financial support by the









University of Florida Alumni Fellowship during the course of this study. This work

benefited from the expertise provided by Drs. Ray Russo, and Phil Neuhoff Many

thanks go to Ray Thomas and Kevin Hartl for their invaluable technical support. The

staff of the geology department, Ron, Mary, and Jodie, have been very helpful during

my study in the University of Florida. I would like to thank all of my friends in the

Department of Geological Sciences for the friendship and support throughout the 4

years of graduate study. Finally, I would like to thank my wife, Katrin, for her love,

patience, and encouragement during this study.















TABLE OF CONTENTS



A C K N O W L E D G M E N T S ................................................................................................. iii

LIST OF TABLES ................................................... vii

LIST OF FIGURES ................................................ viii

A B S T R A C T ........................................................................................................ ............ x i

CHAPTER

1 IN T R O D U C T IO N .................................................. .. ....................................1.. .. ... 1

2 OPTIMIZATION OF MIXED LEAD-THALLIUM SOLUTIONS FOR HIGH
PRECISION ISOTOPIC ANALYSES BY MC-ICP-MS ................. ..................... 6

Introduction ................................................................................ ..........................6
A nalytical Procedures ...... .. ..................................................................................
R e su lts............... ......................................................................................... ........ .. 9
W et P lasm a E xperim ents........................................ ....................... ...............9...
D ry P lasm a E xperim ents ........................................ ....................... ...............9...
D iscu ssio n ......................................................... ...... ... ..................... ... ........... 15
Accuracy and Precision of the Pb and TI Isotope Measurements .................... 15
Production of T3+ ................ .. .... ................................. 16
M ass Discrimination Behavior of Pb and Tl .................................. ................ 18
C o n c lu sio n s................................................................................................................ 2 2

3 HIGH-PRECISION PB ISOTOPE MEASUREMENTS REVEAL MAGMA
RECHARGE AS A MECHANISM FOR ORE DEPOSIT FORMATION:
EXAMPLES FROM LIHIR ISLAND AND CONICAL SEAMOUNT, PAPUA
N E W G U IN E A ........................................................................................................... 24

In tro d u ctio n ................................................................................................................ 2 4
G eological Settings ................................................................... ............. 27
Sam ples and A nalytical M ethods ....................... ............................................... 30
R e su lts....................................................................................................... ....... .. 3 4
D iscu ssion ..................................... ......... .... .. . ............. ........ .. ......................... 39
Physical Constraints on Magmatism and Pb Isotopic Variations in the Volcanic
R o c k s ................................................................ ............................................ . 3 9
Phase Chemical Constraints on Petrogenesis ................................................42









Pb and Sr Isotopic Variations in the Conical and Lihir Mineralizations ..........45
M echanism for O re Form ation ....................................................... ............... 50
C o n c lu sio n s............................................................................................................... .. 5 2

4 DECIPHERING MANTLE AND CRUSTAL CONTROLS IN AN ISLAND ARC
ENVIRONMENT: A SR, ND, AND PB ISOTOPIC STUDY OF SW PACIFIC
ISLAND ARCS AND SUB-ARC XENOLITHS..................................................55

In tro d u ctio n ............................................................................................................... .. 5 5
G eological Settings .................................................................................. ............. 56
Sam ples and A nalytical M ethods .......................................................... ................ 59
R e su lts....................................................................................................... ....... .. 6 3
D iscu ssion ............................................................. ............. ... ............ .. ........... . 80
Major and Trace Element Variations in Seamount and Lihir Lavas................ 80
Major and Trace Element Variations in Mafic and Ultramafic Xenoliths ..........85
Sr and Nd Isotopic Variations in Xenolith and Lava Samples.........................87
Pb Isotopic Variations in the Mafic and Ultramafic Xenoliths ........................ 89
Indian Ocean-Type Mantle Versus Australian Subcontinental Mantle
Incorporation in the Mantle Wedge Beneath TLTF ....................................92
Relationships between Compositions of Xenoliths and Regional Volcanism ...........95
Model for the Alkaline Magmatism in the TLTF Area................. ...................101

5 SUMMARY AND CONCLUSIONS....................................112

6 FU TU RE RE SEA R CH ................................................................. ............... 117

APPENDIX

A MC-ICP-MS ANALYSES OF NBS 981 LEAD ISOTOPIC STANDARD MIXED
WITH QCD ICP-MS THALLIUM STANDARD. TABLE INCLUDES WET
PLASMA AND DSN FRESH AND AGED, EXPOSED TO LIGHT MIXED PB-TL
SO L U T IO N S A N A L Y SE S. ..................................................................................... 119

B LEAD EXTRACTION PROCEDURE (MODIFICATION OF THE TIMS HBR PB
SEPARATION OF MANHES ET AL., (1978), SIMPLIFIED AND ADAPTED
FOR MC-ICP-MS MEASUREMENTS OF PB ISOTOPES BY TL SPIKING .....126

C MIXING CALCULATIONS WITH 207PB/204PB BETWEEN SEDIMENTARY
AND M AFIC SOU RCES ................. ............................................................ 127

LIST O F R EFEREN CE S .. .................................................................... ............... 128

BIOGRAPH ICAL SKETCH ................. ............................................................... 139















LIST OF TABLES


Table page

2-1 Measured SRM 981 Pb and QCD TI isotopic compositions and Pb/TI
inten sity ratio s ..................................................................................................... .. 9

3-1 Location, general description, and strontium and lead isotopic compositions of
sam ples in the study area ....................................................................... .......... ... 31

3-2 Microprobe analyses on seamounts' clinopyroxene phenocrysts......................... 40

4-1 Major and trace element data for lavas and xenoliths.........................................64

4-2 Pb, Sr, and Nd isotopic analyses of lava and xenolith samples. .............................78















LIST OF FIGURES


Figure page

1-1 Regional map of the Western Pacific Ocean, showing some of the Western Pacific
G old Province deposits (in italic).......................................................... ...............2...

2-1 (A) SRM 981 206Pb/204Pb DSN measurements of fresh and aged, exposed to
sunlight mixtures. (B) QCD s205TI DSN measurements of fresh and aged,
exposed to sunlight m ixtures ............................................................... ............... 11

2-2 (A) Changes in measured Pb/Tl intensity ratios in mixed solutions with time and
exposure to sunlight. All solutions prepared with Pb/Tl=6 (B) Changes in measured
;205T1 in mixed solutions with time and exposure to sunlight...............................13

2-3 Chelex-100 resin elution behavior of Pb and TI in fresh and aged, exposed to
sunlight m ixed solutions.......................................... ......................... .............. 14

2-4 (A) Fractionation factors obtained for lead and thallium in T3+-bearing solutions,
fresh mixtures, and wet plasma experiments. (B) Comparison of fractionation
factors for several T3+-solutions analyses collected within a few days with the true
Pb and TI fractionation factor line (dashed line)................................. ................ 19

3-1 Regional map of Northwest Papaua New Guinea showing the Tabar-Lihir-Tanga-
Feni island chain, modified after Taylor, 1979 ................................... ................ 28

3-2 206Pb/204Pb vs Pb concentrations of Conical seamount lavas and high- and low-
tem perature m ineralized zones ............................................................ ................ 35

3-4 Comparison of Pb isotopic compositions of TLTF volcanic rocks with
sedimentary and mafic xenoliths, and Pacific MORB and sediments.. ................. 38

3-5 Representative compositional changes from core to rim in Conical
clinopyroxene show n in figure 3-6...................................................... ................ 42

3-6 Photomicrograph of Conical seamount clinopyroxene. Numbers correspond to
the analysis num bers presented in Table 3-2. ..................................... ................ 44

3-7 87Sr/86Sr vs Sr concentrations for mineralized samples in comparison with
C onical fresh lavas and sedim ents ...................................................... ................ 46









3-8 MgO vs Cl content of seamounts and Lihir island. Arrows show general
direction of changes during magma degassing, fractionation, and fractionation
accom panied w ith degassing ............................................................... ................ 51

3-9 Cartoon depicting the inferred ore formation mechanism ..................................53

4-1 Regional map of the study area, modified after Hall (2001). See chapter 3 for more
detailed m ap of the TL TF area ........................................................... ................ 57

4-2 Classification of the Conical, Edison, Tubaf, and Lihir lavas based on their Si02 vs
N a20 +K 20 concentrations ........................................ ....................... ................ 72

4-3 Comparison of MgO vs CaO, Fe203, Na20, and A1203 between Lihir and
seam ou n t lav as ......................................................................................................... 7 3

4-4 Comparison of MgO vs Ni, V, Ba, and Rb between Lihir and seamount lavas.......74

4-5 Normalized REE patterns of TLTF lavas and mafic and ultramafic xenoliths .......75

4-6 Primitive Mantle normalized trace element patterns of TLTF lavas and mafic
x e n o lith s .. ............................................................................................................. ... 7 6

4-7 Plot CaO/A1203 vs MgO indicating clinopyroxene as a major fractionation phase.
Symbols and data sources are the same as in figure 4-2.................................81

4-9 Primitive Mantle normalized trace element patterns of mafic xenoliths compared
with MORB, altered MORB, and oceanic gabbros.............................................84

4-10 Primitive Mantle normalized trace element patterns of ultramafic xenoliths
compared to TLTF lavas.. ................ ......................................... 86

4-11 Nd vs Sr isotopic compositions of the mafic and ultramafic xenoliths relative to
possible source in the area........................................ ........................ ................ 88

4-12 Pb isotopic compositions of mafic and ultramafic xenoliths relative to possible
sou rces in th e area ................................................................................................... 9 0

4-13 Pb isotopic compositions of xenoliths relative to Indian, Pacific MORB, and
Eastern and N orth Eastern Australia basalts. ...................................... ................ 94

4-14 Nd vs Sr isotopic compositions for island arc and back-arc basins lavas in the
region, compared to Indian and Pacific MORB and Pacific sediments................96

4-15 Pb isotopic compositions of lavas from TLTF, New Britain, Solomon Islands,
M anus and W oodlark back-arc basins ................................................ ................ 98

4-14 Cartoon summarizing the suggested origin of the TLTF volcanism....................102









4-17 Comparison ofK20/Ba vs Ba/La between lavas and mafic xenoliths and pelagic
and carbonate sedim ents...................................... ........................ ............... 105

4-18 Interaction of slab melt (SCHARM) with ultramafic mantle and formation of the
TLTF m agm as. ............. ............................................................................ 108















Abstract of Dissertation Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Doctor of Philosophy

MAGMATISM AND ORE DEPOSIT FORMATION IN SW PACIFIC ISLAND ARCS

By

George Dimitrov Kamenov

December 2004

Chair: Michael R. Perfit
Major Department: Geological Sciences

Majority of the richest base and precious metal deposits on Earth are located in the

Circum-Pacific region and are found either in a close proximity or within subduction-

related volcano-magmatic complexes. It is not clear, however, why some of the volcanic

centers contain ore deposits but others do not. This study utilizes a novel method for

high-precision Pb isotopic measurements by multi-collector-ICP-MS and reveals a

genetical connection between the ore-formation and magmatism in the Tabar-Lihir-

Tanga-Feni (TLTF) island arc, SW Pacific. The developed model suggests that injection

of a volatile-rich magma into an evolving magma body near the surface is the triggering

event that ultimately results in the ore mineralization.

Isotope data for mantle xenoliths suggest that three possible end-members,

including Pacific and Indian mantle, and Pacific sediments, can control the chemistry of

the magmas in the region. Comparison between the isotopic compositions of lavas, and

crustal and mantle xenoliths indicate that the incompatible elements in the TLTF









magmas, including a major part of the ore metals, were ultimately derived from a

subducted oceanic slab with Pacific mantle affinity. Regional and local isotopic trends in

the volcanic rocks in the region indicate that subducted oceanic slabs overall control the

composition of the island-arc magmas. Once the contribution from the subducting slab

decreases, incorporation of a mantle component with Indian affinity can be identified in

the isotopic composition of the lavas in the region.














CHAPTER 1
INTRODUCTION

The most significant geological phenomena on Earth, such as powerful

earthquakes, widespread volcanism, and island-arcs and continents growth, occur in the

zones of subduction. In addition, these zones are of great economic importance because

most of the world's richest base and precious metal deposits are associated with

subduction-related volcanism. A number of subduction zones and back-arc basins are

located in the western Pacific (Fig. 1-1) and a subject of this study is the Tabar-Lihir-

Tanga-Feni (TLTF) island chain, part of the Bismarck Archipelago. The TLTF islands

are part of the Western Pacific gold province (Fig. 1-1), which extends from Japan,

through the Philippines, Papua New Guinea, Solomon Islands, and Fiji to New Zealand

(Sillitoe 1989). The formation of the Bismarck Archipelago is related to the subduction

of the Pacific plate under the Indo-Australian plate and most of the arcs are dominated by

calc-alkaline volcanism, although the TLTF chain is dominated by alkaline volcanoes

(Wallace et al. 1983).

It is generally believed that the widespread magmatic activity in the island arcs is

due to lowering the mantle wedge peridotite melting point as a result of dehydration of

the subducted slab (Tatsumi 1989). However, some fundamental geological

processes, such as what controls the chemistry of the magmas and why there are ore

deposits associated with some of the volcanic systems but none with others, remain

unclear. The TLTF island chain provides an outstanding opportunity to investigate these

processes in detail because a number of xenoliths were recovered from a submarine















































Figure 1-1. Regional map of the Western Pacific Ocean, showing some of the Western
Pacific Gold Province deposits (in italic). Note that only about 1/3 of the
known deposits are shown on the map for clarity purposes, for more details
see Sillitoe (1989). Deposits are characterized by porphyry and epithermal
gold mineralizations and are young, ranging from Middle Miocene to
Quaternary (17-0 Ma). The study area (enclosed in the dashed field) includes
Bismarck Archipelago (New Britain, New Ireland, TLTF, Bougainville), the
Solomon Islands, and Manus and Woodlark back-arc basins, more detailed
maps are available in Chapters 3 and 4.









volcano, located on the flanks of Lihir island, host of one of the richest gold deposits on

Earth (Moyle et al. 1990). The xenoliths provide us with rare samples from the crust and

mantle underlying the islands, and detailed geochemical and isotopic study of xenoliths,

lavas, and ores will allow us to look at details of the processes responsible for the

magmatism and ore deposit formation in the region.

A number of existing and novel techniques, such as petrographic observations,

major and trace element analyses, Sr and Nd isotope data, and high-precision Pb isotope

measurements, are utilized throughout this study. Comparison of existing data from the

area (Wallace et al. 1983; Kennedy et al. 1990a,b; Mclnnes and Cameron 1994; Stracke

and Hegner 1998; Muller et al. 2001, 2003) and new major and trace

element analyses of lavas allow us to look in detail at the nature of the magmas in the

area and processes that led to their formation. Previous studies indicate that the lavas

erupted on the surface are not common for island arc settings because of their high

alkalinity, and was hypothesized that they are products of adiabatic decompression

melting of the subduction-modified mantle wedge (Mclnnes and Cameron 1994).

The mantle wedge beneath TLTF islands is composed of depleted peridotites, probably a

residue from a partial melting event at Mid-ocean ridge settings (Mclnnes et al. 2001,

Gregoire et al. 2001, Franz et al. 2002). Visual and chemical evidences, however,

indicated that the ultramafic xenoliths experienced strong metasomatism by fluids

released probably from the subducted Pacific slab (Mclnnes et al. 2001, Gregoire et al.

2001, Franz et al. 2002). Detailed Sr, Nd, and Pb isotopic comparison between the

metasomatized xenoliths and TLTF lavas, therefore, will provide evidence if the magmas

in the region are a result of partial melting of the metasomatized mantle wedge.









Sr, Nd, and Pb isotopic comparison is a routine technique utilized in geochemical

studies (Faure 1986) however, some recent studies indicate that significant analytical

errors can be generated during the traditional thermal ionization mass-spectrometry

(TIMS) measurements of Pb isotopes (Woodhead et al. 1995, Kamenov et al. 2003).

The problems stem from uncompensated Pb isotopic fractionation behavior during the

TIMS analyses. For example, during the Sr isotopic analyses a non-radiogenic pair

(86Sr/88Sr) is used to correct for the fractionation behavior of Sr during the TIMS

analyses. Such correction is not possible during the Pb analyses due to the existence of

only one non-radiogenic lead isotope (204Pb). A novel technique utilizing 205T1/203T1 to

serve as a "non-radiogenic pair" for mass-bias correction during Pb isotope

measurements was adopted with the development of the multi-collector ICP-MS

(Rehkamper and Halliday 1998). Although some studies have questioned the technique

(Thirwall, 2002), our recent experiments demonstrated that highly precise Pb isotope

measurements can be obtained by preventing the thallium photoxidation to 3+ state

(Kamenov et al. 2004a,b).

In addition to deciphering the relationships between magmas and xenoliths, Pb

isotopic measurements provide valuable information for the formation of the ore

mineralizations in the area. Lead is the only ore metal other than Os which shows large

natural isotopic variability and has similar geochemical behavior to other ore metals, such

as zinc, copper, and silver. Therefore, comparison of ore lead isotope ratios with those of

host rocks and other units in a particular mining district will suggest and/or rule out

possible ore metal sources. Combining the high-precision Pb isotopic measurements with









petrographic and geochemical data will lead to development of a model integrating the

magmatism and the ore-formation in the area.

On a larger scale, the observations and the wealth of data collected during the

course of this study provide valuable information on the composition of the mantle in the

area. Local and regional trends observed in the arc lavas from the region can be related to

a particular source or sources, which can be either a subduction-derived oceanic crust

and/or sediment component, or the local mantle. This will shed new light on the island

arc petrogenesis in the region and will further contribute to our understanding of the

subduction zone magmatism and associated ore deposit formation.














CHAPTER 2
OPTIMIZATION OF MIXED LEAD-THALLIUM SOLUTIONS FOR HIGH
PRECISION ISOTOPIC ANALYSES BY MC-ICP-MS

Introduction

The introduction of the MC-ICP-MS has led to a novel technique for Pb isotope

ratio measurements that utilizes 205T1/203T1 as an internal standard to correct for mass-

dependant fractionation in mixed Pb-Tl solutions (Belshaw et al. 1998, Rehkamper

and Halliday 1999, Collerson et al. 2002). Conversely, precise TI isotope measurements

can be obtained by mixing a Tl-bearing solution with Pb of known isotopic composition

(e.g., 208Pb/206Pb) as an internal standard to correct for mass-dependant fractionation of TI

(Rehkamper and Halliday 1999). Thirwall (2002), however, suggested that the Pb-Tl

isotope measurements determined by MC-ICP-MS are subject to large analytical

uncertainties and that this technique is not suitable for obtaining highly precise data for

Pb or TI isotopes. Thirwall's contribution constituted part of an on-going debate

concerning the reliability of Pb isotope analyses corrected by Tl-normalization

(Rehkamper and Mezger 200,; White et al. 2000, Woodhead 2002). For example,

Rehkamper and Mezger (2000) adjusted the 205T1/203T1 normalization ratio on a daily

basis to obtain results closest to the DS (double spike) TIMS SRM 981 values of Todt et

al. (1996). The need for such adjustments implies inconsistent mass-fractionation of Pb

and TI during the analytical procedure. In addition, Thirwall (2002) and Collerson et al.

(2002) observed that TI intensity was lower than expected in some mixed Pb-Tl solutions

and suggested that it resulted from unspecified and anomalous behavior of thallium in









solution. Rehkamper and Halliday (1998) also observed anomalous behavior of TI in

mixed Pb-Tl solutions and suggested that it resulted from variations in the complexation

of TI that could be avoided if the solutions were allowed to "age" for a period of several

days.

In this chapter I report results of an investigation of the interaction between Pb

and TI in mixed, dilute nitric and hydrochloric acid solutions and its effects on the

elemental and isotopic ratio measurements. I show that very precise MC-ICP-MS isotope

analyses of lead and thallium can be obtained using this system by controlling the analyte

conditions to minimize the formation of T13+

Analytical Procedures

Solutions were prepared in Teflon bottles or vials by mixing SRM 981 Pb standard

solution with commercially available ICP TI stock solutions (QCD Analysts, USA and

Aldrich, USA). The mixed solutions were prepared with Pb-Tl elemental ratios ranging

from 2 to 6 and were diluted to 100 to 150 ppb Pb and 25 to 35 Pb ppb for the wet and dry

plasma experiments, respectively (Appendix A). Most of the experiments were conducted

on mixed solutions diluted in either 0.6, 2, or 5% HNO3 (percent dilution of

concentrated Optima-brand HNO3); several experiments were conducted in 2% HCI

solutions and in 2% HNO3 solutions with traces of HC1. A Nu-Plasma MC-ICP-MS (Nu

Instruments, UK) in the Department of Geological Sciences at the University of Florida

was used for this study. The instrument description, typical operating conditions, and

analytical protocol during Pb-Tl analysis are reported in Belshaw et al. (1998). Sample

and standard solutions were aspirated either through a Nu Instruments desolvating

nebulizer (DSN-100) ("dry plasma" mode) or directly into the plasma source through a

Micromist nebulizer with GE spray chamber ("wet plasma" mode). Measured uptake rate









for both sample introduction methods was about 100 ptL min-1. The instrument settings

were carefully tuned to maximize the signal intensities on a daily basis. Preamplifier gain

calibrations were run before the beginning of each analytical session. Typical sensitivities

for Pb were about 300 V ppm-1 and about 40 V ppm-1 for dry and wet plasma modes,

respectively. Sampler and skimmer cones (Ni) were thoroughly cleaned after 3 or 4

analytical sessions. All analyses reported in this paper were conducted in static mode by

directing 202Hg on low-2, 203T1 on low-1, 204Pb on Axial, 205T1 on high-1, 206Pb on

high-2, 207Pb on high-3, and 208Pb on high-4 (all Faraday detectors). The measured

204Pb beam was corrected for isobaric interference from 204Hg using

204Hg/202Hg=0.2290 (before mass-bias correction), although always negligible. Data

were acquired in blocks of 20 ratios with 10 s integration times. Background

measurements of 30 s durations preceded each block. All of the reported SRM 981 ratios

were normalized using the exponential law for mass-bias correction and

205T1/203T1=2.3875 after Belshaw et al. (1998). All TI isotope analyses were

normalized using the exponential law and SRM 981 208Pb/206Pb=2.1664. This ratio

represents the average of all fresh SRM 981 runs conducted during this study and is

identical to the average of all MC-ICP-MS SRM 981 208Pb/206Pb analyses reported by

Reuer et al. (2003). Tl g-notation values are calculated relative to NIST 997

205T1/203T1=2.3871, following Rehkamper et al. (2002). Note that the 205T1/203T1

(2.3875) used throughout these experiments is 1.68 epsilon units greater than the value of

NIST 997 (205T1/203T1=2.3871), which is why our average s205TI results for QCD are

greater than zero (Table 2-1).










Results

Wet Plasma Experiments

Forty two wet plasma analyses of SRM 981 mixed with QCD TI were conducted

between April and June, 2003 and gave the following results: 206Pb/204Pb=16.9369

(+/-0.0039, 2y), 207Pb/204Pb=15.4904 (+/-0.0034, 2y), and 208Pb/204Pb=36.6949

(+/-0.0087, 2y) (Table 2-1). The reported 2-sigma error (2y) is the external precision,

calculated on the basis of these 42 runs. The internal precision obtained during a single

analysis is usually lower, on the order of +/- 0.001 to 0.002 (2y). Measured F205TI in the

QCD standard during the wet plasma experiments was 0.9 (+/-1.2, 2y) (Table 2-1).

Measured Pb and TI beam intensities for these solutions showed intensity ratios similar

to the elemental ratios in the original mixtures (Table 2-1, Appendix A).


Table 2-1. Measured SRM 981 Pb, QCD TI isotopic compositions, Pb/Ti intensity ratios
experiment /11 Pb 11 i*i .4Pb I.'Pb I. '20Pb s20'5T (QCD)
(mixed) (measured) (
wet plasma 16.9369 15.4904 36.6949 0.9
(fresh and Tl 2 to 6 2 to 6
solutions) n=42 (+-0.0039, 20) (+-0.0034, 2) (+ -0.0087, 2~ (+ -1.2, 20)
dry plasma 16.9373 15.4907 36.6935 1.5
(only fresh mixtures) 2 to 6 2 to 6 (+/-0.0011, 2cy) (+/-0.0012, 2c) (+/-0.0039, 2c) (+/-0.8, 2c)
n=29
dry plasma variable 16.921 15.469 36.630 from -3.9
(onlyTl tions) 2 39 up to 36 (+/-0.039 2C) (+/-0.052 2cy) (+/-0.160 2cy) to +30.1
dry plasma 20 20 16.9375 15.4910 36.6979 0.4
(fresh mix) (+/-0.0018, 20) (+/-0.0018, 20) (+/-0.0052, 20) (+/-1.0, 2y)
dry plasma 40 40 16.9359 15.4886 36.6908 1.4
(fresh mix) (+/-0.0020, 2y) (+/-0.0020, 2y) (+/-0.0062, 20) (+/-1.0, 2y)


Dry Plasma Experiments

Experiments using the DSN-100 were also conducted between April 2003 and

January 2004. During the initial experiments, Pb and TI from single stock solutions were

mixed in 2% HNO3. Solutions were then measured from 48 hours to 60 days after

preparation (mixing), as suggested by Rehkamper and Halliday (1999). Contrary to the









results of Rehkamper and Halliday (1999), however, our initial results with these

solutions revealed poor overall precision and accuracy with 206Pb/204Pb=16.921 (+/-0.039

2y), 207Pb/204Pb=15.469 (+/-0.052 2y), and 208Pb/204Pb=36.630 (+/-0.160 2y) (Table 2-1,

Fig. 2-1A). Similarly, using SRM 981 as a fixed Pb isotopic composition for mass bias

corrections of the TI isotopes yielded relatively large T205TI variations (i.e., from -3.9 to

+30.1) for our QCD standard (Table 2-1, Fig. 2-1B).

Comparison of measured Pb and TI beam intensities for these solutions over this

period showed essentially constant Pb ion beam intensities, but lower TI intensities (i.e.,

higher Pb:Tl intensity ratios compared to the known elemental ratios in the original

mixtures) (Fig. 2-2A). Measured intensities in Pb-only (30 ppb) and Tl-only (5 ppb)

control solutions did not show changes in intensity during the experiments, indicating

that the decrease in the TI signal intensity occurred only in aged, mixed solutions. In

addition, the wash out times were consistently longer for the aged compared to the fresh

mixtures. Condensate (i.e., waste solution) from the DSN collected during analyses of

the aged solutions showed lower Pb-Tl intensity ratios, from 0.6 to 1, indicating a net

increase in TI over Pb in the waste (i.e., during the combined desolvation and washout

processes). Overall, it appeared that extending the interval between mixing the Pb and TI

solutions and analysis produced significant, but irregular, Pb-Tl elemental decoupling

along with poorer precision and accuracy in the isotope ratio measurements for both Pb

and Tl.

To address this problem, we prepared new mixed solutions (also in 2% HNO3) and

analyzed them immediately (<1 h) after mixing. Analyses of these mixed solutions

(n=29) yielded consistent and highly precise results with 206Pb/204Pb= 16.9373 (+/-0.0011,







11


2y), 207Pb/204Pb=15.4907 (+/-0.0012, 2y), and 208Pb/204Pb=36.6935 (+/-0.0039, 2Y)

(Table 2-1, Fig. 2-1A). The average of the measured T205TI in these "fresh" solutions


16.96


16.94


16.92


16.9


16.88

40


30


0 0


0 00 00


- 000


nnoCr-,0


y-t-2t2


0 0 0:
0 00
0 000 0 -
0 0
*
0


April, 2003


August, 2003


January, 2004


Figure 2-1. (A) SRM 981 206Pb/204Pb DSN measurements of fresh and aged, exposed
to sunlight mixtures. (B) QCD s205TI DSN measurements of fresh and aged,
exposed to sunlight mixtures (for more details see the text, Table 2-1, and
Appendix A).


- (B)


-U









prepared with the QCD standard and SRM 981 was 1.5 (+/-0.8, 2G) (Table 2-1,

Fig. 2-1B). In addition, measured Pb and TI ion beam intensities always reflected those

calculated for the mixtures. Analyses of fresh 2%HNO3 mixtures prepared with high

Pb/Tl ratios yielded Pb and TI isotopic values within error with the rest of the fresh

mixtures, suggesting that the observed discrepancies during the analyses of the aged

mixtures is not a result of the decrease in the TI signal (i.e., high Pb/Tl, Table 2-1).

Several experiments were also conducted with 2%HCI and a range of HNO3

concentrations. Aging of mixed Pb:Tl solutions in 2%HC1, 5%HNO3, and

2%HN0O3+0.05%HCI showed similar behavior to the 2%HNO3 experiments, i.e.,

decoupling of the Pb-Tl elemental abundances and imprecise and inaccurate isotope

ratios (Fig. 2-2A and B). Using solutions with a lower HNO3 concentration (0.6%)

showed less Pb-Tl elemental decoupling and more precise reproducibility of isotope

ratios over time compared to the solutions with higher HNO3 concentrations (Fig. 2-2A

and B).

Subsequent testing of the same mixed solutions noted above (0.6 to 5% HNO3) after

several months of storage, however, showed that the solutions reverted to behave as fresh

mixtures; that is, they did not show the Pb-Tl elemental decoupling and poor precision of

Tl and Pb isotope ratio measurements that had characterized them previously

(Fig. 2-2A and B). The source of the "freshening" of the solutions was ultimately traced to

the fact that the solutions were stored in cabinets and were not exposed to sunlight, suggesting

a photochemical effect. To test this hypothesis, the solutions were again moved to their

initial storage location, which allowed exposure to direct sunlight. The subsequent

analyses showed the strong decoupling in the Pb/Tl ratios and the relatively imprecise Pb






13




50

0.6% HN03 2%HN03+HCI
40 A 2% HN03 2% HCI
S- 5% HNO3

30- (A) ,
-j
U) 20
E


S10




(B) no exposure to
20 _------ Initial stages to sunlight sunlight
of experiment
15 light effect
-. unknown
o 10

LL
5


0

-5
1 50 150 160
Time in days (not on scale)


Figure 2-2. (A) Changes in measured Pb/TI intensity ratios in mixed solutions with time
and exposure to sunlight. All solutions prepared with Pb/TI=6 (B) Changes in
measured s205TI in mixed solutions with time and exposure to sunlight. Note
that all (with the exception of 2%HCl) solutions revert to behave as fresh
mixtures after prolonged storage without exposure to sunlight.






14


and TI isotope ratio measurements again characterized these solutions. Repeated

experiments confirmed that exposure to sunlight plays a major role in the interaction of

Pb and TI in the solutions (Fig. 2-2A and B).

The nature of this reaction was constrained using chromatographic columns

(Chelex-100 resin) designed (Lin and Nriagu, 1999) to separate Tl from Tl3+. The TI

from the fresh mixtures and Pb from both fresh and aged, exposed to sunlight mixtures

elute similarly in 2% HNO3. TI from solutions containing Pb and exposed to sunlight,

however, eluted much later, and ultimately required the use of 10% and 20% HNO3 for

complete elution (Fig. 2-3). These results strongly suggest that Tl is oxidized to T3+ in

solutions containing Pb when exposed to sunlight. The reaction is reversible when the

solutions are stored without exposure to sunlight.


70

60

- 50
a)


S2%HN03
40 elution 10%HNO3
S. e;An elutioH 20%HN03
30 eluton elution

020

10

0
0 4 12 20 28
HN03 eluted (ml)

Figure 2-3. Chelex-100 resin elution behavior of Pb and TI in fresh and aged, exposed to
sunlight mixed solutions, for more details see the text.


_ ----. -- Pb 2% fresh TI 2% aged
.... A---....- TI 2% fresh ---- Pb 5% aged -
Pb 2% aged TI 5% aged









Discussion

Accuracy and Precision of the Pb and TI Isotope Measurements

Data for SRM 981 "wet plasma" experiments provide a significant improvement

over unspiked Pb isotopic analyses by TIMS, but are not generally as precise as double

spike results. Analyses, however, completed using the DSN and fresh Pb-Tl mixtures

revealed excellent precision, equivalent or even better than commonly reported for

double spike methods via TIMS (Todt et al. 1996, Thirwall 2000). The generally lower

precision of wet plasma vs dry plasma experiments is probably related to less stable

plasma conditions during wet plasma mode. The absolute value of the SRM 981,

however, is dependant on the 205T1/203T1 ratio used for normalization (Belshaw et al.

1998, Collerson et al. 2002). All of the reported SRM 981 data in this paper, summarized

in Table 2-1, were normalized using 205T1/203TI=2.3875, adapted from Belshaw et al.

(1998). The values observed during wet plasma and DSN fresh mixtures analyses

(Table 2-1) are in excellent agreement with the DS TIMS data of Todt et al. (1996).

In terms of TI isotopes, Rehkamper and Halliday (1999) measured the 205T1/203T1

ratios in commercially available Alfa/AESAR, Aldrich, and NBS 997 TI solutions and

concluded that they all have indistinguishable isotopic compositions. They reported

F205Tl=-1.38 (+/-1.48 3G) for Aldrich and 205TI=0.19 (+/-1.28 3G) for Alfa/AESAR

solutions, relative to NIST 997 205T1/203T1=2.3871. Fresh mixture measurements of our

aliquots of commercially available QCD and Aldrich TI solutions yielded F205T1=1.5

(+/-0.8 2G) and 205T1=0.1 (+/-0.8 2G) respectively, values that are within error of the values

measured by Rehkamper and Halliday (1999). Conversely, the measured TI and Pb









isotopic compositions in the aged Pb-TI mixtures reveal large discrepancies (Fig. 2-1A

and 2-1B) that I attribute to the presence of TI3+ in these solutions.

Production of T13+

Thermodynamic considerations indicate that only monovalent thallium is stable in

the solutions measured for this study (Lin and Nriagu 1999), and this expectation was

confirmed by our experiments on the elution behavior of fresh mixtures with the Chelex-

100 resin (Fig. 2-3). The observed change in the thallium elution behavior for the mixed

solutions exposed to sunlight in the laboratory, however, indicates that Tl was oxidized

to T13+ (Fig. 2-3). The fact that measured ion beam intensities in unmixed Pb and TI

control solutions did not exhibit this behavior suggests that the photo-oxidation of TI was

catalyzed by the presence of Pb. Photoelectrochemical reactions can be driven in an

"uphill" direction by exposure to ultraviolet radiation (Bard 1980). In particular, Switzer

et al. (1982) demonstrated that Tl can be oxidized to T13+ in a liquid photovoltaic cell by

exposure to UV light in the presence of semiconductors such as TiO2, WO3, and ZnO. It

has been proposed that ultraviolet irradiation induces a Tl ++TT3+ electron transfer by

production of a short-lived T12+ chain carrier (Stranks and Yandell 1969). Because Tl

was converted to T13+ upon exposure to solar UV radiation and only in the presence of

Pb2+ during our experiments, it may be that Pb2+ acts as a short-lived electron carrier in a

possible one or two stage reaction, e.g.:

TIl+Pb2+-TI3++Pb0 or Tl++2Pb2+ -T13++2Pb+

It is difficult to confirm the formation of either of the two lead species noted

above due to the very low concentrations and the fact that Pb and Pb+ are not stable in

nitric acid and should be rapidly oxidized to Pb2+. Consequently, the ultimate storage of









the electrons liberated during the oxidation is likely to be accommodated by small

changes in the NO2-/NO3- ratio (i.e., amount of nitric acid reduction). This hypothesis is

supported by the observation that after storage without exposure to sunlight, all of the

solutions returned to their initial state (Fig. 2-2A and B); that is, they behave as Tl+-bearing

mixtures. The exception is the 2%HCI solution, probably because T13+ is stabilized in

presence of Cl (Cotton et al. 1999). In addition, it appears that after aging and exposure

to sunlight, HNO3 solutions with added traces of Cl tend to produce less variable

205T1/203TI isotopic compositions, although very different than the F205TI defined by the

"fresh" and wet plasma experiments (Fig. 2-2B and 2-4B).

The association between T13+ and the generation of increased Pb/Tl ratios,

increased wash-out time for TI, and imprecise isotopic ratios in the post-desolvation

fraction of the analyte suggests that T13+ does not behave in the same manner as Tl or

Pb2+ during the desolvation. The differential behavior of these two TI species may be

related to more than one factor or process. For example, Tl is much more soluble than

T13+ and T13+ (or complex ions/molecules containing T3+) may precipitate or adsorb onto

surfaces inside the DSN more readily than Tl during the concentration that occurs as the

sample is evaporated. T13+ is more likely to be hydrolyzed and form colloids than Tl+,

even at pH 1 to 2.5 (Cotton et al. 1999). If formed, these colloidal particles would be

small enough to be transported downstream from the spray chamber and may adhere,

therefore, to a variety of surfaces inside the DSN, including the membrane. Similarly,

T13+ (or a complex ion containing T3+) may interact differently with the desolvation

membrane. Although composed of relatively inert fluorocarbon, the membrane has a









relatively large surface area compared to the other components of the DSN and is,

therefore, a likely zone of retention.

Mass Discrimination Behavior of Pb and TI

The observed elemental fractionation that occurs during desolvation of T13+

bearing solutions appears to be accompanied by fractionation of TI isotopes. Figure 2-4

shows the fractionation factors derived for TI and Pb for the experiments reported here.

The calculated Pb and TI fractionation factors for fresh mixtures and wet plasma show

very similar behavior and define a common line (indistinguishable slopes) indicating

identical mass fractionation behavior for the Pb and TI isotopes (Fig. 2-4A). Data from

the wet plasma experiments include data generated from both Tl and T13+-bearing

solutions (Table 2-1), which also lie on this common fractionation line. This suggests that

even though thallium is oxidized to T13+, it does not experience differential mass-bias

behavior in the plasma compared to Pb nor does the oxidation process itself appear to

cause isotopic fractionation. No decoupling in the Pb/Tl ratios was observed in wet

plasma mode (Table 2-1) and furthermore, scanning at lower masses (68.3, 102.5,

etc.) failed to detect any peaks where T13+ or T12+ would be present, suggesting that T13+

is efficiently transformed to Tl in the plasma.

Fractionation factors for the T13+-bearing solutions analyzed by DSN, however,

deviate from the factors calculated for the Tl+-bearing solutions and wet plasma

experiments, which suggests mass-dependant fractionation of TI during desolvation. The

variations in the isotopic composition of TI produced during desolvation can be

represented by fractionation lines offset from, but parallel to, the fractionation line

defined by the "fresh" and wet plasma experiments (Fig. 2-4B). These results indicate






19




-1.2


0 -1.4 (A)

S-1.6


-1.8 -o o
SO O0 0
_0- -2 o0 wet plasma
C D 0
0 0o fresh mixtures

-2.2 o aged mixtures _

-1.2U

2 -. (B) +1 _
4- -1.4 -" -
4_ +155
-1.6-
4(0 .

S-1.8 -
S- -






-2.2 -2 -1.8 -1.6 -1.4 -1.2
205TI/203TI fract. factor




Figure 2-4. (A) Fractionation factors obtained for lead and thallium in T13+-bearing
solutions, fresh mixtures, and wet plasma experiments. The data for wet and
DSN fresh mixtures show very similar behavior and lie on a line expected for
identical mass-bias behavior experienced by Pb and TI (dashed line). (B)
Comparison of fractionation factors for several T13+-solutions analyses
collected within a few days with the true Pb and TI fractionation factor line
(dashed line). Solid lines represent calculated fractionation factors using
different TI isotopic compositions (the offset from the wet and fresh analyses
is shown in epsilon notation).









that Pb and TI continue to experience similar mass-bias behavior, but that a net change in

the isotopic composition of thallium in the desolvated Ti3+-bearing solutions occurred.

Changes in the isotopic composition of TI are not, however, consistent. For

example, a 0.6% HNO3 Pb-Tl solution exhibited relatively small changes in F205TI, but at

the same time, a 5%HNO3 Pb-Tl solution exhibited a range of almost 10 F205TI units over

a several days time period (Fig. 2-2B and 2-4B). Over the same period a 2%HNO3+traces

of HCI Pb-Tl solution showed relatively stable T205TI, although the values were very

different than the value obtained for the fresh mixtures (Fig. 2-4B). These observations

suggest that the measured T205TI in the T13+-bearing solutions is influenced by several

factors, including the acid matrix, the conditions during desolvation, and the dose of UV

(sunlight) received (Fig. 2-2 and 2-4).

The exact process or processes which produce differential mass fractionation of

Tl+3-bearing solutions during desolvation is not clear. Mass-dependant isotopic

fractionation during redox reactions involving Fe has been suggested by Zhu et al.

(2002). Nielsen et al. (2004), however, demonstrated that mass-fractionation of TI does

not occur during laboratory handling that includes ion exchange chromatography and

oxidation of Tl to T13+. Our experiments also show no evidence for mass fractionation

during photo-oxidation reaction. It seems most likely, therefore, that the observed

isotopic fractionation (enrichment in 205TI) occurs during desolvation. The fact that the

analyte is enriched in the heavier isotope suggests that adsorption (Rehkamper et al.

2002) is not the controlling process, though we cannot rule out its occurrence. A more

likely process that must be considered for the preferential removal of 203T1 from the

analyte during desolvation is diffusion related mass-dependant fractionation. During the









diffusion process the lighter isotopes have higher average velocities and so diffuse

proportionally faster than the heavier isotopes through a porous membrane (Chopin et al.

2002). This process, known for almost a century, is commercially utilized in uranium

enrichment plants using UF6. The isotope enrichment is accomplished in a series of

diffusion devises consisting of cells divided by a porous membrane (commonly made of

fluorocarbon), which can maintain differential gas pressure (Chopin et al. 2002). The

basic construction of gaseous diffusion devices, therefore, is similar to the desolvators

utilized in this and other studies referred to herein. By analogy with the enrichment of

uranium, it may be that the 203T13+ ions, molecules, or complex ions (e.g., hydrated T3+)

pass through the membrane at a greater rate than the 205T13+ species as they pass along the

length of the desolvating membrane, which results in an enrichment in 205TI in the

desolvated analyte. The greater rate of transfer of TI under these conditions may also lead

to greater interaction with the membrane to the point that more TI is ultimately held on

the membrane, which results in the longer washout times observed. The net effect of

these two processes (adsorption and diffusive fractionation) appears to favor diffusion as

the primary process that determines the isotopic composition of the analyte.

Fractionation during diffusion, however, should affect all species in the analyte

stream to some degree, including Pb. If, however, the fractionation behavior of the

elements of interest is governed by a common fractionation law during the passage

through the DSN and in the plasma, then precise isotopic ratios can be obtained by

applying a single correction factor (e.g., exponential law), as is the case for our fresh Pb-

Tl mixtures. If the fractionation behavior of the elements of interest does not result from

processes adequately described by a single fractionation law, then such a correction will









not result in precise isotopic measurements, as is observed in our aged, exposed to

sunlight mixtures. At this stage, however, it is not clear why T13+ species experience

different fractionation behavior than Tl and Pb2+. Possible explanations include 1) T13+

ions have lower hydration enthalphy (-4184) compared to Tl (-326) and Pb2+(-1480)

(Burgess 1978) and, therefore, form stronger bonds with water molecules (or with Ar, N)

that may persist through the desolvation process and interact with the membrane to a

greater extend; 2) by analogy with the use of the volatile UF6 for U-enrichment, it may be

that the formed T13+ species are more volatile than Tl species and, therefore, are more

susceptible to gaseous diffusion enrichment; and 3) it is also possible that the emerging

from the DSN T13+-bearing dry aerosols experience different mass-bias than the Tl -

bearing dry aerosols in the plasma, although the wet plasma experiments argue against

such possibility. Fractionation may occur also during the vaporization of solvent, which

leads to the formation of compounds and/or complex molecules that may preferentially

incorporate heavier isotopes. In addition, the so formed compounds, molecules, and/or

hydrous ions may have different vapor pressures, and consequently, one species (e.g., Tl)

may be lost preferentially to another (e.g., Pb) and also isotopically fractionated. Further

experiments are required to test these hypotheses.

Conclusions

High-precision Pb isotopic measurements provide valuable geochemical

information, however, significant analytical errors can be introduced during MC-ICP-MS

isotopic analyses of desolvated Pb and Tl-bearing solutions because different species

appear to exhibit different behavior during the liquid-vapor transition and subsequent

desolvation process inside the DSN. My experiments demonstrate that significant TI









isotopic variations, equivalent to the range observed in natural samples by Rehkamper et

al. (2002) can be generated by varying experimental conditions, as reported here.

The interaction of Pb and TI is thermodynamically unexpected and appears to

result from photoelectric driven oxidation of Tl to T13+. The reaction occurs only after

the addition of Pb, which appears to act as an intermediate (short-lived) electron acceptor.

Although no detailed experiments were conducted to constrain the kinetics of the reaction

in detail, this phenomenon was observed in mixed solutions exposed to sunlight for more

than several hours. The demonstrated production of T13+ in solution, which is related to a

complex combination of factors (e.g., differences in the acid matrix and molarity,

desolvation conditions, and duration of UV light exposure), greatly affects the precision

and accuracy of Pb and TI isotopic measurements. Spiking samples intended for Pb

isotope measurements with TI just before the analysis and/or preventing the mixed Pb-TI

solutions from being exposed to sunlight (UV light) are critical for achieving the most

precise Pb isotope ratio measurements from desolvated nitric acid solutions.














CHAPTER 3
HIGH-PRECISION PB ISOTOPE MEASUREMENTS REVEAL MAGMA
RECHARGE AS A MECHANISM FOR ORE DEPOSIT FORMATION: EXAMPLES
FROM LIHIR ISLAND AND CONICAL SEAMOUNT, PAPUA NEW GUINEA.

Introduction

Ore deposits associated with subduction-related volcanism are among the most

important sources of base and precious metals in the world. Although many of these

deposits have received extensive study, several major questions regarding their genesis

remain unanswered. Such major questions include why base metal mineralization is

associated with some magmatic systems, but not with others? What is the mechanism of

incorporation of ore metals into hydrothermal fluids, have these components been derived

from active sub-volcanic melts as a result of magma degassing, or have they been

extracted from the host rocks via meteoric or seawater hydrothermal leaching driven by

thermal energy from magmas? If the latter is the case, magmatic heat will drive the

hydrothermal circulation and, consequently, is the most critical factor for the formation of

an ore deposit. Relatively few magmatic complexes, however, contain world-class

deposits, suggesting that other mechanisms must also play important roles in the

formation of ore deposits associated with magmatic systems. Such mechanisms include

efficient release of metal-bearing fluids from solidifying magmas (Hedenquist and

Lowenstern 1994). Suitable conditions for magmatic fluid saturation and release can be

reached during magma cooling accompanied by crystallization or during recharge by

volatile-rich magma (Sillitoe 1997). Studies suggest that during degassing, magma

chambers maintain relatively oxidizing conditions that inhibit formation of immiscible









sulfide liquids that can sequester ore metals (McInnes and Evans 1996) and thereby

enhance the ore formation potential of the exsolving fluids. Providing clear evidence that

the exsolution of ore-bearing fluids from magmas is a major mechanism for ore

formation remains elusive. Studies utilizing stable isotope tracers, such as oxygen and

sulfur, have shown that the magmatic signature in the ore-forming fluid is often obscured

due to mixing with meteoric fluids in the near surface environment and so considerable

disagreement exists concerning which of the fluids (i.e., magmatic or meteoric) transport

and deposit ore metals (Bodnar 1995).

Another important isotopic tracer that can be directly related to the ore metal

sources are the lead isotopes. Lead is a common ore metal and shares similar

geochemical behavior with other ore metals such as Ag, Cu, and Zn. Numerous studies

have utilized Pb isotopes to investigate the relationship between ores and their possible

sources (Doe and Delevaux 1972, Tilton et al. 1981, Macfarlane et al. 1990,

Richards et al. 1991, Kamenov et al. 2002). Historically, the use of Pb isotopes suffered

from the fact that due to isotopic fractionation, lead isotopic data obtained by thermal

ionization mass spectrometry (TIMS) lack the precision and accuracy readily obtained for

Sr, Nd, Hf, and Os isotopic analyses. For example, for Sr a non-radiogenic isotope pair,

usually 86Sr and 88Sr, is used to correct for the observed isotopic fractionation and thus

highly precise data can be obtained. A similar correction is not possible for lead because

there is only one non-radiogenic isotope (204Pb). So, in the conventional TIMS analysis, a

set of samples is corrected using the fractionation measured in a well-characterized

standard, usually NBS 981, by assuming similar mass-bias behavior in the standard and

the unknowns. It has been shown, however, that samples may exhibit different









fractionation behavior than the pure standards due to the presence of other metals and

varying amounts of sample loaded and the extent of evaporation for individual samples

(Woodhead et al. 1995, Kamenov et al. 2003). In order to deal with this problem

some studies have adopted double (DS) and triple (TS) spike methods to correct for

instrumental mass bias and have successfully produced high-precision Pb isotopic data

(Todt et al. 1996, Abouchami et al. 2000, Thirwall 2000). Recent studies have

demonstrated that important geochemical correlations and trends among Pb isotopes, not

readily observable in the traditional TIMS-derived data, can be revealed by utilizing

high-precision Pb isotopic analyses (Abouchami et al. 2000, Woodhead 2002).

With the development of the MC-ICP-MS a novel technique for Pb isotopic

measurements was adopted that uses 205T1/203T1 as an internal standard to correct for

mass-dependant fractionation of Pb (Rehkamper and Halliday 1998, Collerson et

al. 2002, Woodhead 2002). Although some studies have questioned the applicability of

Tl isotopes for instrumental mass fractionation corrections of Pb isotopes (Thirwall

2002, Baker et al. 2004), our recent experiments have shown that highly precise Pb

isotopic ratios, equivalent to the DS and TS TIMS Pb isotope measurements, can be

obtained using MC-ICP-MS. The key issue in this application is to control the interaction

of Pb and TI in solution so that thallium does not oxidize to the 3+ state (Kamenov et al.

2004).

Here I present high-precision Pb isotopic data (measured by MC-ICP-MS) from

subaerial and submarine alkaline lavas and gold-bearing ores from Papua-New Guinea to

address the origin of these valuable mineral deposits. A number of large epithermal gold

and copper deposits genetically associated with alkaline rocks have recently attracted









considerable interest (Sillitoe, 2002), including the giant Ladolam gold deposit in the

Luise caldera, located on Lihir Island, Papua-New Guinea (Moyle et al. 1990;

Rytuba et al. 1993; Muller et al. 2001, 2003). A group of seamounts (Edi's

Daughter, Edison, Tubaf, and Conical) were recently discovered on the flanks of Lihir

Island and the presence of gold mineralization was discovered at the summit of Conical

seamount in 1998 (Herzig et al. 1999). In this Chapter, I investigate the sources and

processes that may have led to the development of the deposits at both Lihir Island and

Conical seamount.

Geological Settings

Lihir island is one of a group of islands in Papua-New Guinea that lie northeast of

the islands of New Britain and New Ireland in the Southwest Pacific. The Tabar, Lihir,

Tanga, and Feni (TLTF) island chain is located in the former fore-arc region of New

Ireland, part of the Bismarck archipelago (Fig. 3-1). Volcanism on New Ireland and on

most of the islands in the archipelago is dominantly calc-alkaline to high K/calcalkaline,

and was generated as a result of subduction of the Pacific plate under the Indo-Australian

plate along the Manus-Kilinailau trench (Johnson 1979). Subduction ceased about 10 Ma

ago, when the thick and relatively buoyant Ontong-Java plateau collided with the

subduction zone (Coleman and Kroenke 1981). Subsequent to the collision, regional

relocation of stress caused reversal of the subduction direction, formation of the north-

norhtwest facing New Britain-San Cristobal trench, initiation of back-arc spreading in the

Manus Basin at about 3.5 Ma, and development of several microplates (Taylor 1979).

The TLTF island chain extends for about 250 km parallel to the presently inactive

Manus-Kilinailau trench (Fig. 3-1). The islands are equally spaced at about 75 km, and









seismic reflection surveys have delineated horst and sediment-filled grabens bounding the

chain, suggesting extension and crustal thinning (Exon et al. 1986). Volcanism in the


New World 152 1542/
Seamount /"/ / "
North Bismarck >4
microplate''


Searnount ^ Seamount ,, \ -, O t"
-' Lihir
Manus back-arc"Tan
- 4S spreading center ,
South Bismarck Feni
0 100 200kin inicroplate



New Britain
Papua subdX ctin '
New GuineSolomon miicropla ic



Figure 3-1. Regional map of Northwest Papaua New Guinea showing the Tabar-Lihir-
Tanga-Feni island chain, modified after Taylor, 1979. The inset shows the
location of Lihir Island and the seamounts, where samples for this study were
recovered.

TLTF area began on Simberi island, about 3.7 Ma ago (Johnson et al., 1976, Mclnnes,

1992), coeval with the initiation of back-arc spreading in the Manus Basin, and migrated

southward to Feni island (2300 y), as the island of New Britain was transported to the

southeast. Taylor (1979) suggests that the local extension and volcanism are related to the

opening of the Manus back-arc spreading center. TLTF islands are characterized mainly

by high-K calc-alkaline rocks with similar to arc lavas trace element and isotopic

characteristics (Wallace et al. 1983; Kennedy et al. 1990a,b; Stracke and Hegner 1998).

These lavas were probably derived from a number of different parental magmas









generated in a mantle source enriched in incompatible elements during earlier periods of

subduction of the Pacific plate (McInnes and Cameron 1994, Stracke and Hegner 1998).

During a cruise of the RV SONNE in 1994 (SONNE 94) Edi's Daughter, Edison,

TUBAF, and Conical volcanic seamounts were discovered on the flanks of Lihir Island

(Fig. 3-1, Herzig et al. 1994). Sampling (dredge and TV-assisted grab) and observations

using a remote camera indicated the seamounts were volcanically active in the recent past

and that Conical seamount had recently been hydrothermally active (Herzig et al. 1994).

Ultramafic, gabbroic, basaltic, and sedimentary xenoliths were recovered in submarine

lavas from Tubaf and Edison seamounts. The abundance of xenoliths in Tubaf lavas is

remarkable; one TV-grab sample weighing 200 kg contained more than 70 cm-sized

xenoliths (McInnes et al. 2001). During a cruise of the RV SONNE in 1998 (SO-133) the

existence of epithermal-style gold mineralization at Conical seamount was confirmed by

extensive sampling (Herzig et al. 1998). The seamount is a submarine volcanic cone

underlain by a thick (about 5 km) layer of Miocene to Recent sediments (McInnes et al.

2001). The recovered lavas are porphyritic vesicular trachybasalts to basaltic

trachyandesites (Muller et al. 2003). The mineralization is confined to the top of the

seamount along a main eruptive fissure and shows zonal distribution (Herzig et al., 1999).

Three styles of mineralization can be identified (Petersen et al. 2002): (a) intensely

altered clay-silica rich zone containing semi-massive and stockwork-like pyrite, (b) gold-

rich, disseminated, polymetallic sulfides in feldspathic and siliceous veins, and (c) low-

temperature, late-stage, fracture-hosted As-Sb mineralization. Here I have divided the

mineralized samples analyzed in this study into two groups: 1) high-temperature gold-

rich samples (including (a) and (b)) and 2) low-temperature samples (c) (Table 3-1).









Samples and Analytical Methods

General descriptions and locations of samples analyzed in the study are presented

in Table 3-1. Fresh volcanic rocks and altered volcanic rock samples exhibiting various

degrees of mineralization were recovered from the seamounts during the 1998 R/V Sonne

cruise. Conical seamount trachybasalts are composed of about 20 to 25% clinopyroxene

(Cpx), 5% plagioclase (Plag), and rare olivine and phlogopite phenocrysts embedded in a

slightly vesicular, fine-grained groundmass which consists of glass, plagioclase,

clinopyroxene, spinel, and apatite microcrysts (Miuller et al. 2003). The clinopyroxene

phenocrysts are large (up to about 6mm), euhedral to subhedral, and exhibit distinct

zoning. Plagioclase phenocrysts are much smaller, up to about 1mm long. Tubaf and

Edison seamounts' lavas are also trachybasalts and contain phenocrysts of clinopyroxene,

as well as phlogopite and amphibole embedded in a groundmass containing about 20%

vesicles and glass (McInnes et al. 2001). Clinopyroxenes in these samples are smaller

(up to 1mm) and more homogeneous than those from Conical lavas.

Mineralized samples were collected from the Ladolam mine. Two unaltered

monzonite samples were obtained from a drill core taken from different depths in the

Ladolam mine. Sedimentary (limestones and carbonaceous mudstones) and mafic

(gabbro and basaltic) xenoliths were extracted from Tubaf seamount lavas using a

microdrill (Table 3-1).

Mineral analyses in seamount lavas were conducted at the Florida Center for

Analytical Electron Microscopy (Florida International University) using a JEOL EPMA

JXA-8900-R equipped with 5 wavelength dispersive spectrometers. Mineralized samples













Table 3-1. Location, general description, and strontium and lead isotopic compositions of
samples in the study area. Table includes also Pb isotope data for USGS rock
standards prepared and analyzed together with the samples.
Sample # Location Description Sr 87Sr/86Sr Pb *i i.2Pb i ri., Pb i ri. Pb


15GTVA
2DSpGTVA Conical
2DSp

15GTVA
2DSp
23GTVA Conical
Sp
25GTVA
2SGTVA Conical
6BSp

25GTVA
6BSp
25GTVA
8C2 Conical


53GTVA
53GTVA Conical
2AGn

53GTVA
2Agn


39GTVA
39GTVA Conical
2Xpy

40GTVA
GTV2Py2 Conical
2Py2

42GTVA
42GTVA Conical
2Py

53GTVA
53GTVA Conical
1Bpy


7DR Conical
12DR2A Conical
12DR2A -
13DR4 Conical
36GTVA
36GTVA Conical

50DR1C Conical
52GTVA
2GTVA Conical

52GTVA
1
54GTVA Tubaf
Tubaf
4-2
56GTVA Tubaf
Tubaf
2C
56GTVA
Tubaf
2M
10GTVA Edison
5A2 Edison
5A2
11GTVA Edison
2A1 Edison
2A1
33GTVA Edison
Edison
2J1
34GTVA Edison
Edison
3-1


LH98-3 Lihir,
Ladolam


High temperature
mineralization
(Gn*, Sp)
Duplicate
High temperature
mineralization (Sp)
High temperature
mineralization
(Cp, Sp, Stbn, Plag, Kfs)
Duplicate
High temperature
mineralization
(Gn, Sp, Cp, Chd, Plag)
High temperature
mineralization
(Gn, Py, Cp, Kfs, Qtz)
Duplicate


25 0.70401 29300


490 0.70411 13500


18.764


15.546 38.372


18.765 15.549 38.374

18.768 15.548 38.382


18.763 15.549 38.380


18.761


172 0.70457 34100


212 0.70575 25700


Low temperature
mineralization 846 0.70745 298
(Py, Mrc, Qtz, Kfs, Rlg, Jar)
Low temperature
mineralization (Chl, Am-Si, 66 0.70493 563
Py, Anat)
Low temperature
mineralization (Qtz, Chd, 928 0.70421 286
Py, Hm, Plag)
Low temperature
mineralization 411 0.70863 33
(Py, Hm, Qtz, Plag, Smec)


Fresh lava
Fresh lava
Duplicate
Fresh lava
Fresh lava
Fresh lava
Fresh lava

Duplicate

Fresh lava

Fresh lava

Fresh lava

Fresh lava

Fresh lava

Fresh lava

Fresh lava


Mineralization (Kfs, Py,
Cp)


4
4

6


15.547 38.371


18.767 15.551 38.379


18.764


15.546 39.372


18.766 15.547 38.371



18.756 15.554 38.377


18.742 15.543 38.350


18.764 15.546 38.366


18.754


18.742
18.733
18.732
18.734


15.547 38.359


15.542
15.551
15.549
15.539


38.341
39.351
38.344
38.335


5 18.737 15.548 38.364
4 18.746 15.548 38.360


5 18.721


1504 0.70397 10

1698 0.70397 10


15.547 38.332


18.720 15.544 38.327

18.759 15.554 38.391

18.756 15.550 38.377


11 18.766 15.550 38.366


5 18.761


15.547 38.371


4 18.762 15.546 38.373

5 18.766 15.550 38.384

5 18.765 15.549 38.378


18.718 15.545 38.339


0.70395













Table 3-1. Continued
LH98-6 Lihir, Mineralization (Qtz, Py, Cp, 0.70405
Ladolam Gn)
LH98-7 Lihir, Mineralization (Qtz, Py) 0.70409
Ladolam
LH98-7 Duplicate -
LH98-8 Lihir, Mineralization (Qtz, Py) -
Ladolam
LH98-9 Lihir, Mineralization (Qtz, Py, Cp, -
Ladolam Gn)


Fresh lava -


Fresh lava


LIH- 10 Lihir,
Londolov
it
LIH- 11 Lihir,
Londolov
it
LIH- 18 Lihir,
Luise
LIH-21 Lihir,
Luise
LIH-23 Lihir,
Luise
LIH-25 Lihir,
Luise
LIH98-1 Lihir,
Luise
LIH98-2 Lihir,
Luise
471-265 Lihir,
Luise
471-3395o Lihir,
Luise
471-3395o -

56GTVA Tubaf
30
54GTVA Tubaf
3E
55GTVA Tubaf
2E
56GTVA Tubaf
3I
54GTVA Tubaf
6B
54GTVA Tubaf
6G
54GTVA Tubaf
6E
54GTVA Tubaf
6F
56GTVA Tubaf
4D
56GTVA Tubaf
4F
72GTVA Near
Conical


Fresh lava

Fresh lava

Fresh lava

Fresh lava

Fresh lava

Fresh lava

Monzonite

Monzonite

Duplicate

Basaltic xenolith

Gabbro xenolith

Gabbro xenolith

Gabbro xenolith

Sedimentary xenolith

Sedimentary xenolith

Sedimentary xenolith

Sedimentary xenolith

Sedimentary xenolith

Sedimentary xenolith

Surface sediment


USGS rock standard
Duplicate
Duplicate
Duplicate
Duplicate
USGS rock standard
Duplicate
Duplicate
Duplicate
Duplicate


18.715

18.726

18.726
18.727

18.720

18.765


18.766


18.759

18.762

18.757

18.747

18.748

18.745

18.737

18.727

18.728

18.692

18.747

18.722

18.647

18.627

18.640

18.901

18.771

18.712

18.770

18.743


18.767
18.765
18.767
18.763
18.762
18.942
18.944
18.940
18.944
18.940


15.544

15.543

15.542
15.543

15.539


38.337

38.342

38.336
38.340

38.331


15.546 38.357


15.554 38.377


15.549

15.548

15.549

15.553

15.543

15.541

15.553

15.544

15.544

15.509

15.509

15.533

15.529

15.557

15.568

15.560

15.567

15.528

15.583

15.585


15.624
15.622
15.618
15.620
15.620
15.653
15.655
15.652
15.655


38.368

38.367

38.368

38.366

38.347

38.341

38.369

38.342

38.344

38.163

38.196

38.280

38.191

38.302

38.325

38.495

38.441

38.307

38.495

38.512


38.728
38.721
38.738
38.729
38.730
38.548
38.549
38.541
38.550


15.652 38.545


*Gn-galena; Sp-sphalerite; Stbn-stibnite; Cp-chalcopyrite; Chd-chalcedony; Rig-realgar; Jar-jarosite;


BCR-2
BCR-2
BCR-2
BCR-2
BCR-2
AGV-1
AGV-1
AGV-1
AGV-1
AGV-1


795 0.70396



144 0.70297 0.33

120 0.70302 -



130 0.70293 0.26

892 0.70751 2

820 0.70828 6

818 0.70759 2

456 0.70669 4

1221 0.70791 5

2









from Conical Seamount were carefully micro-drilled and the mineral phases (Table 3-1)

were determined by XRD at the Geological Survey of Canada in Ottawa. Trace element

concentrations were determined by ICP-MS at the Geological Survey of Canada

(http://www.nrcan.gc.ca/gsc/mrd/labs/chem-e.html).

Whole-rock powders (free of xenoliths and alteration) were leached in Optima

grade 2N HCI at about 700C for several hours, then rinsed several times with 4x distilled

H20 to remove leachate residue, and then dissolved in a mixture of HF+HNO3. Micro-

drilled sample powders from the mineralized zones were dissolved (but not leached)

similarly to the whole rock powders. Sr and Pb for isotopic analyses were separated using

standard chromatographic methods under clean lab environment (Chapter 4 and

Appendix B). Sr isotopic analyses were performed on a Micromass Sector 54 Thermal

Ionization Mass Spectrometer equipped with seven Faraday collectors and one Daly

detector at the University of Florida. The Sr was loaded on oxidized W single filaments

and run in triple collector dynamic mode. Data were acquired at a beam intensity of about

1.5V for 88Sr, with corrections for instrumental discrimination made assuming

86Sr/"Sr=0.1194. Errors in measured 87Sr/86Sr are better than +/- 0.00002 (2c) based on

long-term reproducibility of NBS 987 (87Sr/86Sr=0.71024).

Pb isotopic analyses were conducted on a Nu Plasma multi-collector ICP-MS

(Nu Instruments, UK), in the Department of Geological Sciences, University of Florida, using

the TI normalization technique (Chapter 1). Sample and standard solutions were aspirated

into the plasma source through a Micromist nebulizer with a GE spray chamber. The

instrument settings were carefully tuned to maximize the signal intensities on a daily

basis. Preamplifier gain calibrations were determined before each analytical session. All









analyses reported in this paper were conducted in static mode acquiring simultaneously

202Hg on low-1, 203TI on low-2, 204Pb on Axial, 205TI on high-1, 206Pb on high-2, 207Pb on

high-3, and 208Pb on high-4 Faraday detectors. 42 analyses of NBS 981 conducted with

the sample analyses in the period between April and June, 2003 gave the following

results 206Pb/204Pb=16.9369 (+/-0.0039 2y), 207Pb/204Pb=15.4904 (+/-0.0034 2Y), and

208Pb/204Pb=36.6949 (+/-0.0087 2y). All standard and sample Pb data were normalized

with 205T1/203TI=3-38750 (for more details see Chapter 2). Several samples were prepared

and analyzed for Pb isotopes as duplicates to evaluate the reproducibility of the results

(Table 3-1). In addition, 10 separate dissolutions of USGS rock standards (BCR-2 and

AGV-1, Woodhead and Hergt, 2000) were prepared and analyzed together with the

samples to further verify the precision and accuracy of the analytical protocol

(Table 3-1).

Results

Isotopic analyses of rocks, xenoliths, and mineralized samples from the study area

are presented in Table 3-1. Sr isotopic compositions of fresh lavas from the area show

relatively small variations with 87Sr/86Sr, close to 0.704, within the range of 0.7037 to

0.7044 observed in previous studies of TLTF islands and seamounts (Wallace et al.

1983, Kennedy et al. 1990b, Stracke and Hagner 1998). Measured 87Sr/86Sr in

sedimentary xenoliths recovered from Tubaf seamount lavas exhibit much more

radiogenic values than the lavas in the area (Fig.3-2). Sr isotopic compositions of the

Conical seamount mineralized samples have values either similar to or more radiogenic

than the host lavas (Fig. 3-2). Lihir mineralized samples also have 87Sr/86Sr values very

similar to the host lavas (Table 3-1).














-- seawater

0
o

0
-



- o



-


o sedimentary
xenoliths


A






0 -
-


Pb (ppm)


Figure 3-2. (A) 206Pb/204Pb vs Pb concentrations of Conical seamount lavas and high- and
low-temperature mineralized zones (based on data from Table 3-1). Note that
the high-temperature mineralized samples exhibit less variations in their Pb
isotopic compositions compared to the lavas and the low-temperature
mineralized samples. (B) 87Sr/86Sr vs Pb concentrations of Conical seamount
lavas, high- and low-temperature mineralized zones, and sedimentary
xenoliths (for discussion see the text). Sr data for fresh lavas from this study
and Stracke and Hagner (1998).


0.709

0.708

0.707

0.706

0.705

0.704


0.703


18.76


18.75


18.74


18.73


18.72


18.71


... .I


S2 S.E.


B
B


High-temp. mineralization
n O Low-temp. mineralization
a Fresh lavas


10 100 1000 10000 1


; T T T r T T i !


. I i


. .i









Fresh volcanic rocks exhibit small, but distinguishable variations in their Pb

isotopic compositions, with Conical seamount lavas having the least radiogenic ratios

with 206Pb/204Pb=18.721 to 18.746, 207Pb/204Pb= 15.539 to 15.551, and 208Pb/204Pb=38.332

to 38.364. Tubaf and Edison seamount lavas are slightly more radiogenic than Conical

with 206Pb/204Pb=18.756 to 18.766, 207Pb/204Pb=15.546 to 15.550, and 208Pb/204Pb=38.366

to 38.391 (Fig. 3-3). Most of the analyzed Lihir Island lavas exhibit lead isotopic

signatures indistinguishable from Edison and Tubaf lavas with several samples having

slightly less radiogenic values, notably the two monzonite drill core samples from the

Ladolam mine, with 206Pb/204Pb=18.727 and 18.737, 207Pb/204Pb=15.544 and 15.553, and

208Pb/204Pb=38.342 and 38.369 (Table 3-1). Nine low- and high-temperature mineralized

samples from the Conical seamount mineralized zones show slightly more radiogenic

ratios than the fresh host lavas, ranging from 206Pb/204Pb=18.742 to 18.768,

207Pb/204Pb=15.543 to 15.554, and 208Pb/204Pb=38.350 to 38.382. The measured lead

isotopic compositions in the mineralized Conical seamount samples exhibit a positive

correlation between Pb content and 206Pb/204Pb ratios (Fig. 3-2A). Mineralized samples

from Lihir also show slightly different Pb isotopic ratios from the host fresh lavas

exposed on the surface with 206Pb/204Pb= 18.711 to 18.727, 207Pb/204Pb=15.539 to 15.545,

and 208Pb/204Pb=38.331 to 38.340. A surface sediment sample from New Ireland basin

and several sedimentary and mafic xenoliths recovered from the Tubaf seamount lavas

were also analyzed for Pb isotopes (Table 3-1). They exhibit larger range in their lead

isotopic compositions compared to the fresh volcanic rocks in the area with values

ranging from 206Pb/204Pb= 18.627 to 18.901, 207Pb/204Pb=15.509 to 15.585, and
208Pb/204Pb=38.163 to 38.512 (Fig. 3-4).














+
2S.E.


4+1
A


38.4


38.38


38.36


38.34


38.32


15.56


15.555


15.55


15.545


* Em


* Conical mineral.
R Conical lavas
A Tubaf and Edison
lavas
O Lihir lavas
* Lihir mineral.


+1
4:,
2S.E.



NHR
itS-


NHRL
^-' -


18.72 18.74 18.76 18.78 18.8


206 Pb204Pb
2Pb/2Pb



Figure 3-3. MC-ICP-MS Pb isotopic data for lavas and mineralized samples. Note the
similarity between the Tubaf and Edison lavas and Conical high-temperature
mineralization, suggesting magma with similar Pb isotopic composition as a
source for the ore metals. Similarly, Lihir mineralized samples plot at the non-
radiogenic end of the array, suggesting Pb source similar to the least
radiogenic Conical lavas or the monzonite intrusion. Note that the ores and
lavas from a trend parallel to the NHRL (for more discussion see the text).


A



mE A
*
mFF


15.54 -


15.535


15.53
18.7


.* . .


. &
















38.5 sediments 0 0 0
CL



38


Pacific MORB

37.5


15.65 -


15.6
C- NHRL o a
k 15.55 0 .


15.5


15.45
Pacific MORB
15.4 l l
18.2 18.4 18.6 18.8 19
206pb204pb
MPb/0Pb



Figure 3-4. Comparison of Pb isotopic compositions of TLTF volcanic rocks with
sedimentary and mafic xenoliths, and Pacific MORB and sediments. Note the
homogeneous isotopic character of the TLTF samples compared to the
possible sources in the area. TLTF volcanic rocks field is based on MC-ICP-
MS data from this study and TIMS data from Stracke and Hagner (1998),
Pacific MORB after White et al. (1987), and Pacific sediments after Othman
et al. (1989).









Analyses of clinopyroxene phenocrysts from Conical, Tubaf, and Edison

seamounts, completed by electron microprobe, provide more constraints on changing

magmatic conditions (Table 3-2). Typically, Conical clinopyroxenes are larger

compared to the clinopyroxenes in the other two seamounts and show concentric

zoning with oscillatory chemical variations. The analyzed clinopyroxenes from Tubaf

and Edison seamounts are smaller, homogeneous, and do not exhibit significant chemical

variations.

Discussion

Physical Constraints on Magmatism and Pb Isotopic Variations in the Volcanic
Rocks

Seamount volcanism is younger than volcanism associated with TLTF island

formation. Tubaf seamount probably records the youngest volcanic event in the area

(222 +/-34 Ka, one biotite Ar-Ar age determination). Conical seamount yielded slightly

older age: 287 (+/-20) Ka (M. Hannington, personal communication). Edison seamount

may be older than the other two seamounts, based on field observations (Herzig et al. 1998).

Lihir Island is a Pliocene to Holocene volcanic complex built from several coalescent

volcanoes (Wallace et al., 1983). Volcanism and hydrothermal activity in the area of the

Ladolam deposit have been active during the last 1 Ma with biotite samples yielding

K-Ar ages from 900 (+/-100) to 340 (+/-40) Ka in altered intrusions, and 610 (+/-250) to

150 (+/-20) Ka for hydrothermal adularia and alunite samples (Carman 2003 and

references therein). The northernmost part of Lihir (Londolovit block) was formed during

the oldest volcanic event on the island and shows slightly elevated 206Pb/204Pb ratios

compared to the younger Luise volcano, which hosts the Ladolam deposit (Table 3-1).










Conical seamount has Pb isotopic ratios similar to, but extending to lower 206Pb/204Pb

ratios compared to Luise volcano (Fig. 3-3, Table 3-1).

Table 3-2. Microprobe analyses on seamounts' clinopyroxene phenocrysts. Conical
seamount data are for the CPx shown on figure 3-6, numbers correspond to
the analysis points.
Conical Conical Conical Conical Conical Conical Tubaf Tubaf Edison Edison
Smpl# 50DR1 50DR1C. 50DR1C. 50DR1C. 50DR1C. 50DR1C. 21DR1 21DR1 11GTVA2 11GTV
C.1 2 3 4 5 6 A B A A2B
SiO2 51.20 48.30 49.91 52.02 48.20 49.90 52.01 51.9 53.32 53-1
TiO2 0.25 0.53 0.38 0.2 0.44 0.52 0.15 0.19 0.15 0.14
A1203 3-76 5.74 4.14 3-13 4.06 3.89 1.99 3-1 1.91 3-1
Cr2O3 0.25 0.03 0.14 0.36 0.04 0.18 0.2 0.29 0.27
FeO 5.16 9.10 8.80 5.51 9.51 9.37 5.44 5.21 4.12 5.8
MnO 0.25 0.32 0.29 0.10 0.26 0.44 0.25 0.25 0.20 0.2
MgO 15.66 13.85 13.88 15.57 14.37 14.31 15.91 15.85 17.59 16.8
CaO 23.31 21.89 23.48 23.92 21.91 23.15 23.11 23.0 23.78 23.9
Na20O 0.23 0.57 0.32 0.19 0.30 0.38 0.32 0.33 0.25 0.25
K20 0.05 0.03 0.01 0.02 0.01 0.03
Total 99.12 100.3 100.3 100.0 99.05 101.0 99.3 99.1 100.6 100.5



The two other seamounts, Tubaf and Edison have elevated lead isotopic ratios

compared to Conical, but similar to the northernmost Londolovit block (Fig. 3-3,

Table 3-1). Overall, there appears to be no correlation between the relative ages and the

Pb isotopic compositions of the volcanic rocks. The variations can be either a result of

small Pb isotopic variations in the magma sources or a result of magma modification

processes during ascent. The presence of a large number of sedimentary, mafic, and

ultramafic xenoliths in Tubaf and Edison seamount lavas allows me to address this

possibility. The common occurrence of xenoliths in alkaline rocks is attributed to the high

ascent velocities of the magmas, on the order of several kilometers per hour (Spera 1984,

Morin and Corriveau 1996). Calculated minimum ascent velocity (after Spera 1984) for

Tubaf magma required to lift an ultramafic xenolith (3.3 g/cm3, 15 cm diameter) to the

surface is on the order of 2 to 3 km/h. McInnes et al., (2001) argue on the basis of the observed









mineral assemblages that some of the ultramafic xenoliths come from about 60 to 70 km

depth. Assuming an ascent velocity of 3 km/h, the Tubaf and Edison magmas must have

ascended to the surface in about 20 to 24 h from their mantle source region. Rapid

ascent rates calculated for the two seamounts' magmas suggest that existence of a magma

chamber and/or upper crustal modification of the magmas close to the surface is highly

unlikely. In addition, sharp contacts between the sedimentary and mafic xenoliths and

host lavas provide no evidence for melting or resorption of the xenoliths, suggesting that

no significant assimilation of sedimentary or basaltic/gabbro material occurred. This

suggests that the lead isotopic compositions of the two seamounts are probably inherited

from their source region.

The mantle wedge beneath the TLTF island chain has been metasomatically

modified by fluids released during earlier subduction of the Pacific plate beneath the

Indo-Australian plate (McInnes et al. 2001, Gregoire et al. 2001) and geochemical

studies of fresh lavas suggest multiple parental melts (Kennedy et al. 1990a,b). Lead

isotopic compositions of volcanic rocks from TLTF islands show relatively uniform

206Pb/204Pb ratios, ranging from 18.7 to 18.8 and slightly elevated 207Pb/204Pb ratios

relative to Pacific MORB (Fig. 3-4) and were interpreted to be a result of a small

contribution (ca. 1 to 2%) of subducted sediment to the mantle source region (Stracke and

Hagner 1998). The fresh lavas shown on Figure 3-4 fall within the field defined by the

sediment and oceanic crust samples, but overall are confined to the Pacific MORB field,

which suggests that the Pb budget of the lavas is probably controlled mainly by oceanic

crustal lead (Chapter 4).







42


Phase Chemical Constraints on Petrogenesis

The paucity of xenoliths in the lavas from Lihir Island and Conical seamount and


the more fractionated chemistry of the lavas (Chapter 4) are consistent with slower


magma ascent and evolution in a magma chamber close to the surface. Muller et al.


0




tM




0
a)
LL







0
M








0
Oil
5.


L111"'" 111"'1""I""1t1
" D -
: --Tubaf and Edison Cpx

A %.

%
A ---A












A





-A-
2 \' C a











-* Tubaf and Edison Cpx
C I






and Edison Cpxumber

- A I -

- -A Conical A"
L .
L- ft --A


- Tubaf and Edison Cpx -

1 2 3 4 5 6
analysis number


Figure 3-5. Representative compositional changes from core to rim in Conical
clinopyroxene shown in Figure 3-6. Bars represent the observed variations in
Tubaf and Conical clinopyroxenes.









(2001) conducted an extensive study of Lihir volcanic rocks and minerals exposed in the

vicinity of the Ladolam gold deposit and the P-T conditions inferred from mineral

analyses suggested that their main evolution stage occurred at a shallow crustal level.

Muller et al. (2003) also investigated P-T conditions of crystallization of Conical

seamount lavas and calculated crystallization pressures of 10-14 kbar based on Al content

in hornblende cores and 0-4.5 kbar (+/-2 kbar) crystallization pressure for

clinopyroxenes. This data suggests magma evolution in 2 stages: one deeper, controlled

by early hornblende crystallization, and a second stage, shallower (crustal levels)

associated with clinopyroxene and plagioclase crystallization (Muller et al., 2003). The

presence of plagioclase in Conical lavas also suggests relatively shallower crystallization

compared to the Tubaf and Edison magmas. Plagioclase crystallization is suppressed in

magmas with high concentration of water (Carmichael et al., 1996), therefore, its

presence is consistent with the model that Conical seamount lavas evolved at a shallower

level under relatively low water fugacity, probably in a degassing magma chamber close

to the surface. Microprobe elemental analyses of Conical clinopyroxenes reveal chemical

zoning (Fig. 3-5), whereas Tubaf and Edison clinopyroxenes do not show zoning and

their compositions are similar to the central parts of the Conical clinopyroxenes (Table 3-2

and Fig. 3-5). Stracke and Hagner (1998) argue that clinopyroxene was the major

fractionating phase during the magmatic evolution of the seamounts and TLTF islands.

As a result, Mg and Ca will behave compatibly and decrease in the more evolved lavas.

Furthermore, other elements, such as Al and Fe, will not be incorporated preferentially in

the clinopyroxenes and their concentrations will increase in the evolving liquid. The

observed chemical variations in the interiors of the analyzed clinopyroxenes are









consistent with such an evolutionary trend. The cores of the Conical clinopyroxenes are

higher in Mg and Ca and lower in Al, Fe, and Ti, similar to the compositions of Tubaf

and Edison clinopyroxenes (Fig. 3-5). The darker colored zones surrounding the lighter

zones (Fig. 3-6) show lower Ca and Mg and elevated Al, Fe, and Ti reflecting changes

likely occurring in the evolving liquid. Light colored areas toward rims of the crystals,

however, have elevated Mg and Ca and lower Fe, Al, and Ti contents and this reverse

zoning is consistent with a mafic magma recharge episode in the evolving magma

chamber beneath Conical seamount. In addition, careful observation reveals that the

darker zones (also along the borders between the lighter and darker zones) contain a

number of small inclusions (Fig. 3-6). The inclusions are composed of a glass and fluid

phase, and often contain clusters of needle-shaped apatite crystals. The later observation

suggests that during the growth of the dark zones the clinopyroxenes were in contact with

highly evolved magma. Apatite usually forms toward the end of the crystallizing




















Figure 3-6. Photomicrograph of Conical seamount clinopyroxene. Numbers correspond
to the analysis numbers presented in Table 3-2 and shown in Figure 3-5.









sequences and is observed in the Conical lavas groundmass (Miuller et al. 2003). The

presence of a fluid phase in the inclusions along the zones provides evidence for episodes

of volatile saturation and magmatic fluid exsolution, processes considered to play

important roles in the ore formation.

Pb and Sr Isotopic Variations in the Conical and Lihir Mineralizations

Petersen et al. (2002) observed that the early, high-temperature mineralization

stages at Conical seamount are overprinted by a low temperature stage that mainly

produced amorphous silica and arsenic sulfides. Sr isotopic compositions of high and low

temperature mineralized samples compared with Conical seamount lavas and sediments

from the area are shown in Figures 3-2 and 3-7. The high-temperature ore-rich samples

with high Cu, Ag, Au, and Pb contents show overall similar 87Sr/86Sr to Conical, Lihir,

and the other seamount lavas, suggesting that the Sr is mainly derived from the alkaline

lavas in the area. One of the high-temperature mineralization samples exhibits elevated

87Sr/86Sr, which indicates incorporation of more-radiogenic Sr. Two of the analyzed 3

Lihir mineralization samples have Sr isotopic compositions that are slightly elevated

compared to the analyzed monzonite sample from Luise volcano (Table 3-1), which is

also suggestive of minor incorporation of more-radiogenic Sr. Conical low-temperature

mineralized samples with relatively lower Pb, Zn, Cu, and Au contents show either

similar or elevated 87Sr/86Sr compared to the fresh lavas (Table 3-1, Fig. 3-7). Seawater

and/or sediments are the two possible sources for more radiogenic Sr in the area,

however, simple mixing can not explain the observed variations (Fig. 3-7, see caption for

more discussion). Observations at MOR black smoker systems show only minor increase

in Sr concentrations in the emanating fluids compared to the ambient seawater

(Teagle et al. 1998). Experiments by Seewald and Seyfried (1990) indicate KD Sr values






46


between 0.017 and 0.027 during high-temperature (300 to 5000C) fluid/basalt interactions,

which also suggests that Sr will not be strongly enriched in the hydrothermal fluids.

Therefore, ore-forming fluids probably will not contain very high Sr content, then, even

small amounts of seawater incorporated in the Conical or Ladolam ore-forming

hydrothermal fluids will result in elevated 87Sr/86Sr in the mineralized zones compared to

the host lavas. In addition to seawater, the thick Miocene-Recent sedimentary sequences

beneath the seamounts also can be a possible source of more radiogenic Sr. Due to their


0.709

0.708

0.707

0.706

0.705

0.704

0.703


200 400 600 800

Sr (ppm)


1000 1200 1400


Figure 3-7. 87Sr/86Sr vs Sr concentrations for mineralized samples in comparison with
Conical fresh lavas and sediments. Symbols are the same as in figure 2.
Arrows show possible mixing trends between the possible end-members in the
region: A exsolution and/or dissolution of Sr from magma/lava without
significant seawater or sediment Sr incorporation; B mixing between
magmatic Sr and seawater Sr (note that Sr has low fluid-rock Kd (less than
0.1) and that is why the fluid Sr concentration is expected to be lower that the
lavas (for more discussion see the text); C simple Sr mixing between lava
and sediment; D mixing between lava, sediment, and seawater Sr. Sr data
from Table 3-1 and from Stracke and Hagner (1998).


seawater



D 0




B Conical lavas
- !

\ C

A[.









calcareous character, the sediments in the area have 87Sr/86Sr values either close to or

lower than the present day seawater (Fig. 3-7) and this fact prevents unequivocal

distinction based solely on Sr isotopic data whether the more radiogenic end-member in

the mineralized zones is seawater or sediment derived Sr.

Seawater contains negligible amounts of Pb compared to the lavas and sediments

in the area and, although it may affect the Sr budget, it will not affect the Pb isotopic

compositions of the altered and mineralized zones. Overall, the mineralized samples

exhibit Pb isotopic compositions similar to the volcanic rocks in the area (Fig. 3-3),

suggesting that Pb and possibly other ore metals with similar geochemical behavior were

derived from the latter. An complicating issue, however, for the isotopic composition of

the lead in the mineralized samples is that their suggested source (i.e., the volcanic rocks

in the area) originated from a mantle that has been metasomatically altered by

components released from subducted sediments and oceanic crust (Stracke and Hagner

1998, Mclnnes et al. 2001, Gregoire et al. 2001). These components most probably had

Pb isotopic compositions varying between the Pacific sediments (including the

sedimentary xenoliths), and the mafic xenoliths (Fig. 3-4), and so make it very difficult to

resolve if the isotopic signatures observed in the lavas and mineralizations were acquired

in the mantle or in the crust. Although the above arguments suggest that the Pb isotopic

signature of the lavas is inherited directly from their mantle source, the same arguments

can not be applied to the origin of the ore metals in the Conical and Lihir mineralizations.

Therefore, two possible scenarios for the origin of the Pb in the mineralized zones must

be considered: (1) lead in the mineralized samples can be derived from the host lavas

(either hydrothermally leached or exsolved from solidifying magmas at depth), and (2) it









can be hydrothermally scavenged from the thick layer of sediments and underlying

oceanic crust.

An Os isotopic study conducted on Au ores from the Ladolam deposit indicates

that the primary source of the ore metals is the mantle underlying the TLTF island chain

(Mclnnes et al. 1999), providing evidence that material derived from the Miocene-

Recent sediments in the area has not contributed significantly to the metal budget of the

ore deposits. Overall, the sediments exhibit much higher 207Pb/204Pb and 208Pb/204Pb

compared to the TLTF lavas (Fig. 3-4), therefore, incorporation of sedimentary Pb in the

hydrothermal system will shift the isotopic compositions of the mineralized samples to

higher 207Pb/204Pb and 208Pb/204Pb at relatively constant 206Pb/204Pb, which is not the case

(Fig. 3-3). Pb isotopic compositions of the lavas and mineralized samples do show a

minor increase in 207Pb/204Pb and 208Pb/204Pb, however, accompanied with an increase in

206Pb/204Pb and form a trend parallel to the NHRL (Fig. 3-3, note that only high-precision

MC-ICP-MS data are shown). This trend, and the fact that the mineralized samples fall

within the field of the Pacific MORB (Fig. 3-4, note that at the scale of the figure the

mineralized samples can not be distinguished from the lavas), suggest mainly oceanic

crust (or mantle) control on their Pb isotopic compositions. In addition, the relatively

non-radigenic Pb isotopic composition of the Ladolam deposit further argues against

scavenging of sedimentary Pb within the hydrothermal system (Fig. 3-3).

Hydrothermal scavenging of Pb from the oceanic crust underlying the thick layer

of sediments also must be considered as a possible process for incorporation of ore metals

in the mineralizations. The mafic xenoliths recovered from Tubaf lavas are less-

radiogenic than the lava and mineralized samples (Table 3-1, Fig. 3-4), but mixing of Pb









from the sediments and the underlying mafic crust within the hydrothermal system may

generate Pb isotopic values similar to the observed in the mineralized samples and fresh

lavas. However, a Pb isotopic study on Escanaba Trough sulfide deposits indicates that

the source for the ore metals is the thick, up to 1 km layer of sediments, without any

significant contribution from the underlying Gorda Ridge MORB (German et al. 1995).

It is hard to believe, therefore, that the mafic crust that underlies the sediments in the

TLTF area, with its low Pb content will have significant impact on the Pb isotopic

composition of hydrothermal fluids that must pass through 5 km of sediments and/or

volcanic rocks with much higher Pb content (Table 3-1). The most probable sources for

the ore metals in the giant Ladolam deposit and the Conical seamount mineralization,

therefore, are the magmas in the area.

Intriguingly, mineralized zones bracket the host lavas (Fig. 3-3), with Lihir

mineralization plotting at the low-radiogenic end, and Conical mineralization plotting at

the high-radiogenic end of the array, suggesting distinct sources for each mineralization.

In addition, both mineralizations exhibit less variation in their Pb isotopic compositions

than the Conical and Lihir host lavas, suggesting a relatively homogeneous source and/or

homogenizing process (Fig. 3-3). The Tubaf and Edison magmas, which rapidly ascended

from their source region, also show relatively homogeneous Pb compared to Conical and

Lihir lavas, indicating that in some instances single magmatic pulses in the area were

composed of relatively homogeneous Pb. The Conical high-temperature mineralized

samples have Pb isotopic compositions similar to the nearby Tubaf and Edison seamount

lavas (Fig. 3-3), suggesting that a single pulse of magma with similar isotopic

composition could be the metal source for the high-temperature mineralization.









Conical low-temperature mineralized samples containing relatively low Pb

concentrations and elevated 87Sr/86Sr, have Pb isotopic compositions scattered between

those of the host lavas and the high-temperature, ore-rich samples (Fig. 3-2). This

suggests that Pb was remobilized from both the host lavas and from the high-temperature

ore zones during the waning stages of the hydrothermal system in order to form the late-

stage low-temperature zones.

Mechanism for ore Formation

Sillitoe (1997) suggested that the gold deposits associated with alkaline rocks may

be related to formation of highly oxidized magma generated by quenching of a volatile-

rich mafic melt during injection into a shallow magma chamber. The resultant volatile

phase will be charged with ore-forming metals such as Au, Ag, Cu, Pb, Zn, and Fe.

In addition, alkaline rocks usually contain elevated amounts of sulfur and chlorine,

important ligands for hydrothermal gold and base metal transport (Candela and

Piccoli 1995). Sulfur isotopic ratios in sulfide minerals from mineralized Conical

seamount samples are consistent with mineralization from a magmatic fluid (Herzig et

al. 1999, Petersen et al. 2002). Chemical compositions of clinopyroxene and

petrological observations described above, provide evidence for the presence of a

relatively shallow magma chamber beneath Conical seamount and an episode of magma

recharge involving a mafic magma similar in composition to Tubaf and Edison lavas.

Conical and Lihir lavas are more evolved compared to Tubaf lavas, however, the latter

are more volatile-rich (Fig. 3-8). In addition to being highly enriched in chlorine, Tubaf

lavas also show about two times higher Pb content than Conical lavas (Table 3-1). The






51


8000

7000

6000 [


fract A A -
5000 -



3 0 degas A A
3000 [ a AA -
S[] AB

2000 AB A

1000 [- B

0 I I I .I I .
2 3 4 5 6 7 8

MgO (wt%)


Figure 3-8. MgO vs Cl content of seamounts and Lihir island. Arrows show general
direction of changes during magma degassing, fractionation, and fractionation
accompanied with degassing. Note the highest Cl content in Tubaf lavas.

presence of chlorine greatly increases the fluid/magma partitioning coefficients of base

metals, and fluids with 103-104 ppm base-metal concentrations can be rapidly generated

from magmas with tens of ppm base-metal concentrations (Candela 1989).

Mafic, volatile-rich magmas travel upward through subvertical cracks in brittley

fractured rocks as dikes, and in extensional tectonic settings over-pressured magmas may

ascend through such self-generated cracks for tens of kilometers, even through less dense

rocks (Best and Christensen 2001). Once such rapidly ascending magma encounters

magma stored in a shallow magma chamber, then its upward motion will be retarded due


B Conical A Tubaf
A Edison 0 Lihir


A
A
A A


. . .


-

-









to lack of significant density contrast and because the mechanism of brittle fracturing will

not operate in the fluid medium. Quenching of this volatile-rich magma that is recharging

the system will cause rapid fluid saturation and metal-bearing solutions will exsolve (note

the higher Cl and Pb content in Tubaf compared to the other lavas (Table 3-1, Fig. 3-8)

from the magma body at depth (Fig. 3-9). The ore-bearing fluids, therefore, will be

primarily derived from the mafic magmas (similar to Tubaf) recharging the system, not

from the lavas that host the mineralization close to the surface (Fig. 3-9).

A similar ore-forming mechanism can be proposed for the Ladoloam gold deposit.

Sulfur isotopic data, similar to the sulfur data for the Conical seamount hydrothermal

system, indicate mineralization from a magmatic fluid (Carman 2003). Similar to the

observed relationships between Conical ores and lavas, a small offset between the Pb

isotopic compositions of the Ladolam ores and surficial Lihir lavas is apparent (Fig. 3-3),

suggesting that the ore metals were also probably not hydrothermally leached from the

host lavas, but possibly exsolved from a recharging volatile-rich magma at depth

(Fig. 3-9).

Conclusions

High precision lead isotopic measurements conducted with MC-ICP-MS on ores,

lavas, and sedimentary and mafic xenoliths in the area of Lihir Island provide a clear

picture of the sources of metals in the Ladolam gold deposit and in mineralized zones on

Conical seamount. Although Sr isotopic data suggest some involvement of seawater,

particularly during the waning stages of the hydrothermal system, Pb isotopes suggest

that neither seawater nor the sediments in the area have significantly contributed to the

metal budget of the ore mineralizations discovered on the island and the seamount. The

ore Pb in the hydrothermal systems and presumably other ore metals with similar












TUBAF


- -


Rapid ascent
to the surface of.
xenolith bearing.
mafic magmas







", '. .' '

,; '. ;.. '' ..;


EDIS


Figure 3-9. Cartoon depicting the inferred ore formation mechanism. Evidences suggest
that a magma chamber under Conical seamount was recharged by a mafic
magma and the exsolved fluids formed the ore deposit. Pb isotopes in the ore
mineralization will inherit their signature from the recharging magma. Note
that the Tubaf magmas have the highest Pb content (Table 3-1). Barren
seamounts' magmas (Tubaf and Edison) ascended rapidly to the surface from
their mantle source region and no ore mineralization was formed. However,
during their ascent a number of xenoliths were trapped, thus providing
representative samples from the possible lithosphere sources beneath the
TLTF volcanoes.

geochemical behavior were derived from alkaline magmas in the area. The extent of

mineralization during a magmatic event can be related to the volcano-magmatic evolution

of the volcanic complex. I propose that the magmas forming the two barren Tubaf and

Edison seamounts ascended rapidly to the surface and preserved their volatile content

close to the moment of eruption and thus precluded significant exsolution of metal-

bearing fluids and consequent potential for ore formation. On the other hand, Conical

seamount, composed of similar lavas to Tubaf and Edison, shows evidences for magmatic


&
C.
8




~


C


CONICAL
ON


S., FormationT orf gold
~ : : ." : '.: : mineralization
~ Exsolution of metal-earn (Pb isotopes in the.
.Exsolution of metal-bearing o0 ores similar to the
Fluids from recharging .' recharging magma)
volatile rich mafic magma .:. .;.: '


0 00 0
0 00
"*' :*.. .'. *. : ",^' ,f ,0, 0,: ^", ., ', '
*'*" .> .'\ ', .- ',', '. '**-'^ '^ ^ ^ F r ^ ^ '. *'--, o o O -, .,.*'


U .'.









evolution at shallower levels, and thus enhanced ore-forming potential. The triggering

ore-mineralization event, however, appears to be related to a mafic, volatile-rich magma

recharging event in the evolving magma chamber beneath the seamount. The latter

hypothesis is supported by petrologic observations indicative of a complex magmatic

history including magma recharge and fluid saturation and exsolution at Conical

seamount. In addition, small but distinguishable differences between the Pb isotopic

compositions of ores and host lavas were observed, suggesting that the ore metals were

not hydrothermally leached out from the host lavas but most probably exsolved from

quenched mafic magmas injected into magma chambers existing beneath the Conical

seamount and Luise volcano.














CHAPTER 4
DECIPHERING MANTLE AND CRUSTAL CONTROLS IN AN ISLAND ARC
ENVIRONMENT: A SR, ND, AND PB ISOTOPIC STUDY OF SW PACIFIC ISLAND
ARCS AND SUB-ARC XENOLITHS.

Introduction

In the classical model for magma generation in continental and island arc settings it

is believed that magmatism results from lowering the melting point of the mantle wedge

peridotite as a result of fluid and/or melt introduction from the subducted slab (Poli

and Schmidt 2002, Stem 2002). It can be considered, then, that complex interaction

between components derived from the subducted slab, the crust underlying the island arc,

and the subarc mantle wedge play major role in arc petrogenesis. Samples from these

sources in the form of mantle and crustal xenoliths brought to the surface by arc lavas,

therefore, can provide important constrains on the composition and evolution of the

mantle wedge and the contribution of the crust to the chemistry of arc lavas. Such

samples, however, are rare and limited to only a few localities: Japan, Cascades,

Philippines, Mariana, and Kamchatka arcs (McInnes et al. 2001).

The Tabar-Lihir-Tanga-Feni (TLTF) arc, Papua New Guinea provides an

outstanding opportunity to look in detail at the components directly involved in the

petrogenesis of the island arc because numerous sedimentary, mafic, and ultramafic

xenoliths were recovered from TUBAF seamount, located on the flank of Lihir island

(Chapter 3). Detailed studies on the mineralogy and trace element geochemistry of the

xenoliths indicated that the mantle wedge and crust underlying the TLTF island chain

resembles oceanic lithosphere possibly formed at mid-ocean ridge (MOR) settings (Franz









and Wirth 2000, McInnes et al. 2001, Gregoire et al. 2001, Franz et al. 2002).

Many of the ultramafic xenoliths exhibit evidence for metasomatic alteration by hydrous

fluids, possibly released during the subduction of the Pacific plate underneath the Indo-

Australian plate (McInnes et al. 2001, Gregoire et al. 2001, Franz et al. 2002).

TLTF lavas are characterized mainly by silica undersaturated, alkaline rocks and

two mechanisms can be invoked to explain their origin: 1) the lavas can be a product of

high pressure, small degree melting of a MORB-like mantle source, or 2) the lavas can be

derived from partial melting of a metasomatized mantle wedge. Elemental and isotopic

data from the xenoliths and the lavas from the region will allow us to determine whether

the ultramafic xenoliths are the source or a component for the alkaline magmas. In

addition, the mantle xenoliths will provide information for the isotopic composition of

the mantle in the region, which will be used to constrain the origin of the isotopic

signatures in New Britain and Solomon island arcs and the Manus and Woodlark

back-arc basins.

Geological Settings

The oceanic region to the north and east of northeastern Australia includes the

island arcs of New Britain, New Ireland, and the Solomon Islands and numerous small

basins and seas (Fig. 4-1). The Tabar, Lihir, Tanga, and Feni (TLTF) island chain is

located in the former fore-arc region of New Ireland, extending for about 250 km parallel

to the presently inactive Manus-Kilinailau trench. Some studies suggest that the

Solomons, New Britain, and New Ireland arcs rifted away from the eastern Australian

margin around 40 Ma ago, in contrast to other interpretations that indicate that the arcs

originated as intra-oceanic arcs within the Pacific plate (Hall 2001 and references

therein). The formation of the arcs led to the formation of the Solomon Sea, and recovered

































Figure 4-1. Regional map of the study area, modified after Hall (2001). See chapter 3 for
more detailed map of the TLTF area.

seafloor basalts show geochemical similarity to evolved MORB from the East Pacific

Rise (Davies and Price 1987). Solomon Islands, New Britain, and New Ireland

volcanism began during the Eocene as a result of south-westerly subduction of the Pacific

plate under the Indo-Australian plate along the Manus-Kilinailau trench (Johnson 1979).

During the Oligocene New Ireland and New Britain islands formed continuous arc with

the Solomon Islands (Hall 2001). Subduction ceased about 20-10 Ma ago, when the thick

and relatively buoyant Ontong Java oceanic plateau collided with the subduction zone

(Coleman and Kroenke 1981, Petterson et al. 1997). After the collision, regional

relocation of stress caused reversal of the subduction direction and caused

Solomon plate subduction beneath New Britain to the north, and beneath Solomon

Islands to the east, along the newly formed New Britain-San Cristobal trench (Petterson









et al. 1997). New Britain began to migrate southwestward and as a result, Manus back-

arc basin opened at about 3.5 Ma (Taylor 1979). The floor of Manus basin is composed

of diverse magmatic rocks, ranging from normal MORB to BAB and lavas with arc-type

compositions (Sinton et al. 2002). Woodlark Basin spreading began about 5 to 6 Ma ago

and at present, typical MORB erupt at the spreading center, although some of the lavas

exhibit back-arc characteristics (Perfit et al. 1987, Trull et al. 1990).

The main part of the New Britain arc is composed of Eocene-Oligocene calc-

alkaline rocks, generated during the previous subduction of the Pacific plate along the

currently inactive Manus-Kilinailau trench (Woodhead and Johnson 1993). The arc

migrated southward after the opening of the Manus back-arc basin as the subduction

began along the New Britain trench. A number of Quaternary volcanic centers exposed

along the north rim of the New Britain are related to the present Solomon Sea plate

subduction and Woodhead et al. (1998) divided the volcanic centers into zones with

specific chemical characteristics related to the depth of the subducted slabs. The zones

closer to the trench show a Solomon plate slab contribution dominated by altered oceanic

crust, and the zones further away from the trench exhibit decreasing slab influence and an

increase in the Manus basin mantle component (Woodhead and Johnson 1993,

Woodhead et al. 1998).

The TLTF island groups, located about 30 to 70 km off-shore New Ireland, are equally

spaced at about 75 km apart. Volcanism in the TLTF area began on Simberi island (part

of Tabar island group) about 3.7 Ma ago (Johnson et al., 1976), coeval with the initiation

of back-arc spreading in the Manus Basin, and migrated southward to Feni island (2300 y),

as the island of New Britain was transported to the southeast. Taylor (1979) suggests









that the local extension and volcanism are related to the opening of the Manus back-arc

spreading center. Pliocene-Pleistocene TLTF islands are characterized mainly by silica

undersaturated, alkaline rocks, ranging from basanites to trachyandesites with rare

occurrences of more evolved tephriphonolites and phonolites (Johnson et al. 1976;

Wallace et al. 1983; Kennedy et al. 1990a,b; Mclnnes and Cameron 1994; Stracke and

Hegner 1998). More detailed description of the TLTF geological settings is available in

Chapter 3.

Samples and Analytical Methods

Fresh volcanic rocks used in this study were recovered from the seamounts during

the "SONNE-133" cruise in 1998. Conical seamount trachybasalts are composed of about

20 to 25% clinopyroxene, 5% plagioclase, and rare olivine and phlogopite phenocrysts

embedded in a slightly vesicular, fine-grained groundmass which consists of glass,

plagioclase, clinopyroxene, spinel, and apatite microcrysts (Muller et al. 2003).

The clinopyroxene phenocrysts are large (up to about 6 mm), euhedral to subhedral, and

exhibit distinct zoning. Plagioclase phenocrysts are much smaller, up to about 1mm long.

Tubaf and Edison seamounts' lavas are also trachybasalts and contain phenocrysts of

clinopyroxene, as well as phlogopite and amphibole embedded in a groundmass

containing about 20% vesicles and glass (Mclnnes et al. 2001). Clinopyroxene crystals

in these samples are smaller (up to 1 mm) and more homogeneous than those in Conical

lavas.

Sedimentary (limestones and carbonaceous mudstones), mafic (gabbroic and

basaltic), and ultramafic (lherzolite, harzburgite) xenoliths were recovered from Tubaf

seamount lavas and have been described in detail in several recent publications (Franz

and Wirth 2000, Mclnnes et al. 2001, Gregoire et al. 2001, Franz et al. 2002).









Sedimentary xenoliths are mainly composed of coralline and coralgal limestones,

interbeded with layered carbonaceous and volcanoclastic sediments, similar to uplifted

sedimentary sequences exposed on New Ireland, and some pelagic deep-sea sediments

(McInnes et al. 2001).

The mafic xenoliths are composed mainly of gabbros and few fine-grained basalts.

Representative gabbro samples 543E and 5631 contained uralitized pyroxene and altered

plagioclase common low-temperature alteration products that probably formed as the

oceanic lithosphere was transported away from the ridge axis (McInnes et al. 2001).

Sample 553E shows little to no alteration of the plagioclase and pristine pyroxene,

suggesting that the oceanic crust beneath the arc is not entirely altered.

The ultramafic xenoliths are course-grained spinel lherzolites composed mainly of

olivine and lesser amount of orthopyroxene (Opx) crystals, with minor clinopyroxene

(Cpx) and spinel. In some cases deformed exsolution lamellae in pyroxenes and kink-

banded olivines are present, suggesting a period of ductile deformation (McInnes et al.

2001). Peridotite samples 542D, 562B, 562P, 542S, and 542K contain numerous vein and

veinlets composed mainly of fibrous Opx, some amphibole, phlogopite, spinel, and Fe-Ni

sulfides. These vein components are secondary in origin and were formed as a result of a

release of metasomatic agents from the subducted Pacific oceanic crust underneath the

TLTF chain (McInnes et al. 2001, Gregoire et al. 2001, Franz et al. 2002). Samples

562F, 542G, and 552H are spinel harzburgites that do not contain vein assemblages,

although 542G and 552H show some areas of irregular metasomatic alteration.

Clinopyroxene separates 611B Cpx, 611C Cpx, and 671 Cpx are from ultramafic

xenoliths collected from Tubaf seamount during the 1994 SONNE cruise (B. McInnes,









pers. comm.). The clinopyroxens were separated from lherzolites, discussed in detail by

McInnes et al. (2001) and Gregoire et al. (2001).

For this study, xenolith samples were separated from the host lavas using a

microdrill being particularly careful to avoid any contamination by the host lavas (R.

Chopra, pers. comm.). Before isotope analyses fresh lava, mafic and ultramafic xenolith

samples were leached for several hours in warm 2N HC1. Then, the residues were rinsed

several times with 4xH20 and were dissolved in sealed Teflon vials for several days at

100C in a HF-HNO3 mixture (see also Chapter 3). Major and trace element

concentrations Table 1) were determined by ICP-AES and ICP-MS at the Geological

Survey of Canada (http://www.nrcan.gc.ca/gsc/mrd/labs/chem-e.html).

Radiogenic isotopic analyses were performed at the Department of Geological

Sciences at the University of Florida. Sr, Pb, and Nd were separated using standard

chromatographic methods used in our lab (Heatherington and Mueller 1999). Procedural

blanks determined several times during sample preparation were maximum 70 ppt for Sr,

80 ppt for Pb, and 30 ppt for Nd. Sr isotope measurements were collected using a

Micromass Sector 54 Thermal Ionization Mass Spectrometer equipped with seven

Faraday collectors and one Daly collector. Sr samples were loaded on oxidized W single

filaments and run in dynamic collection mode. Data were acquired at a beam intensity of

1.5V for 88Sr, with corrections for instrumental discrimination made assuming

86Sr/"Sr=0.1194. Errors in measured 87Sr/86Sr are better than +/- 0.00002 (2c) based on

long-term reproducibility of NBS 987 (87Sr/86Sr=0.71024). Nd isotopic analyses were

performed on a Nu Plasma multiple-collector magnetic-sector inductively coupled mass

spectrometer (MC-ICP-MS). Samples and standard solutions were aspirated into the









plasma source either via a Micromist nebulizer with GE spray chamber (wet plasma) or

through DSN-100 desolvating nebuliser (dry plasma). The instrument settings were

carefully tuned to maximize the signal intensities on a daily basis. Preamplifier gain

calibration was performed before each analytical session. Nd isotope measurements were

conducted for 60 ratios in static mode simultaneously acquiring 142Nd on low-2, 143Nd on

low-1, 144Nd on Axial, 145Nd on high-1, 146Nd on high-2, 147Sm on high-3, 148Nd on high-

4, and 150Nd on high-5 Faraday detectors. The measured 144Nd, 148Nd, and 150Nd beams

were corrected for isobaric interference from Sm using 147Sm/144Sm = 4.88, 147Sm/148Sm

= 1.33, and 147Sm/150Sm = 2.03. All measured ratios were normalized to 146Nd/144Nd =

0.7219 using an exponential law for mass-bias correction. The mean value of 143Nd/144Nd

for our Ames Nd in-house standard based on 23 repeat analyses during the samples

analyses was 0.512140 (2c = 0.000012). Three repeated analyses of the JNdi-1 and

LaJolla Nd standards during the same time interval produced mean values of 0.512106

(25 = 0.000013) and 0.511856 (25 = 0.000013), respectively. Three separate dissolutions

of USGS SRM BCR-1 were prepared and analyzed for Nd isotopes together with the

samples in order to further evaluate the analytical protocol. The mean value of

143Nd/144Nd for the analyses of BCR-1 was 0.512645 (2c = 0.000011), which is

indistinguishable from the published TIMS value of 0.51264 (Gladney et al. 1990).

Pb isotopic analyses were conducted on a Nu Plasma multi collector ICP-MS using

the TI normalization technique on fresh mixtures to prevent oxidation of thallium to T13+

(for more details see chapter 2 and 3). Analyses of NBS 981 conducted in wet plasma

mode during the period of time the samples were analyzed gave the following results

(n=42): 206Pb/204Pb=16.937 (+/-0.004 2y), 207Pb/204Pb= 15.490 (+/-0.003 2y), and









208Pb/204Pb=36.695 (+/-0.009 2y). Due to their low Pb content the ultramafic xenoliths

were analyzed in dry plasma mode and the reported data are relative to the following

NBS 981 values (n=29): 206Pb/204Pb=16.937 (+/-0.001, 2y), 207Pb/204Pb=15.491

(+/-0.001, 2y), and 208Pb/204Pb=36.694 (+/-0.004, 2y)

Results

Major and trace element concentrations of lava, xenolith, and sediment samples

from the study area are shown in Table 4-1. Lihir and the nearby seamount lavas vary

from subalkaline basalts to trachyandesites, with the majority of the samples plotting in

the field of trachybasalts on an alkali-silica diagram (Fig. 4-2), similar to the lavas found

on the other TLTF islands (Wallace et al. 1983; Kennedy et al. 1990a,b; McInnes and

Cameron 1994; Stracke and Hegner 1998). Tubaf and Edison lavas are characteristically

more primitive, whereas Lihir lavas vary from subalkaline basalts to trachyandesites and

include a few phono-tephrites. The seamount lavas have more restricted MgO content,

ranging between about 8 and 4 wt % in comparison to Lihir lavas, which have quite

variable MgO content, ranging from as high as about 8 wt% to as low as about 1 wt%

(Table 4-1, Fig. 4-3). Both, the seamounts and the island exhibit decreasing CaO and

Fe203 and increasing SiO2, A1203, Na20 and K20 with the decrease in the MgO (Fig. 4-3).

TiO2 data, on the other hand, exhibit large scatter and do not show clear correlation

with MgO variation. Compatible trace elements, such as V, Ni, and Cr have relatively

moderate concentrations and show an overall decrease with decreasing MgO

concentration (Fig. 4-4). Incompatible trace elements, such as Ba, Rb, Cs, Nb, and La

show increase with decreasing MgO (Fig. 4-4), although most of the elements exhibit

large scatter when plotted on correlation diagrams.








64



Table 4-1. Major and trace element data for lavas and xenoliths

Location: Conical Conical Conical Conical Conical Conical Conical
Sample: 7DR 12DR 2A 13DR 4 36GTVA 1 42GTVA-1 42GTVA-3 50DR 1C
Type: lava Lava lava lava lava lava lava
Si02 47.2 48.5 47.6 50.2 51.6 48.7 49
TiO2 0.69 0.75 0.72 0.77 0.84 0.77 0.74
Al203 19.3 15.4 18.4 16.5 16.6 16.1 15.4
Fe203T 9.6 10.1 9.8 10.2 8.45 10.3 10.5
MnO 0.18 0.19 0.2 0.19 0.14 1.08 0.19
MgO 5.99 6.36 5.84 5.68 4.2 3.83 6.49
CaO 10.6 11.4 10.3 11 10.3 9.5 11.7
Na20 2.35 2.66 2.94 3.03 3.67 3.41 2.64
K20 2.89 2.71 3.02 2.94 2.45 2.81 2.84
P205 0.31 0.32 0.42 0.36 0.52 0.52 0.36
Total 99.3 98.6 99.5 101.1 99.2 97.4 100.1
LOI 1.3 1.2 1.1 1.3 3.3 3.6 0.9
Ba 202 221 234 245 294 279 210
Be 1 0.9 1.2 1.1 0.9 1.3 1.1
Co 28 30 27 30 27 73 31
Cr 141 124 97 90 98 89 157
Cs 0.77 0.77 0.84 0.85 1.2 0.89 0.82
Cu 84 117 116 107 148 246 102
Hf 1.6 1.7 2.4 1.7 1.5 1.5 1.6
Nb 1.3 1.4 1.6 1.6 1.8 1.8 1.3
Ni 29 30 30 25 22 35 31
Pb 4 4 6 5 10 10 4
Rb 57 56 57 63 32 56 63
Sc 38 40 33 36 30 27 40
Sr 869 936 1195 1011 1180 1100 915
Ta 0.08 0.09 0.1 0.11 0.09 0.09 0.08
Th 0.82 0.75 1.2 0.91 1 0.97 0.84
U 0.52 0.51 0.85 0.64 1.7 1.5 0.58
V 279 302 297 307 247 239 290
Zn 77 82 85 86 75 111 82
Zr 54 57 92 63 55 55 60
Ce 19 20 34 23 23 21 20
Dy 2.8 2.9 3.8 3.2 3 2.9 3.1
Er 1.5 1.5 1.7 1.6 1.6 1.5 1.6
Eu 1.2 1.2 1.6 1.2 1.3 1.2 1.2
Gd 3.6 3.7 4.7 3.8 3.8 3.5 3.9
Ho 0.57 0.58 0.67 0.63 0.6 0.56 0.62
La 8.7 8.6 14 9.8 11 10 8.9
Lu 0.25 0.24 0.29 0.26 0.26 0.25 0.25
Nd 13 14 20 15 14 14 14
Pr 2.8 2.9 4.3 3.2 3.2 3.1 2.9
Sm 3.5 3.6 4.9 3.9 3.7 3.4 3.7
Tb 0.5 0.54 0.66 0.56 0.52 0.49 0.54
Tm 0.22 0.23 0.26 0.25 0.24 0.22 0.23
Y 17 17 20 18 17 17 17
Yb 1.5 1.6 1.8 1.7 1.6 1.6 1.6












Table 4-1.


Continued


Location: Conical Edison Edison Edison Edison Edison Edison
10GTVA 10GTVA 10GTVA 11GTVA 11GTVA 33GTVA
Sample: 52GTVA 1 5A2 6B1 6B2 2A1 2A2 2J2
Type: lava lava lava lava lava lava lava
SiO2 48.4 45.7 46.7 47 47.1 47.1 46.5
TiO2 0.77 0.79 0.78 0.78 0.8 0.82 0.79
Al203 16.4 14.2 16.9 14 14 14 15
Fe203T 8.98 11.2 11 11.3 11.3 11.3 11.3
MnO 0.16 0.19 0.19 0.19 0.18 0.19 0.19
MgO 5.14 7.5 7.41 7.09 6.83 7.36 7.3
CaO 11.3 11.9 11.5 11.7 12 12.1 12
Na20 2.37 2.22 2.28 2.3 2.13 2.25 2.46
K20 2.88 2.31 3.38 2.01 2.17 1.98 2.39
P205 0.31 0.44 0.44 0.44 0.43 0.43 0.47
Total 96.9 96.7 100.8 97 97.2 97.8 98.6
LOI 3.1 2.2 1.5 2.2 2.6 2.5 1.1
Ba 230 242 250 280 246 252 245
Be 1 0.9 1.1 1 0.9 0.9 1.1
Co 24 35 34 34 32 33 34
Cr 89 98 99 115 110 127 107
Cs 0.64 0.61 0.68 0.65 0.62 0.64 0.74
Cu 108 118 139 127 107 111 141
Hf 1.7 1.9 1.7 1.9 1.9 1.8 1.9













Table 4-1. Continued

Location: Edison Edison Tubaf Tubaf Tubaf Tubaf Tubaf
54GTVA 54GTVA 55GTVA
Sample: 34GTVA 3.1 34GTVA 3.2 2R1 2R2 54GTVA 4.1 54GTVA 4.2 5F1
Type: lava lava lava lava lava lava lava
Si02 45.9 45.6 47.1 47.8 47.1 46.4 47.1
TiO2 0.78 0.78 0.75 0.8 0.76 0.75 0.76
Al203 14.1 14.6 16.2 16.2 16.9 17.7 16.3
Fe203T 11 10.8 10.1 10.6 10.1 10.2 10.4
MnO 0.19 0.18 0.21 0.2 0.21 0.2 0.2
MgO 7.16 7.11 7.24 7.64 7.24 7.16 7.8
CaO 11.7 12.2 9.22 9.19 9.07 9.04 9.01
Na20 2.29 2.38 3.31 3.34 3.3 3.19 3.3
K20 2.35 2.06 2 2.16 2.18 2.1 2.05
P205 0.44 0.48 0.54 0.34 0.46 0.43 0.47
Total 96.1 96.4 97 98.6 97.6 97.5 97.7
LOI 2.9 3.1 2.2 2 1.6 1.8 2
Ba 245 239 233 238 245 230 240
Be 1 0.9 1.9 1.9 1.9 2 1.9
Co 32 31 31 32 30 32 31
Cr 125 107 140 154 159 180 145
Cs 0.67 0.66 0.53 0.51 0.56 0.55 0.37
Cu 126 124 69 66 82 86 89
Hf 1.8 1.9 2.9 3 2.8 3 2.9
Nb 1.5 1.5 2.8 2.9 2.8 2.8 2.8
Ni 48 45 87 99 89 105 84
Pb 5 5 11 10 10 10 10
Rb 30 28 20 21 23 22 23
Sc 42 43 29 30 29 30 29
Sr 1161 1169 1920 1942 1901 1927 1961
Ta 0.09 0.08 0.16 0.15 0.16 0.16 0.16
Th 1.1 1.1 2.1 1.9 2.1 2.1 2
U 0.75 0.69 1.4 1.4 1.4 1.5 1.3
V 307 299 297 307 297 302 294
Zn 83 80 89 90 89 91 87
Zr 63 65 103 101 100 100 100
Ce 26 27 42 39 39 42 39













Table 4-1. Continued

Location: Tubaf
55GTVA
Sample: 5F2
Type: lava

Si02 47.9
TiO2 0.78
Al203 16.4
Fe203T 10.5
MnO 0.2
MgO 7.37
CaO 9.42
Na20 3.37
K20 2.13
P205 0.49
Total 98.9
LOI 1.7
Ba 240
Be 2
Co 31
Cr 135
Cs 0.49
Cu 92
Hf 3.2
Nb 2.9
Ni 85
Pb 11
Rb 20
Sc 29
Sr 1962
Ta 0.18
Th 2.2
U 1.5
V 300
Zn 89
Zr 107
Ce 43
Dy 4.5
Er 2.2
Eu 2
Gd 6.2
Ho 0.86
La 20
Lu 0.37
Nd 26
Pr 5.9
Sm 6.4
Tb 0.84
Tm 0.34
Y 24
Yb 2.3


Tubaf


56GTVA 2C
lava

49.5
0.76
16.3
10.4
0.21
7.77
9.2
3.5
2.6
0.45
101
1.5
247
2
31
166
0.68
79
3.2
2.8
109
10
29
29
1963
0.15
2.2
1.5
297
93
103
43
4.4
2.3
2
6
0.85
20
0.35
25
5.8
6.2
0.83
0.32
24
2.2


Tubaf Tubaf Tubaf


Tubaf
56GTVA
3F1
lava

47.7
0.79
16
10.5
0.2
7.35
9.23
3.42
2.04
0.43
98
1.8
229
2
30
147
0.66
125
3.2
2.8
92
10
24
30
1874
0.18
2.2
1.5
301
92
103
42
4.4
2.3
2
6
0.87
20
0.38
26
5.8
6.2
0.83
0.33
26
2.3


Tubaf
56GTVA
3F2
lava

47
0.74
16
10.1
0.19
7.63
9.13
3.33
2.08
0.43
97
2.3
232
2
32
175
0.63
142
3.2
2.9
103
11
22
30
1959
0.18
2.3
1.6
298
93
102
43
4.5
2.2
2
6.1
0.84
20
0.36
26
6
6.3
0.82
0.34
25
2.3


54GTVA2D
lherzolite

41.40
0.03
0.90
11.80
0.17
39.00
5.20
0.10
0.05
bdl
98.1
0.0
399
bdl
131
3890
0.04
20
0.17
0.08
2050
19
0.52
18.0
bdl
bdl
0.02
0.03
69
68
6.8
0.4
0.15
0.09
0.03
0.10
0.03
bdl
bdl
0.2
0.03
0.06
bdl
bdl
0.83
0.09


54GTVA2G
harzburgite

37.30
bdl
2.30
9.50
0.13
48.70
0.38
bdl
bdl
bdl
98.2
bdl
bdl
bdl
145
4400
bdl
11
0.55
0.10
2940
2
0.13
4.4
bdl
0.03
0.03
0.02
39
51
22.0
1.7
0.15
0.03
bdl
bdl
bdl
bdl
bdl
bdl
0.02
0.02
bdl
bdl
0.38
0.05


54GTVA2J
lherzolite

38.60
0.02
1.70
11.10
0.16
47.20
2.10
0.04
bdl
bdl
100.8
bdl
bdl
bdl
170
7410
bdl
16
0.18
0.10
2120
3
0.21
12.0
10
0.04
0.03
0.04
58
57
6.6
0.4
0.07
0.05
0.02
0.07
bdl
0.2
bdl
0.2
0.06
0.06
bdl
bdl
0.49
0.06








68



Table 4-1. Continued

Location: Tubaf Tubaf Tubaf Tubaf Tubaf Tubaf Tubaf
Sample: 54GTVA2L 55GTVA2A 55GTVA2D 55GTVA2F 55GTVA2H 56GTVA2A 56GTVA2B
Type: lherzolite lherzolite lherzolite lherzolite harzburgite lherzolite lherzolite
Si02 42.00 42.60 42.70 41.50 42.60 43.80 39.50
TiO2 bdl bdl bdl 0.02 bdl bdl 0.02
Al203 2.10 1.20 1.90 4.90 3.50 1.00 1.50
Fe203T 9.14 9.10 8.45 8.16 9.12 8.83 12.00
MnO 0.13 0.13 0.13 0.12 0.14 0.13 0.17
MgO 46.40 45.70 44.40 42.80 43.60 46.20 46.00
CaO 0.75 0.72 0.72 0.97 0.79 1.02 1.59
Na20 bdl 0.03 bdl 0.09 bdl 0.11 0.06
K20 bdl bdl bdl 0.09 bdl bdl 0.05
P205 0.01 bdl bdl bdl bdl bdl bdl
Total 100.2 99.4 98.1 98.4 99.4 100.9 100.4
LOI bdl bdl 0.1 bdl bdl bdl bdl
Ba bdl 60 23 bdl bdl bdl 159
Be bdl bdl bdl bdl bdl bdl bdl
Co 130 124 116 114 120 125 167
Cr 2210 2870 3000 3100 2730 2850 4970
Cs bdl bdl bdl 0.04 bdl 0.06 0.07
Cu 14 23 14 12 10 15 17
Hf 0.46 0.21 0.37 0.71 0.57 0.14 0.94
Nb 0.07 bdl bdl 0.17 0.07 0.07 0.12
Ni 2710 3930 2510 2380 2480 3210 2040
Pb 1 bdl bdl bdl bdl bdl bdl
Rb 0.14 0.07 0.10 1.10 0.25 0.34 1.00
Sc 9.1 9.0 10.0 10.0 10.0 12.0 8.4
Sr bdl bdl bdl 48 bdl bdl bdl
Ta bdl bdl bdl 0.03 bdl 0.04 bdl
Th 0.03 bdl 0.02 0.08 0.03 bdl 0.17
U 0.02 bdl 0.02 0.06 0.03 bdl 0.05
V 45 45 50 52 43 54 52
Zn 47 43 43 45 41 40 66
Zr 19.0 9.2 16.0 30.0 25.0 5.4 43.0
Ce 0.1 0.7 1.1 3.9 2.3 bdl 5.0
Dy 0.03 0.08 0.10 0.35 0.19 bdl 0.38
Er 0.03 0.02 0.02 0.08 0.02 bdl 0.05
Eu bdl bdl bdl 0.06 bdl bdl bdl
Gd bdl bdl bdl 0.16 0.02 bdl 0.08
Ho bdl bdl bdl 0.04 bdl bdl 0.02
La bdl bdl bdl 0.6 0.1 bdl 0.5
Lu bdl bdl bdl 0.02 bdl bdl bdl
Nd bdl bdl bdl 0.7 0.1 bdl 0.4
Pr 0.02 0.03 bdl 0.18 0.04 bdl 0.12
Sm bdl 0.03 bdl 0.19 0.04 bdl 0.06
Tb bdl bdl bdl 0.03 bdl bdl bdl
Tm bdl bdl bdl bdl bdl bdl bdl
Y 0.22 0.27 0.24 1.10 0.48 0.12 0.96
Yb 0.05 0.04 0.04 0.10 0.05 0.03 0.06













Table 4-1.

Location:
Sample:
Type:

Si02
TiO2
A1203
FeAO3T
MnO
MgO
CaO
Na20
K20
P205
Total
LOI
Ba
Be
Co
Cr
Cs
Cu
Hf
Nb
Ni
Pb
Rb
Sc
Sr
Ta
Th
U
V
Zn
Zr
Ce
Dy
Er
Eu
Gd
Ho
La
Lu
Nd
Pr
Sm
Tb
Tm
Y
Yb


Continued

Tubaf
56GTVA2T
lherzolite

47.10
bdl
3.10
7.94
0.14
41.90
0.87
0.05
bdl
bdl
101.0
0.1
bdl
bdl
101
4350
0.03
14
0.47
0.08
1430
bdl
0.34
10.0
bdl
bdl
0.03
bdl
46
44
21.0
2.1
0.19
0.04
bdl
0.04
bdl
bdl
bdl
bdl
0.03
0.03
bdl
bdl
0.55
0.06


Tubaf
56GTVA2P
lherzolite

41.40
bdl
2.40
8.84
0.13
47.20
0.79
0.03
0.05
bdl
100.6
bdl
bdl
bdl
129
3030
0.06
19
0.58
0.06
3090
3
0.50
9.5
bdl
bdl
0.05
0.02
45
42
27.0
0.1
0.03
0.03
bdl
0.02
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
0.23
0.05


Tubaf
56GTVA2G
dunite

40.30
bdl
1.70
11.70
0.17
45.90
1.00
0.07
0.06
bdl
100.0
bdl
bdl
bdl
165
682
0.08
15
0.70
bdl
1870
bdl
0.88
7.9
bdl
bdl
0.04
0.02
29
55
32.0
3.7
0.33
0.05
bdl
0.05
0.02
0.1
bdl
0.1
0.03
0.04
bdl
bdl
0.82
0.06


Tubaf
56GTVA2H
dunite

43.40
bdl
1.10
8.72
0.13
45.70
0.84
bdl
bdl
bdl
99.7
0.2
bdl
bdl
128
3200
bdl
bdl
0.18
bdl
2650
bdl
0.15
11.0
bdl
bdl
bdl
bdl
53
45
7.3
bdl
bdl
0.02
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
0.14
0.04


Tubaf
56GTVA2X
dunite

40.60
0.03
2.30
7.53
0.11
44.80
2.25
0.19
0.38
bdl
99.1
0.0
90
bdl
120
4250
0.35
71
0.23
0.08
4060
1
11.00
11.0
24
bdl
0.10
0.03
50
38
9.0
0.9
0.24
0.12
0.07
0.21
0.05
0.3
0.02
0.4
0.11
0.18
0.04
bdl
1.30
0.14


Tubaf
56GTVA2F
dunite

38.80
bdl
1.60
10.10
0.14
48.00
0.32
bdl
bdl
bdl
98.6
bdl
bdl
bdl
153
3460
0.02
bdl
0.17
0.06
2510
8
0.14
5.0
bdl
bdl
bdl
bdl
32
46
7.3
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
bdl
0.12
0.03


Tubaf
54GTVA3A
gabbro

57.70
1.76
15.50
8.89
0.10
2.75
6.70
5.17
0.30
0.13
98.5
0.2
bdl
1.0
17
bdl
0.10
bdl
6.50
3.00
bdl
bdl
3.20
28.0
142
0.21
0.32
0.17
161
73
242.0
23.0
12.00
6.60
2.10
9.60
2.40
6.5
1.10
21.0
3.90
7.20
1.70
1.10
72.00
7.20








70



Table 4-1. Continued

Location: Tubaf Tubaf Tubaf Tubaf Tubaf Tubaf Tubaf
Sample: 54GTVA3D 54GTVA3E 54GTVA3I 55GTVA3E 55GTVA3F 56GTVA3I 56GTVA3H
Type: gabbro gabbro gabbro gabbro gabbro gabbro gabbro
Si02 57.30 50.50 52.60 42.60 42.50 50.10 47.00
TiO2 1.98 1.44 2.20 2.38 2.14 0.86 0.43
Al203 16.00 15.70 13.50 15.80 15.30 16.60 16.40
Fe203T 9.90 10.80 15.70 17.40 17.30 11.20 8.12
MnO 0.14 0.17 0.16 0.19 0.20 0.21 0.14
MgO 2.69 6.39 5.26 7.57 8.83 8.00 9.29
CaO 5.55 10.80 4.85 12.60 11.80 11.80 14.40
Na20 4.73 2.93 4.06 1.58 1.65 2.18 1.91
K20 0.69 0.30 0.36 0.11 0.30 0.09 1.56
P205 0.07 bdl 0.09 bdl bdl bdl 0.01
Total 98.4 98.2 98.0 99.4 99.1 100.2 98.8
LOI 0.9 0.3 1.2 bdl 0.4 0.1 0.3
Ba 29 36 93 bdl bdl 21 38
Be 1.1 0.5 0.9 bdl bdl bdl bdl
Co 24 38 40 56 66 40 42
Cr bdl 112 11 12 200 253 135
Cs 0.16 0.07 1.40 0.04 0.35 0.02 0.75
Cu 11 29 bdl 80 82 36 66
Hf 5.50 2.50 4.70 0.93 0.62 0.98 0.32
Nb 3.00 1.90 2.30 0.40 0.21 0.31 0.13
Ni 40 51 24 68 129 61 85
Pb bdl bdl bdl 5 4 bdl bdl
Rb 6.60 2.80 13.00 1.00 8.50 0.57 30.00
Sc 29.0 41.0 35.0 50.0 52.0 49.0 52.0
Sr 148 120 102 161 106 130 316
Ta 0.25 0.15 0.19 0.04 0.04 0.04 0.02
Th 0.30 0.12 0.20 0.07 0.10 0.03 0.05
U 0.18 0.10 0.14 0.06 0.07 0.03 0.06
V 169 330 382 1030 1120 301 299
Zn 60 105 60 128 121 123 122
Zr 207.0 81.0 169.0 31.0 22.0 35.0 9.2
Ce 17.0 14.0 21.0 4.6 3.9 5.7 1.7
Dy 11.00 7.70 9.50 2.80 1.80 3.70 1.50
Er 6.50 4.30 5.30 1.50 0.99 2.20 0.90
Eu 2.20 1.40 1.90 0.67 0.42 0.97 0.43
Gd 9.20 6.10 8.00 2.30 1.30 3.00 1.20
Ho 2.40 1.60 2.00 0.56 0.36 0.79 0.33
La 4.3 3.7 6.3 1.2 1.2 1.1 0.7
Lu 1.10 0.70 0.87 0.23 0.16 0.36 0.13
Nd 19.0 14.0 18.0 3.8 1.9 4.8 1.7
Pr 3.30 2.50 3.50 0.67 0.41 0.76 0.28
Sm 6.60 4.50 5.90 1.40 0.78 1.90 0.77
Tb 1.60 1.10 1.40 0.40 0.25 0.56 0.24
Tm 1.00 0.71 0.85 0.23 0.16 0.34 0.12
Y 67.00 49.00 57.00 16.00 11.00 22.00 8.40
Yb 7.00 4.70 5.70 1.50 1.00 2.20 0.80













Table 4-1. Continued

Location: Tubaf Tubaf Tubaf Tubaf Tubaf Tubaf Tubaf
Sample: 56GTVA30 56GTVA4D 56GTVA.4E 56GTVA.4F 54GTVA.6F 54GTVA.6B 54GTVA.6E
Type: basaltic sedimentary sedimentary sedimentary sedimentary sedimentary sedimentary
Si02 51.00 42.20 46.50 15.30 48.40 41.30 32.70
TiO2 2.56 0.19 0.51 0.12 0.68 0.43 0.34
Al203 14.10 10.90 13.90 4.20 16.00 9.80 6.70
FeAO3T 15.00 2.34 6.29 1.21 5.80 4.31 3.31
MnO 0.28 0.07 0.32 0.04 0.12 0.12 0.11
MgO 4.89 2.14 2.89 0.76 2.25 1.60 1.15
CaO 9.14 22.30 17.40 43.10 13.10 23.50 29.70
Na20O 3.52 2.34 2.21 0.67 2.82 1.77 1.26
K20 0.12 1.02 0.64 0.33 2.27 0.78 0.74
P205 0.20 0.12 0.15 0.12 0.08 0.16 0.11
Total 100.9 83.8 90.9 66.0 91.6 83.9 76.3
LOI bdl 16.1 9.9 32.3 7.5 15.9 22.7
Ba bdl 163 92 64 212 70 201
Be 0.5 0.5 0.5 bdl 0.5 bdl bdl
Co 34 bdl 11 bdl 12 6 7
Cr 37 16 19 12 24 30 22
Cs 0.04 0.69 10.00 0.52 4.00 0.57 0.44
Cu 47 96 223 25 123 38 49
Hf 2.20 1.70 1.40 0.70 1.30 1.50 1.10
Nb 3.40 2.50 1.40 0.99 1.30 1.50 1.20
Ni 15 13 23 bdl 18 12 11
Pb bdl 5 3 2 4 2 2
Rb 1.00 15.00 42.00 7.70 45.00 15.00 13.00
Sc 36.0 3.9 18.0 2.1 24.0 12.0 9.0
Sr 144 1221 518 911 537 892 818
Ta 0.25 0.16 0.07 0.06 0.09 0.10 0.07
Th 0.05 1.20 0.59 0.48 0.37 0.52 0.40
U 0.04 0.66 0.68 0.65 0.79 1.00 1.40
V 454 28 114 23 165 83 74
Zn 134 65 81 68 80 67 68
Zr 81.0 64.0 51.0 28.0 46.0 61.0 42.0
Ce 14.0 24.0 13.0 7.5 7.5 17.0 8.3
Dy 9.30 1.50 3.00 1.10 2.60 2.50 2.00
Er 5.40 0.86 1.90 0.69 1.60 1.40 1.20
Eu 2.40 0.54 0.80 0.28 0.63 0.72 0.41
Gd 8.20 1.80 2.90 1.10 2.30 2.60 1.80
Ho 2.00 0.31 0.65 0.24 0.56 0.52 0.44
La 3.3 13.0 7.2 5.9 4.0 9.2 5.2
Lu 0.88 0.18 0.32 0.12 0.25 0.25 0.22
Nd 15.0 12.0 9.6 5.4 5.9 11.0 5.9
Pr 2.40 3.30 2.00 1.30 1.20 2.50 1.30
Sm 5.90 2.00 2.40 1.00 1.80 2.50 1.40
Tb 1.50 0.26 0.47 0.16 0.41 0.43 0.30
Tm 0.81 0.14 0.30 0.11 0.26 0.22 0.20
Y 53.00 12.00 22.00 11.00 16.00 17.00 14.00
Yb 5.40 0.96 1.90 0.72 1.60 1.50 1.30















Tephri-phonolite


Phono-tephrite


Trachybsit a -

-A~m


S/


Trachyandesite


Basaltic


I racnyan


aesne -


d e t 1 Andesite


S- Mafic Xenoiths -




- Basalt


-

0 Conical
Basaltic
A Edison
Andesite N Ediso
Tubaf
+ Lihir


SiO2 (wt.%)





Figure 4-2. Classification of the Conical, Edison, Tubaf, and Lihir lavas based on their
SiO2 vs Na20+K20 concentrations (after LeMaitre, 1989). Data from this
study and from Kennedy et al. (1990a), Stracke and Hegner (1998), and
Muller et al. (2001). The field for the mafic xenoliths is based on the several
analyses presented in Table 4-1. Note that although the mafic xenoliths show
relatively wide range in their SiO2 content, they plot at lower alkali contents
relative to the TLTF lavas.


. . . . .


. . . . . . . .


AP



















S10
0
0) 9
L
8

7


16


14 -


12

S10

0 8
o


0 ,0
0 8Y







0 w

1 2 3 4 5 6 7 8 9

MgO (wt.%)


22


20


18


160


14


IIIIIIIIIIIIIIIIIIIIII.IIIIII1 5


3.5
0
32


2.5


2


2 3 4 5 6 7 8


MgO (wt%)


Figure 4-3. Comparison of MgO vs CaO, Fe203, Na20, and A1203 between Lihir and
seamount lavas. Note that CaO and Fe203 decrease, and Na20, and A1203
increase with the decrease in the MgO, suggesting that lavas were affected by
fractionation processes (for more discussion see the text). Data sources and
symbols are the same as in Figure 4-2.


0-

0 A0

0 0 to

0

0 0


0

a--
Go O2
+< *








-~ ~ 0A-
;Isl l
gAA


0


. i












. I
.: .



















- '5


''1''"1 'n" .1'"'"'1



S00


0
<* I


I I I I I I I I I .I I .III











0 0



W.. ..I....I....I, ,

1 2 3 4 5 6 7 8 9
MgO (wt%)


. I . . I . . I . I. I .


2 3 4 5 6 7 8 9
MgO (wt%)


Figure 4-4. Comparison of MgO vs Ni, V, Ba, and Rb between Lihir and seamount lavas.
Ni and V behave compatible and decrease with the decrease in MgO, and Ba
and Rb behave incompatible and increase with the decrease in MgO, both
trends indicating fractination (see the text for more discussion). Data sources
and symbols are the same as in Figure 4-2.


0 0
CDo



OP'


o Q

O 0
0
0


~ Fs




+ + 0 A

. I 0


120


100


80


60


40


20


600


500


400 1


300


200


100










100
TLTF lavas Mafic
10. xenoliths
10 -



1
2542D
562B

S 0.1



0.01

Ultramafic
xenoliths
0.001
La Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu


Figure 4-5. Normalized REE patterns of TLTF lavas and mafic and ultramafic xenoliths.
TLTF data from this study and from Stracke and Hegner (1998). Ultramafic
xenoliths data from Gregoire et al. (2001) and Franz et al. (2002).

Primitive mantle-normalized rare-earth elements (REE) patterns for the seamounts

and TLTF islands are subparallel and show enrichments in light rare-earth elements

(LREE) relative to heavy rare-earth elements (HREE) (Fig. 4-5). Similarly, the primitive

mantle-normalized trace element variations (spider diagram) for the seamount lavas

follow the same pattern as the Lihir and the other TLTF island lavas (Fig. 4-6). They are

characterized by negative Nb and Ti anomalies, and positive U, K, Pb, and Sr anomalies,

typical for island arc volcanics (Perfit et al. 1980).












1000


100





10





1


, TLTF lavas


-4


Mafic xenoliths


0.1
Cs Rb Ba Th U Nb K La Ce Pb Sr Nd Sm Hf Zr Ti Eu Gd Dy Y Er Yb Lu

Figure 4-6. Primitive Mantle normalized trace element patterns of TLTF lavas and mafic xenoliths. Primitive Mantle values from Sun
and McDonough (1989).


, A."









The mafic xenoliths contain 42.5 to 57.7 wt % Si02 (i.e., from gabbro to diorite,

Table 4-1, Fig. 4-2). A few plagiogranite xenoliths also have previously been recovered

(McInnes et al., 2001). Overall, the mafic xenoliths are subalkaline with low total alkalis

relative to the Lihir and seamount lavas (Fig. 4-2). They exhibit depleted LREE patterns

(Fig. 4-5) and enrichments in some fluid-mobile elements, such as Ba, K, U, and Pb

(Fig. 4-6).

Ultramafic xenoliths have high MgO content, ranging from 26% to 48.7% and low

contents of CaO, Na20, A1203 (Table 4-1). They are also enriched in compatible

elements, such as Ni and Cr, and depleted in incompatible elements, most below

detection limits (Table 4-1).

Isotopic analyses of lavas, xenolith, and sediment samples from the study area are

presented in Table 4-2 and also in Kamenov et al. (2004b) (Chapter 3). Sr isotopic

compositions of fresh lavas from the region, including TLTF, New Britain, and Solomon

Islands and Manus and Woodlark Basins, show relatively small 87Sr/86Sr variations with

values close to 0.704 (Table 4-2), similar to previously published data (Wallace et al.

1983; Johnson et al. 1988; Trull et al. 1990; Kennedy et al. 1990b; Stracke and Hegner

1998; Woodhead et al. 1993, 1998). Measured 87Sr/86Sr in sedimentary

xenoliths recovered from Tubaf seamount lavas have more radiogenic 87Sr/86Sr values,

ranging from 0.7067 to 0.7083, compared to the lavas in the area (Table 4-2). Sr isotopic

compositions of the analyzed ultramafic xenoliths exhibit a wider range than the TLTF

lavas, ranging from 0.7037 to 0.7059 (Table 4-2).








78



Table 4-2. Pb, Sr, and Nd isotopic analyses of lava and xenolith samples


Location Type sample 206Pb/204Pb 207 Pb/204Pb 208Pb/204Pb 87Sr/86Sr 143Nd/ 144Nd


Tabar
Tabar
Tanga
Feni
Rabaul
Rabaul
Bouganville
Solomon
Solomon
Savo
Kavachi
Woodlark
Woodlark
Woodlark
Woodlark
Woodlark
Woodlark
Woodlark
Tubaf
Tubaf
Tubaf
Xenoliths:
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf
Tubaf


TAB 3
TAB 4
DR11.1/2A
DR12.1/8A
RAB 2
RAB 981
69B
24-12
31-1
SAVO 2
35-2
4 29-H
426-5
426-14
429-8
432-10
432-16
432-17
544-2
56 2C
56 2M


54 6G
54 6B
56 4F
54 6E
54 6F
56 4D
54 2D
562F
56 2B
54 2G
56 2P
55 2H
54 2S
54 2K
55 2F
61 1C CPx
67 1 CPx
61 1B CPx
5630
55 3E
56 31
54 3E
56 3H
55 3F


18.773
18.714
18.735
18.679
18.813
18.815
18.573
18.622
18.443
18.424
18.394
18.273
18.288c
17.947c
17.890c
18.117c
18.116c


18.759
18.756
18.767


18.640
18.627
18.770
18.901
18.771
18.712
19.120
18.694
18.340
18.499
18.800
18.514
18.853
19.099


17.859
17.591
17.892
18.692
18.722
18.647
18.747


0.70355 0.513037


15.553
15.547
15.542
15.555
15.543
15.545
15.533
15.580
15.509
15.515
15.518
15.484
15.522
15.492
15.511
15.494
15.530


15.554
15.550
15.550


15.568
15.557
15.583
15.560
15.567
15.528
15.652
15.557
15.567
15.641
15.577
15.524
15.672
15.759


15.567
15.561
15.567
15.509
15.533
15.529
15.509


38.377
38.345
38.342
38.370
38.423
38.423
38.283
38.448
38.252
38.225
38.203
37.927
38.005
37.629
37.591
37.758
37.910


38.391
38.377
38.366


38.255
38.302
38.495
38.494
38.441
38.307
38.633
38.321
38.263
38.523
38.414
38.192
38.652
38.781


37.672
37.429
37.716
38.163
38.280
38.191
38.196


A-data from Johnson et al. 1988; B-data from Trull et al. 1990; C-TIMS Pb analyses;


0.704195A
0.704005A
0.70367


0.70370
0.703916B
0.703503B
0.70408
0.703724B
0.702826B
0.70258B
0.70270
0.70282
0.70267
0.703311B
0.70351
0.70397
0.70397




0.70828
0.70751
0.70669
0.70759


0.70791
0.70374
0.70398
0.70437
0.70592
0.70453
0.70404




0.70398
0.70446
0.70411
0.70457
0.70297
0.70333
0.70293
0.70302
0.70366
0.70343


0.51301


0.513006


0.512988




0.512971




0.513095


0.513078
0.513151
0.513063
0.513028
0.5130017
0.513003




0.5129289
0.5129586
0.5129378
0.5128994
0.5129111


0.513010
0.512910


0.512920








0.51298
0.51292
0.512963
0.513077
0.513030


0.513056


sedimentary
sedimentary
sedimentary
sedimentary
sedimentary
sedimentary
veined lherzolite
harzburgite
veined lherzolite
harzburgite
veined lherzolite
harzburgite
veined lherzolite
veined lherzolite
lherzolite
CPx separate
CPx separate
CPx separate
basaltic
gabbro
gabbro
gabbro
gabbro
aabbro









Sr isotopic compositions in the analyzed 3 clinopyroxene separates from the ultramafic

xenoliths range from 0.7041 to 0.7046. The mafic xenoliths have slightly less radiogenic

87Sr/86Sr values, ranging from 0.7029 to 0.7036 (Table 4-2).

Fresh volcanic rocks from Lihir and the seamounts exhibit small, but

distinguishable variations in their Pb isotopic compositions, with Conical seamount lavas

having slightly less, and Tubaf and Edison seamount lavas having slightly more

radiogenic ratios (Kamenov et al. 2004b). Lavas from Tabar and Tanga show overall

similar Pb isotopic compositions to Lihir and the seamounts (Table 4-2). Feni lavas have

slightly less radiogenic, whereas Rabaul lavas have slightly more radiogenic Pb isotope

values compared to the other TLTF lavas (Table 4-2). Solomon Islands and Woodlark

Basin samples show less radiogenic Pb isotopic ratios than the TLTF islands (Table 4-2).

Sedimentary xenoliths recovered from the Tubaf seamount lavas exhibit a larger range in

their lead isotope compositions compared to the fresh volcanic rocks in the area (Table 4-2,

Chapter 3) with values ranging from 206Pb/204Pb= 18.627 to 18.901, 207Pb/204Pb=15.509

to 15.585, and 208Pb/204Pb=38.163 to 38.512. Surprisingly, the ultramafic xenoliths

exhibit a very wide range in their Pb isotopic compositions, ranging from

206Pb/204Pb= 18.340 to 19.120, 207Pb/204Pb=15.524 to 15.759, and 208Pb/204Pb=38.192 to

38.781. The 3 clinopyroxene separates, however, show distinct Pb isotopic compositions

from the ultramafic xenoliths, with 206Pb/204Pb=17.591 to 17.892, 207Pb/204Pb=15.561 to

15.567, and 208Pb/204Pb=37.429 to 37.716. The mafic xenoliths, on the other hand, show

Pb isotopic compositions from similar to the lavas to slightly less-radiogenic, ranging

from 206Pb/204Pb= 18.647 to 18.747, 207Pb/204Pb=15.509 to 15.533, and

208Pb/204Pb=38.163 to 38.280 (Table 4-2).









Nd isotopic compositions of the lavas from the region show small variations,

ranging from 0.51297 to 0.51315, similar to previously published data (Stracke and

Hegner 1998). The mafic and ultramafic xenoliths overall exhibit similar Nd isotopic

compositions to the lavas (Table 2). Sedimentary xenoliths have slightly lower

143Nd/144Nd compared to the lavas and igneous xenoliths (Table 4-2).

Discussion

Major and Trace Element Variations in Seamount and Lihir Lavas

Although the lavas in the TLTF area are relatively alkaline, their overall major and

trace element abundances are typical for potasic magmatic rocks recovered in island arc

settings (Muller et al. 2001). Elemental variations in the seamount and Lihir samples

suggest that most of the lavas were modified to some degree by fractional crystallization.

The observed decrease in CaO and increase in A1203 with decreasing MgO (Fig. 4-3)

suggests that plagioclase fractionation did not play a major role in magmatic evolution.

Although a common phenocryst in the Conical seamount and Lihir lavas, plagioclase is

not common in Edison and is rare in Tubaf lavas. The Cpx is the most abundant

phenocryst observed in the lavas from the area, and Lihir island samples form a positive

correlation between the MgO and CaO/A1203 ratio (Fig. 4-7), indicating Cpx as a

dominant phase in the crystallization sequence, in agreement with previous observations

(Stracke and Hegner 1998). An experimental study on Lihir lavas also provided

evidences that the magmatic evolution was mainly controlled by clinopyroxene,

accompanied by amphibole, magnetite, and minor olivine fractionation (Kennedy et al.

1990a).













0.8 -L


0 0.6 03 O


0 0.4- -


0.2
0
-+-




1 2 3 4 5 6 7 8 9

MgO (wt.%)

Figure 4-7. Plot CaO/A1203 vs MgO indicating clinopyroxene as a major fractionation
phase. Symbols and data sources are the same as in Figure 4-2.

Miller et al. (2001 and 2003) conducted extensive P-T studies on mineral phases in

Lihir and Conical seamount lavas and suggested that magma evolution occurred in two

stages: one deeper, controlled by early hornblende crystallization, and a second shallower

stage, associated with clinopyroxene and plagioclase crystallization. Plagioclase is not a

stable phase at higher pressures and in addition, the plagioclase stability field is

suppressed in magmas with high H20 concentrations (Carmichael et al. 1996). The

absence of plagioclase in the more primitive Tubaf lavas is consistent with the Tubaf

lavas containing the highest volatile contents of the TLTF suite (Chapter 3). Furthermore,

the abundance of ultramafic xenoliths in Tubaf suggest that this magma rose rapidly to

the surface, without much time for low-pressure evolution close to the surface (Chapter 3),

another potential explanation for absence of plagioclase from the phenocryst

assemblage. Decreasing Fe203 with decreasing MgO indicates removal of a Fe-rich









phase, most probably magnetite, during fractional crystallization. Magnetite is abundant

in the lavas and also was suggested to play a role in the magma evolution by the

experimental study of Kennedy et al. (1990a). It is a common feature for arc magmas to

show a decrease in Fe203 with evolution indicating high f02, which stabilizes magnetite

early in the crystallization sequence. Indeed, several previous studies confirmed the high

f02 state of the magmas in the area (Kennedy et al. 1990a, McInnes and Cameron 1994,

McInnes et al. 2001). In addition, the positive correlation between MgO and Ni (Fig. 4-4)

also indicates that olivine was present in the early fractionation sequence, even though

it is not observed as a phenocryst in the lavas.

The major element data for Lihir Island and the seamounts show considerable

scatter on Harker diagrams, which suggests that the magmas did not evolve along a single

liquid line of descent from a common parental magma. Incompatible trace elements, such

as Rb, Ba, La, Ce, U, Th, and Pb plotted versus MgO also show considerable scatter

indicating multiple parental magmas (Fig. 4-4), similar to the observations derived from

the major element variations. This is further supported by the observed small, but

significant differences in the lavas' isotopic compositions (Stracke and Hegner 1998,

Kamenov et al. 2004b). Variations in parental melt compositions, on the other hand, are

probably a result of small differences in the source materials, as indicated by the isotope

data, and as well as a result of different degrees of partial melting. For example, Sr is

higher in the relatively more primitive lavas from Tubaf seamount, and the higher Sr

content correlates with higher La/Yb, suggesting smaller degrees of partial melting

created parental Tubaf magmas (Fig. 4-8). Alternatively, these differences can be






83


explained by magma derivation from sources with different Sr, La, and Yb

concentrations.







0
7.5
0
8-




7.5



6- 0 -
0
5.5 o o


500 1000 1500 2000 2500

Sr (ppm)

Figure 4-8. La/Yb vs Sr for Lihir island and seamount lavas. For more discussion see the
text, data symbols same as Fig. 4-2.

Overall, the TLTF lavas exhibit trace element signatures typical for subduction-

related volcanism, in particular exhibiting high LILE/HFSE and elevated LREE relative

to HREE ratios (Fig. 4-5 and 4-6). Although currently not associated with active

subduction, this observation indicates that the TLTF volcanism is related to a previous

episode of subduction, as suggested previously by a number of studies (Kennedy et al.

1990b, Mclnnes et al. 1994; Stracke and Hegner 1998; Franz et al. 2002).

















Altered basalt


Indian Ocean gabbros


Cs Rb Ba Th U


Nb K La Ce Pb Sr Nd Sm Hf Zr Ti Eu Gd Dy Y


Figure 4-9. Primitive Mantle normalized trace element patterns of mafic xenoliths compared with MORB, altered MORB, and oceanic
gabbros. Primitive Mantle values from Sun and McDonough (1989), MORB data from Kelley et al. (2003), Indian Ocean
gabbros from Coogan et al. (2001), and Cayman Rise gabbros from Perez-Suarez (2001).


1000




100-




10 -




1 -


Mafic xenoliths


0.1 MORB


0.01


Cayman Rise gabbros


-I.


Er Yb Lu









Major and Trace Element Variations in Mafic and Ultramafic Xenoliths

The wealth of xenoliths recovered from Tubaf seamount allow us to look more

closely at the processes and sources in the crust and mantle and/or subducted slab and

their role in the formation of the TLTF magmas. The gabbroic xenoliths have major and

trace element abundances similar to gabbros recovered from oceanic settings. This

observation is in agreement with the McInnes et al. (1999) study that suggested that the

mafic crust underlying the TLTF islands originated at an oceanic spreading system. The

mafic xenoliths exhibit depletion in the LREE (Fig. 4-5) indicating origin from a depleted

mantle source. Their extended trace element patterns exhibit overall lower abundances

than typical MORB (Fig. 4-9) indicating that the gabbros are cumulates, similar to other

mid-ocean ridge gabbros (Coogan et al. 2001). The elevated concentration of Ti in

some samples (Fig. 4-9) is due to Ti-magnetite accumulation, a common accessory

mineral observed in the mafic xenoliths (McInnes et al. 2001). The mafic xenoliths trace-

element patterns exhibit enrichments in fluid-mobile elements, such as Ba, U, K, and Pb

(Fig. 4-9) suggesting that part of the mafic crust was affected by alteration probably after

its formation at an oceanic spreading system. Indeed, a majority of the studies on ODP

cores penetrating into the basaltic and gabbro layer of the oceanic crust provide evidences

for chemical exchange between the oceanic lithosphere and seawater (Bach et al.

2001, Kelley et al. 2003).

The overall high MgO and low CaO, Na20, A1203 content observed in the

ultramafic xenoliths (Table 4-1) indicates that they are residues after partial melting event

(Gregoire et al. 2001, McInnes et al. 2001, Franz et al. 2002, for detailed

discussion). Almost all of the xenoliths exhibit metasomatic alteration features, such as

veins and irregular alteration areas (Gregoire et al. 2001, Franz et al. 2002). Overall, the
















TLTF lavas


-' >
'llt Lamaicxenoith
_RA U 4;n. ,.:EE:
VV
.. .,ramafic xnlt



Ultramafic xenoliths


0211'dl
.t. S :


I l I I I I I A I A I I I A I A I I


I I I A I A I I I A


Cs Rb Ba Th U Nb K La Ce Pb Sr Nd Sm Hf Zr Ti Eu Gd Dy Y Er Yb Lu

Figure 4-10. Primitive Mantle normalized trace element patterns of ultramafic xenoliths compared to TLTF lavas. Primitive Mantle
values from Sun and McDonough (1989), ultramafic xenoliths data from Gregoire et al. (2001).


1000



100


10


1


0.1


0.01


0.001


I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I I


I I I A I I I a I I I A I I I A I









ultramafic xenoliths have strongly depleted trace element abundances compared to the

TLTF lavas and mafic xenoliths. The REE patterns observed in the ultramafic xenoliths

range from LREE and HREE enriched, so called "spoon-shape" pattern, to almost flat,

slightly LREE enriched pattern (Fig. 4-5). The LREE enrichment can be explained by a

metasomatic enrichment from a hydrous fluid phase (Gregoire et al. 2001, Franz et al.

2002). In addition, the metasomatized xenoliths exhibit high LILE/HFSE and

LREE/HFSE ratios (Fig. 4-10), also indicative of enrichment by hydrous fluids (Gregoire

et al. 2001).

Sr and Nd Isotopic Variations in Xenolith and Lava Samples.

The measured Sr and Nd isotopic compositions in the mafic xenoliths are similar to

the Pacific-Indian MORB (Fig. 4-11), indicating derivation from a depleted mantle

source which is consistent with their trace element and REE patterns. Some of the

ultramafic xenoliths, on the other hand, exhibit elevated 87Sr/86Sr at relatively high

143Nd/144Nd (Table 4-2, Fig. 4-11) and their trace element patterns provide evidence for

metasomatic alteration by hydrous fluid released most probably from the subducted slab.

Experiments show that Sr is partitioned in hydrous fluid phase while the REE are

relatively immobile and do not partition into the fluid (Brenan et al. 1995). Therefore,

fluids released during dehydration of subducted sediments and/or altered oceanic crust

are expected to have relatively high Sr and low Nd contents. Metasomatism of the TLTF

mantle wedge by such fluids will shift the Sr isotopic compositions of the ultramaifc

xenoliths to more radiogenic ratios, but at the same time will not have strong effect on the

Nd isotopic ratios, which appears to be the case (Fig. 4-11).

143Nd/144Nd measured in the lavas from the region are generally similar to Indian

and Pacific MORB, indicating that Nd reflects derivation from a depleted mantle source.









0.5134 ..1111 .iiiI III 1111

TLTF lavas Pacific
0.5132 sediments


0.513 -

T:*- ,^lOTonga "
0.5128 ,,o-
Indian, -, '.d
Pacific
0.5126 MORB Banda

ultramafic xenol. ,
0.5124 CPx separates
V mafic xenol.

0.5122
0.702 0.704 0.706 0.708 0.71

87Sr/8Sr

Figure 4-11. Nd vs Sr isotopic compositions of the mafic and ultramafic xenoliths relative
to possible source in the area. Data from this study; and White (1987), Othman
et al. (1989), Ewart and Hawkesworth (1987) Vroon et al. (1993), and
Stracke and Hegner (1998).

Although the TLTF lavas have a MORB-type Nd isotopic signature, some of them are

displaced to slightly elevated 87Sr/86Sr values compared to the Indian and Pacific MORB.

The elevated Sr isotopic ratios suggest involvement of sedimentary and/or altered oceanic

crust material. Altered oceanic crust is expected to acquire higher 87Sr/86Sr as a result of

interaction with seawater, but at the same time it will preserve its 143Nd/144Nd due to the

relative immobility of the REE during hydrothermal alteration. Incorporation of material

in the TLTF magmas from altered oceanic crust, therefore, may produce the trend exhibit

by the TLTF volcanoes (Fig. 4-11). In addition, incorporation of small amount of