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Construction of the Oceanic Crustal Layer 2A: A Detailed Petrographic and Geochemical Study of Lavas from 9 Degrees 30 M...


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CONSTRUCTION OF THE OCEANIC CRUSTAL LAYER 2A: A DETAILED PETROGRAPHIC AND GEOCHEMICAL STUDY OF LAVAS FROM 9 30 N AND 9 50 N EAST PACIFIC RISE By JILLIAN SARAY HINDS A THESIS PRESENTED TO THE GRADUATE SCHOOL OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF MASTER OF SCIENCE UNIVERSITY OF FLORIDA 2005

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ii ACKNOWLEDGMENTS I am in great debt to a number of friends and colleagues who provided technical, analytical, financial, and mental support during the conceptualization, development, and realization of my masters research. My greatest thanks go to my advi sor, Mike Perfit, for bringing me to the University of Florida to continue studying a subject I love, as well as providing a safe, constructive environment to work in over the last two years. I would also like to thank Dave Foster, Ray Russo, Ellen Martin and Phil Neuhoff, whose doors were always open when I needed their help. Many thanks are due to the scientists and crew involved in collecting and preparing lava samples during the AT11 7 cruise, especially Hans Schouten, the chief scientist, along with fellow scientists Dan Fornari and Adam Soule. Continued involvement by the latter two gentlemen was most welcome and resulted in two published abstracts. Without the help of Ian Ridley, George Kamenov, Matt Smith, and Susanna Blair, it is questionable wh ether the processing and analysis of the many recovered lavas would have been completed in a timely matter or at an acceptable scientific level. I thank them for all their help! Thanks go again to Adam Soule in addition to John Jaeger, and Ray Thomas for their patience and technical help related to my learning and the operation of ARC/GIS. This research would not have been completed without the financial support of a NSF grant and the mental support provided by my family and friends. I love them!

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iii TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ .............................. ii LIST OF TABLES ................................ ................................ ................................ .......... v LIST OF FIGURES ................................ ................................ ................................ ....... vii ABSTRACT ................................ ................................ ................................ .................... x CHAPTER 1 INTRODUCTION ................................ ................................ ................................ .... 1 Introduction to the Problem ................................ ................................ ...................... 1 Sample Localities: AT11 7 Cruise and Dive Information ................................ .......... 5 2 BACKGROUND ................................ ................................ ................................ .... 15 Construction of Oceanic Crust at the Fast Spreading East P acific Rise ................... 15 Regional Geology of The 9 10 N segment of the East Pacific Rise ...................... 18 3 ANALYTICAL METHODS ................................ ................................ ................... 24 4 PETROGRAPHY ................................ ................................ ................................ ... 27 Petrography ................................ ................................ ................................ ............ 27 Flow Units With Fronts, Dives 3963 and 3974 ................................ ................. 27 Channels, Dive 3968 ................................ ................................ ........................ 46 Off Axis Mounds, Dive 3970 ................................ ................................ ........... 46 Phase Chemistry ................................ ................................ ................................ ..... 49 Summary ................................ ................................ ................................ ................ 58 5 MAJOR AND TRACE ELEMENT CHEMISTRY ................................ ................. 59 Major Element Data ................................ ................................ ................................ 59 Flow Units With Fronts, Dives 3963 and 3974 ................................ ................. 69 Channels, Dive 3968 ................................ ................................ ........................ 70 Off Axis Mounds, Dive 3970 ................................ ................................ ........... 71 Trace Element Data ................................ ................................ ................................ 71 Flow Units With Fronts, Dives 3963 and 3974 ................................ ................. 85

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iv Channels, Dive 3968 ................................ ................................ ........................ 86 Off Axis Mounds, Dive 3970 ................................ ................................ ........... 86 Summary ................................ ................................ ................................ ................ 86 6 PETROGENESIS ................................ ................................ ................................ ... 88 Major Element Models ................................ ................................ ........................... 88 Flow Units, Di ves 3963 and 3974 ................................ ................................ .... 89 Off Axis Mounds, Dive 3970 ................................ ................................ ........... 97 Trace Element Models ................................ ................................ ............................ 99 Flow Units, Dives 3963 and 3974 ................................ ................................ .. 100 Off Axis Mounds, Dive 397 0 ................................ ................................ ......... 102 Magma Mixing and Source Characteristics ................................ ........................... 103 Su mmar y ................................ ................................ ................................ .............. 105 7 DISCUSSION ................................ ................................ ................................ ...... 109 Combining and Evaluating Petrographic and Geochemical Res ults ....................... 109 Heterogeneity Index for Mid Ocean Ridge Lava Flows ................................ ........ 113 Models for Crustal Construction ................................ ................................ ........... 118 8 CONCLUSIONS ................................ ................................ ................................ .. 123 APPENDIX A E2 ICP MS ANALYTICAL PRO TOCOL ................................ ............................ 125 B ERROR ANALYSIS ................................ ................................ ............................ 129 Accuracy ................................ ................................ ................................ .............. 129 Precision ................................ ................................ ................................ ............... 133 LIST OF REFERENCES ................................ ................................ ............................. 137 BIOGRAPH ICAL SKETCH ................................ ................................ ....................... 149

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v LIST OF TABLES Table page 4 1. Thin Section Petrography of Lavas Collected during Alvin Dive 3963 Near 9 50N East Pacific Rise ................................ ................................ ...................... 28 4 2. Thin Section Petrography of Lavas Collected Duri ng ALVIN Dive 3974 Near 9N East Pacific Rise ................................ ................................ ...................... 32 4 3. Thin Section Petrography of Lavas Sampled During ALVIN Dive 3968 Near 9N East Pacific Rise ................................ ................................ ...................... 36 4 4. Thin Section Petrography of Lavas Sampled During ALVIN Dive 3970 Near 9N East Pacific Rise ................................ ................................ ...................... 37 4 5. Petrologic Characteristics of Flow Units From ALVIN Dives 3963 and 3974 ......... 47 4 6. Olivine Chemical Compostions by Electron Microp robe for 3970 9 and 3974 11t .. 50 4 7. Plagioclase Chemical Compositions by Electron Microprobe for 3970 9 ................ 52 4 8. Clinopyroxene Chemical Compositions by Electron Microprobe for 3970 9 ........... 53 5 1: Major Element Chemistry of Lavas Collected From 9 10N East Pacific Rise During AT11 7 Cruise ................................ ................................ .......................... 60 5 2. Select Trace elements For Lavas Collected During Dives 3963, 3968, 3970, and 3974 ................................ ................................ ................................ ..................... 72 5 3. Rare Earth Element Concentrations for Dives 3963, 3968, 3970, and 3974 ............. 74 6 1. Petrogenetic Parameters and Conditions Used in Flow Unit (Dive 3963) ................ 90 6 2. Petrogenetic Param eters and Conditions Used in Flow Unit (Dive 3974) ................ 91 6 3. Petrogenetic Parameters and Conditions Used in Off Axis Mounds (Dive 3970) Calculations ................................ ................................ ................................ ......... 9 2 6 4. Partition Coefficients Used in Mode ling o f Trace Element ................................ ... 101 7 1. Characterisitics of Mid Ocean Ridge Lavas ................................ .......................... 114

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vi 7 2. Heterogeneous Index Calculations for The Flow Units (9 50' N) and Off Axis Mounds (9 30' N) ................................ ................................ ............................... 115

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vii LIST OF FIGURES Figure page 1 1. Location and bathymetry of 9 to 10 N segment of the East Pacific Rise between the Clipperton and Siqueiros fracture zones. ................................ ........................... 2 1 2. Microbathymetry map show ing the location of Alvin dives during AT11 7 along the East Pacific Rise ................................ ................................ ............................... 6 1 3. Location of lavas sampled during dives 3963 and 3974 ................................ ............ 7 1 4. Typical lava morphologies observed at the East Pacific Rise ................................ .... 9 1 5. Location of lavas sampled during dive 3968.. ................................ ......................... 10 1 6. Location of lavas sampled during dive 3970 ................................ ........................... 13 1 7. Bathymetric profile for dive 3963 ................................ ................................ ........... 14 2 1. Interpretative cross section of the subaxial crustal structure beneath the East Pacific Rise ................................ ................................ ................................ .......... 17 2 2. Cross axial profiles for two sections of the East Pacific Rise ................................ .. 19 2 3. Ty pical lava morphologies and stratigraphic positions within the axial summit collapse trough at the East Pacific Rise ................................ ................................ 20 4 1. Photomicrographs of common textures found in basalts collected near 9 30 and 9 51N EPR ................................ ................................ ................................ ......... 41 4 2: Photomicrographs of olivine, clinopyroxene and plagioclase crystal morphologies, all in crossed polars. ................................ ................................ ...... 43 4 3. Photomicrographs of xenoglomerocrysts found in basalts collected during dive 3963 near 9 51N EPR.. ................................ ................................ ....................... 45 4 4. Photomicrographs of basaltic samples retrieved during Alvin dive 3970 to a series of off axis mounds 1.23 1.58 km west of the AST near 9 30N EPR .................... 48 4 5. Olivine compositions ................................ ................................ .............................. 54

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viii 4 6. Zoning in olivine, plagioclase, and clinopyroxene ................................ ................... 56 5 1. Major element oxide variation diagrams for lavas sampled during dive 3963, 3968, 3970, and 3974. ................................ ................................ .......................... 66 5 2. MgO content of lavas from dives 3963, 3968, 3970, and 3974 as a function of their latitude. ................................ ................................ ................................ ........ 68 5 3. Compositional distributions for sheet, lobate, and pillow lavas ............................... 69 5 4. Spider diagram and REE plot for lavas from dive 3963 ................................ .......... 76 5 5. Spider diagram and REE plot for lavas from dive 3974 ................................ .......... 77 5 6. Spider diagram and REE plot for lavas from dive 3968 ................................ ........... 78 5 7. S pider diagram and REE plot for lavas from dive 3970 ................................ ........... 79 5 8. Trace element variation diagrams for lavas from dives 3963, 3968, 3970, and 3974. ................................ ................................ ................................ .................... 80 6 1. Liquid lines of descent calculated for flow un its sampled during dives 3963 and 3974 under both anhydrous and hydrous conditions ................................ .............. 93 6 2. Liquid lines of descent calculated for off axis pillow mounds sampled during dive 3970 under both anhydrous and hydrous conditions ................................ ...... 98 6 3. Liquid lines of descent for select trace elements calculated for the flow units sampled during dives 3963 and 3974 under anhydrous (A) and hydrous (H) conditions ................................ ................................ ................................ ........... 102 6 4. Liquid lines of de scent for select trace elements calculated for the off axis pillow mounds sampled during dive 3970 ................................ ................................ ...... 103 6 5. Zr v. Y for the flow units (dive 3963 and 3974), channels (dive 3968), and off axis mounds (dive 3970) ................................ ................................ ..................... 104 6 6. Zr/Y v. Y for the flow units (dive 3963 and 3974), channels (dive 3968), and off axis mounds (dive 3970) ................................ ................................ ..................... 107 6 7. Ce/Yb v. Ce for the flow units (dive 3963 and 3974), channels (dive 3968), and off axis mounds (dive 3970). ................................ ................................ .............. 108 7 1. The heterogeneity Index calculated for individual flows v. their flow volume and spreading rate. ................................ ................................ ................................ .... 119 7 2. Three dimensional diagram of oceanic crust formation along the northern East Pacific Rise ................................ ................................ ................................ ........ 122

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ix B 1. Measured and accepted trace element concentrations of standard BHVO 1. ......... 130 B 2. Spider diagrams for in house standard ENDV ................................ ...................... 131 B 3. Spider diagram and REE plots for standard 2392 9 ................................ .............. 132 B 4. Distributions of major element analyses for standard 2392 9. ............................... 134 B 5. Spider diagram and REE plots for repeat analyses of several s am ples ................. 136

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x A bs t r a c t of T he s i s P r e s e nt e d t o t he G r a dua t e S c hool of t he U ni ve r s i t y of F l or i da i n P a r t i a l F u l f i l l m e nt o f t he R e qui r e m e nt s f or t he D e g r e e of M a s t e r of S c i e nc e C O N S T R U C T I O N O F T H E O C E A N I C C R U S T A L L A Y E R 2A : A D E T A I L E D P E T R O G R A P H I C A N D G E O C H E M I C A L S T U D Y O F L A V A S F R O M 9 30 N A N D 9 50 N E A S T P A C I F I C R I S E B y J i l l i a n S a r a y H i nds D e c e m be r 2005 C ha i r : M i c ha e l P e r f i t M a j or D e pa r t m e nt : G e ol ogi c a l S c i e nc e s L a ye r 2A of oc e a ni c c r us t i s c om pos e d of a c ont i nuum of ove r l a ppi ng l a va f l ow s e r upt e d f r o m bot h on a xi s a nd of f a xi s s i t e s a t t he E a s t P a c i f i c R i s e ( E P R ) B e t w e e n 9 25 N a nd 9 55 N E P R m i c r oba t hy m e t r y a nd s i de s c a n s ona r m a ps i n a ddi t i on t o i n s i t u obs e r va t i ons r e ve a l m os t f l ow s f or m l a r ge l obe s 1 2 km w i de e xt e ndi ng up t o 1 km f r om t he r i dge a xi s T he body o f e a c h l obe d f l ow uni t m os t l y c ons i s t s of s h e e t a nd l oba t e f l ow s t ha t t r a ns i t i on i nt o pi l l ow s a t s t e e p f l ow f r ont s C ha nne l s ys t e m s a r e f ound i nt e r s pe r s e d a m ongs t t h e s e f l ow uni t s i n 50 100 0 m l ong s e gm e nt s a nd s e r ve t o m ove l a va up t o 3 km of f a xi s ont o t he c r e s t a l pl a t e a u. A ddi ng t o t hi s c om pl e xi t y a r e pi l l ow m ounds e r upt e d f r om f i s s ur e s > 1 km f r om t he s pr e a di ng r i dge T he a c c um ul a t i on of l a va s c l os e t o t he a xi s e xpl a i n s bot h t he a ppa r e nt doubl i ng i n t hi c kne s s of l a ye r 2A w i t hi n 1 2 km of t he a xi s a nd t he obs e r va t i ons a nd i s ot opi c e vi de nc e t ha t s ugge s t a c t i ve

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xi vol c a ni s m oc c ur s ove r a m uc h w i de r z one a bo ut t he E P R a xi s ( up t o 4 km ) t ha n pr e vi ous l y t hought L a va s f r om a s e r i e s of f l ow f r ont s on e i t he r s i d e of t he a xi a l s um m i t c ol l a ps e t r ough ( A S C T ) a t 9 50 N E P R a nd t w o c ha nne l s a t ~ 9 29 N E P R a r e m a i nl y a phy r i c t o s pa r s e l y pl a gi oc l a s e phyr i c ( < 2% by vol um e ) ba s a l t s w i t h nor m a l i nc om pa t i bl e e l e m e nt de pl e t i ons ( N M O R B ) I n c ont r a s t t he m or e e vo l ve d ( l ow e r M gO ) l a va s f r om of f a xi s m ou nds a t ~ 9 30 N E P R a r e r e l a t i ve l y m o r e i nc o m pa t i bl e e l e m e nt de pl e t e d a nd c ont a i n a s m uc h a s 10% m i c r ophe noc r ys t s of ol i vi ne pl a gi oc l a s e a nd c l i nopyr oxe ne T he d a t a s ugge s t t he s e l a va s m a y ha ve c om e f r om t he c ool e r e dge s of t he a xi a l m a gm a c h a m be r a nd w e r e de r i ve d f r om a di s t i nc t l y di f f e r e nt s our c e t ha n t he f l ow s e m a na t i ng f r om t he a xi s P i l l ow ba s a l t s c om pr i s i ng t he f l ow f r ont s of e a c h f l ow uni t a r e c ons i s t e nt l y m or e e vol ve d ( 0. 2 0. 77 l ow e r w t % M gO ) t ha n t he i r a s s oc i a t e d l oba t e a nd s he e t f l ow s i n t he body of t he f l ow a nd ha ve s l i ght l y hi ghe r c onc e nt r a t i ons of i nc om pa t i bl e e l e m e nt s T he qua nt i t i e s of m i c r ophe noc r ys t s i n pi l l ow l a va s a t t he f l ow f r ont a r e ~ 1. 0 2. 4 vol % gr e a t e r t ha n t hos e i n t he body of t he f l ow F r a c t i ona l c r ys t a l l i z a t i on m ode l s a t l ow pr e s s ur e pr e di c t 4 14 vol % c r ys t a l l i z a t i on of pa r e nt a l m a gm a s i s r e qui r e d t o pr oduc e t h e obs e r ve d c ha nge s i n c he m i s t r y w i t hi n i ndi vi dua l f l ow uni t s a t odds w i t h t he m e a s ur e d c r ys t a l l i ni t y va r i a t i on. F u r t he r m or e s om e m i xi ng be t w e e n pr i m i t i ve a nd e vo l ve d m e l t s i s r e qui r e d t o e xpl a i n a l l of t he va r i a t i ons i n m a j or e l e m e nt a nd t r a c e e l e m e nt c ha r a c t e r i s t i c s of t he f l ow uni t s M i xi ng i s s uppor t e d by t h e pr e s e nc e of xe noc r ys t s a nd ot he r di s e qui l i br i um r e l a t i ons hi ps be t w e e n phe noc r ys t s a nd t he i r hos t m e l t T he r e s ul t s of our s t udy s ugge s t t ha t t he m a j or i t y of c he m i c a l i m pr i nt i ng of l a va s pr e c e de s e r upt i on ont o t he s e a f l oor a nd t ha t l i t t l e va r i a t i on oc c ur s onc e m a gm a s ha ve be e n e r upt e d.

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1 C H A P T E R 1 I N T R O D U C T I O N I n t r od u c t i on t o t h e P r ob l e m O c e a ni c c r us t m a ke s up ne a r l y 70% of t he r oc ks on E a r t h s s ur f a c e ye t t he na t ur e a nd s i gni f i c a nc e of t he m a gm a t i c a nd vol c a ni c pr oc e s s e s t ha t c r e a t e t he m a r e i n de t a i l unc e r t a i n. M os t oc e a ni c c r us t i s t hought t o f o r m w i t hi n t he na r r ow ne ovol c a ni c z one ( 1 4 km w i de ) c e nt e r e d a l ong t he a xi s of a m i d oc e a n r i dge ( M O R ) S pa nni ng ne a r l y 65, 000 km a l ong t he oc e a n bot t om s t he r e m ot e ne s s a nd l a r ge l y i na c c e s s i bl e na t ur e of t he M O R s ys t e m ha s l i m i t e d i t s s t udy t o onl y a f e w r e pr e s e nt a t i ve r i dge s ( e g. E a s t P a c i f i c R i s e G or da R i dge J ua n de F uc a R i dge a nd M i d A t l a nt i c R i dge ) T he 9 10 N s e gm e nt of t he E a s t P a c i f i c R i s e ( E P R ) i s one s e c t i on of t he M O R w he r e t he i nt e gr a t i on of num e r ous di s c i pl i ne s s uc h a s s e a f l oor m or phol ogy [ F or nar i e t al 1998; K ur r as e t al 2000 ] l a va ge oc he m i s t r y [ B at i z a and N i u 1992; P e r f i t e t al 1994] hydr ot he r m a l a c t i vi t y [ H ay m on e t al 1991; V on D am m e t al 1992, 1995 ] de t a i l e d m a ppi ng [ M ac donal d e t al 1984; A l e x ande r and M ac donal d 1996; C r ow de r a nd M ac donal d 2000] a nd s e i s m i c i nve s t i ga t i ons [ D e t r i c k e t al 1987; C hr i s t e ns on e t al 1994 1996; H oof t e t al 1996 ; C r aw f or d and W e bb 2002] ha s r e s ul t e d i n on e of t he m os t e xt e n s i ve m ul t i di s c i pl i na r y da t a ba s e s of a ny M O R A t t he 9 10 N s e gm e nt of t he E P R ( F i gur e 1 1) l a va s t ha t a r e e r upt e d f r om w i t hi n or c l os e l y pr oxi m a l ( < 100 m ) t o t he a xi a l s um m i t c ol l a ps e t r ough ( A S C T ) a m a s s t o f or m t he e xt r us i ve por t i on of oc e a ni c c r us t ( L a ye r 2A ) A c c or di ng t o s e i s m i c s t udi e s t h i s l a ye r doubl e s i n t hi c kne s s w i t h 1 2 k m of t he r i dge a xi s [ C hr i s t e ns on e t al 1994,

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2 F r om Soul e e t al ( 2005) F i gur e 1 1 L oc a t i on a nd ba t hym e t r y of 9 t o 1 0 N s e gm e nt of t he E a s t P a c i f i c R i s e be t w e e n t he C l i ppe r t on a nd S i que i r os f r a c t ur e z one s T h e a xi a l s um m i t c ol l a ps e t r ough i s m a r ke d by a dot t e d l i ne a nd t he l oc a t i on of t he D S L 120A s i de s c a n s ona r s ur ve y i s out l i ne d i n bl a c k. T he M ul t i be a m ba t hym e t r y c ont our i nt e r va l i s ~ 84 m ( R e vi s e d f r om Soul e e t a l [ 2005 ] ) 1996; H oof t e t al 1996 ] I n a r e a s up t o 4 km f r om t he s pr e a di ng a xi s a nom a l ous l y young l a va f l ow s a nd pi l l ow m ounds ha ve be e n i de nt i f i e d t hr ough bot h f r e s he r a ppe a r a nc e a s w e l l a s younge r U s e r i e s m ode l e r upt i on a ge s t ha n w oul d be pr e di c t e d ba s e d on t he i r di s t a nc e f r o m t he a xi s [ G ol ds t e i n e t al 1994; P e r f i t e t al 1994; P e r f i t and C hadw i c k 1998; Sm i t h e t al 2001; Si m s e t al 2003] T hus r e c e nt s t udi e s s ugge s t a c t i ve vol c a ni s m o c c ur s ove r a m uc h w i d e r z one a t t he E P R t ha n pr e vi ous l y b e l i e ve d a n d

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3 of f a xi s vol c a ni c a c t i vi t y c ont r i but e s not a bl e vol um e s t ow a r ds c r us t a l c ons t r uc t i on [ G ol ds t e i n e t al 1 994; P e r f i t e t al 1994; S i m s e t al 2003] T he r e l a t i ve vol um e s of l a va s t ha t e r upt of f a xi s c om pa r e d t o t hos e t ha t or i gi na t e i n t he A S C T a nd t he n f l ow l ong di s t a nc e s of f a xi s i s s t i l l unc l e a r O nl y a f e w i ndi vi dua l f l ow s ha ve be e n i de nt i f i e d, m a ppe d, a nd s a m pl e d a t M O R s [ e g. E m bl e y e t al 1991; H ay m on e t al 1993; C hadw i c k and E m bl e y 1994 ; R ubi n e t al 1994, 2001; G r e gg e t al 1995, 1996; Si nt on e t al 2002 ] a nd a l l a r e t he c ons e que nc e of r e c e nt vol c a ni s m ( pr oba bl y w i t hi n a f e w ye a r s of e r up t i on) w i t hi n t he ne ovol c a ni c z one A s f l ow s a ge younge r f l ow s on t he a xi a l c r e s t a r e h a r de r t o di s t i ngui s h f r om ne i ghbor i ng ol de r f l ow s due t o i nc r e a s e d s e di m e nt c ove r s e a f l oo r w e a t he r i ng, a nd c om pl e x s t r a t i g r a phi c r e l a t i ons hi ps A l t hough m a ny s e c t i ons of t he M O R ha ve be e n s a m pl e d a nd t hous a nds of l a va s ha ve be e n c he m i c a l l y a na l yz e d ve r y f e w i ndi vi dua l M O R f l ow s ha ve be e n une qui voc a l l y i de nt i f i e d ( s e e R ubi n e t al [ 2001] ) C on s e que nt l y, s c i e nt i s t s do not ha ve m uc h i nf or m a t i on r e ga r di ng t he e xt e nt of c he m i c a l h e t e r oge ne i t y i n f l ow s a nd i ns t e a d us e r e gi ona l c he m i c a l t r e nds i n c onj unc t i on w i t h t h e r e l a t i ve pos i t i on o f a s a m pl e t o t he s pr e a di ng a xi s t o c ha r a c t e r i z e t he s pa t i a l a n d t e m por a l a s pe c t s of pr oc e s s e s ope r a t i ng be ne a t h t he r i dge a xi s W i t hout pl a c i ng l a va s i nt o t he c ont e xt of s i ngl e e r upt i ons i t c a nnot be know n i f t he obs e r ve d c he m i c a l va r i a t i ons ove r s m a l l s pa t i a l s c a l e s a r e r e l a t e d by one or m or e pr oc e s s e s i nc l udi ng m i xi ng of di f f e r e nt m a gm a s dur i ng e r upt i on f r a c t i ona l c r ys t a l l i z a t i on dur i ng s ur f i c i a l c ool i ng, f l ow c ont a m i na t i on dur i ng e r upt i on, or i nc om pl e t e m i xi ng o f m a gm a s pr i o r t o e r upt i on

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4 C a r e f ul i ns pe c t i on of s i de s c a n s ona r a nd m i c r ob a t hym e t r y m a ps of t he 9 25' t o 9 55' N r e gi on of t he E P R r e ve a l s t he ne ovol c a ni c z one i s dom i na nt l y c om pr i s e d of ove r l a ppi ng l obe s ha pe d r e f l e c t or s 0. 25 1 km i n l e ngt h a nd 1 2 km i n w i dt h t ha t e m a na t e f r om t he A S C T a nd e xt e nd up t o a f e w km dow n t he r i dge f l a nks i n a n ove r l a pp i ng a nd a na s t om os i ng pa t t e r n. E a c h l obe ha s be e n i nf e r r e d t o r e pr e s e nt l a va f l ow s ur f a c e s e ndi ng i n a r i nge d f l ow f r ont [ F or nar i e t al 1998; K ur r as e t al 2000; Sc hout e n e t al 2001] I nt e r s pe r s e d w i t hi n t hi s vol c a ni c t e r r a i n a r e s i nuou s t o l i ne a r c h a nne l s ys t e m s ( 50 1000 m s e gm e nt s ) t ha t a r e or i e nt e d r oughl y pe r pe ndi c ul a r t o t he A S C T a nd a r e f ound up t o 3 km f r o m t he a xi s [ Sc hout e n e t al 2001; Soul e e t al 2005 ] I n a ddi t i on, e l onga t e r i dge pa r a l l e l pi l l ow m ounds ~ 10 30 m hi gh, ~ 100 m w i de ~ 1 km l ong, a nd l oc a t e d > 1 km of f a xi s a dd t o t h e c om pl e x vol c a nol ogi c pa nor a m a of t h e s e a f l oor T he s e f e a t ur e s s ugge s t t hr e e m a i n t y pe s of vol c a ni s m a c t i ve w i t hi n t h e ne ovol c a ni c z one of t he E P R : ( 1) ove r l a ppi ng e r upt i ons of l oba t e s a nd s he e t f l ow s or i gi na t i ng f r om t he a xi s a nd e xt e ndi ng up t o 1 km ont o t he c r e s t a l pl a t e a u, ( 2) s l ope dr i ve n l a va c ha nne l s a nd t ube s w hi c h m ove l a va f l ow s gr e a t e r di s t a nc e s a nd ( 3) of f a xi s e r upt i ons f r om f i s s ur e s a t t he e dge o f t he ne ovol c a ni c z one I n or de r t o be t t e r unde r s t a nd t he di s t r i but i on of l a v a s t ha t c om pr i s e t he ne ovol c a ni c z one of t he E P R a nd t o be t t e r c ons t r a i n m ode l s of uppe r oc e a ni c c r us t a l c ons t r uc t i on a t f a s t s pr e a di ng m i d oc e a n r i dge s l i ke t he nor t he r n E P R t he de gr e e of ge oc he m i c a l he t e r oge ne i t y, pe t r ogr a phi c di ve r s i t y, a nd m or phol ogi c a l va r i a t i on w i t hi n c a r e f ul l y m a ppe d a nd s a m pl e d l a v a f l ow uni t s f r om t he r i dge c r e s t w e r e de t e r m i ne d. I n a ddi t i on t o s t e e p pi l l ow f l ow f r ont s a nd s he e t a nd l oba t e f l ow t ops t ha t c om pr i s e t he f l ow uni t s

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5 a dj a c e nt t o t he A S C T s a m pl e s f r om di s t r i but a r y l a va c ha nne l s a nd of f a xi s pi l l ow m ounds ha ve a l s o be e n i nve s t i ga t e d. S am p l e L oc al i t i e s : A T 11 7 C r u i s e an d D i ve I n f or m at i on I n J a nua r y a nd F e br ua r y of 2004 dur i ng V oya ge 1 1, L e g 7 of t he R / V A t l a nt i s t he s ubm e r s i bl e A l vi n c ol l e c t e d 107 s a m pl e s of l a va a l ong 9 25' 55'N of t he E P R ( F i gur e 1 2) G e ol ogi c s e t t i ngs a nd s pa t i a l r e l a t i ons of t he s a m pl e s i t e s w e r e doc um e nt e d us i ng phot ogr a phs a nd vi de o i m a ge s w hi l e t he D oppl e r na vi ga t i on s ys t e m i n A l vi n pr ovi de d s a m pl e pos i t i ons on t he s e a f l oo r T he s e l e c t i on of s a m pl e s i t e s w a s gui de d by A B E m i c r oba t hym e t r y a nd D S L 120A s i de s c a n s ona r i m a ge s f or t he a r e a w i t h t he pr i m a r y goa l t o m a p a nd s a m pl e f e a t ur e s t ha t a ppe a r t o b e di s c r e t e f l ow uni t s e m a na t i ng w i t hi n or ne a r t he A S C T a nd e xt e ndi ng of f a xi s O f pa r t i c ul a r i nt e r e s t w e r e f l ow uni t s s i t ua t e d a l ong t he c r e s t a l pl a t e a u t ha t a ppe a r a s br i ght l y r e f l e c t e d s c a l l ope d f e a t ur e s on t he s i de s c a n i m a ge s ( F i gur e 1 3 ) E a c h f l ow uni t i s pr e s u m e d t o r e pr e s e nt e i t he r a s i ngl e e r up t i ve uni t or s e ve r a l l a v a f l ow s t ha t a r e c l os e l y r e l a t e d i n t i m e a nd s pa c e T he body of f l ow uni t i s c om pos e d of m a i nl y s he e t a nd l oba t e f l ow s w hi l e pi l l ow l a va s c om pr i s e t he f r ont s of t he f a r t he s t r e a c hi ng f l ow s ( F i gur e 1 4) M or e t ha n 25 i nd i vi dua l l obe s w e r e d i r e c t l y obs e r ve d a nd/ or s a m pl e d t o e v a l ua t e t he s e c onj e c t ur e s A t m a ny s i t e s l a va s a m pl e s w e r e c ol l e c t e d f r om t he body a nd t he f l ow f r ont o f a s i n gl e l obe a s w e l l a s f r om a dj a c e nt l obe s A l s o of i nt e r e s t w e r e i nf e r r e d l a va c ha nne l s ys t e m s i de nt i f i e d on m i c r oba t hym e t r y m a ps a s pr e va l e nt non r e f l e c t i ve l i ne a r f e a t ur e s s i t ua t e d l a r ge l y pe r pe ndi c ul a r t o t h e A S C T ( F i gur e 1 5) T he s e c ha nne l s a r e t hought t o r e pr e s e nt di s t r i but a r y c ha nne l s t ha t s e r ve t o m ove l a va s of f a xi s S e ve n s i t e s i n f i ve s e pa r a t e c ha nne l s ys t e m s w e r e e xa m i ne d i n de t a i l a nd s a m pl e d ( di s c us s e d by Soul e e t al [ 2 005] ) L i ke t he f l ow uni t s va r i a bl e

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6 F i gur e 1 2. M i c r oba t hym e t r y m a p s how i ng t he l oc a t i on of A l vi n di ve s dur i ng A T 11 7 a l ong t he E a s t P a c i f i c R i s e T he c ont our i nt e r va l i s ~ 25 m T he bl a c k t r a ns e c t s a l ong or a c r o s s t he a xi a l s um m i t c ol l a p s e t r ough i nc l ude di ve s w he r e s a m pl e s of l a va s w e r e r e c ove r e d. T he nor t he r nm os t c l us t e r of di ve s 3963, 3965, 3973 3974 a nd 3976 r e c ove r e d a t ot a l of 50 s a m pl e s f r om 9 50. 7' 49' N T he m i ddl e c l us t e r of 3966, 3967, 3971, 3976 di ve s r e c ove r e d 24 s a m pl e s f r om 9 44' 43 3' N T he s out he r nm os t c l us t e r of di ve s 3968, 3970 3975 r e c ove r e d 33 s a m pl e s f r om 9 30. 8' 28. 7' N

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7 A F i gur e 1 3. L oc a t i on of l a va s s a m pl e d dur i ng di ve s 3963 a nd 3974. A ) M i c r oba t hym e t r y a nd B ) s i de s c a n s on a r m a ps of f l ow f r ont s s how i ng l oc a t i ons of l a va s a m pl e s c ol l e c t e d by A l vi n dur i ng di ve s 3963 ( num be r s ym bol s E a s t of A S C T ) a nd 3974 ( num be r s ym bol s W e s t of A S C T ) a nd by e a r l i e r r oc k c or e s ( c i r c l e s ) a nd A l vi n di ve s ( s qua r e s ) A s ym bol s c ol or r e f e r s t o t he w t % M gO f or t he s a m pl e T he A S C T i s l oc a t e d i n t he c e nt e r of e a c h m a p. D i ve 3963 s a m pl e d a f l ow f r ont ( s a m pl e s 10 a nd 11) a nd t hr e e f l ow un i t s : ( 1) s a m pl e s 7 a nd 9, ( 2) s a m pl e s 4, 5, a nd 8, a nd ( 3) s a m pl e s 3 a nd 6. D i ve 3974 s a m pl e d t hr e e f l ow uni t s : ( 1) s a m pl e s 10 a nd 11, ( 2) s a m pl e s 5, 6, a n d 9, a nd ( 3) s a m pl e s 1 5 C ) P hot om os a i c of f l ow f r ont f r om d i ve 3963.

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8 B C F i gur e 1 3 C ont i nue d

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9 A B C F i gur e 1 4. T ypi c a l l a va m or phol ogi e s obs e r ve d a t t he E a s t P a c i f i c R i s e A ) R opy s he e t l a va s B ) L oba t e l a va s C ) D e c or a t e d p i l l ow l a va s

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10 A F i gur e 1 5 L oc a t i on of l a va s s a m pl e d dur i ng di ve 3968. A ) M i c r oba t hym e t r y a nd B ) s i de s c a n s ona r m a p s c ha nne l s s how i ng l oc a t i ons of l a va s a m pl e s c ol l e c t e d by A l vi n dur i ng di ve 3968 a nd by e a r l i e r r oc k c or e s ( c i r c l e s ) a nd A l vi n di ve s ( s qua r e s ) A s ym bol s c ol or r e f e r s t o t he w t % M gO f or t he s a m pl e T he A S C T i s l oc a t e d on t he r i ght ha nd s i de o f e a c h m a p. S a m pl e s 6 10 c om e f r om t he nor t he r n m os t c ha nne l w hi l e s a m pl e s 1 5 w e r e t a ke n f r o m t he s out he r n c ha nne l C ) P hot om os a i c of a c ha nne l f r om di ve 3968

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1 1 B C F i gur e 1 5 C ont i nue d

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12 l a va m or phol ogy pr om pt e d t he s ys t e m a t i c s a m pl i ng a c r os s a nd a l ong t he s i de s of t he c ha nne l D i s c r e t e pi l l ow m ounds l oc a t e d on t he c r e s t a l pl a t e a u w e r e a l s o i nve s t i ga t e d due t o t he i r pos s i bl e of f a xi s o r i gi ns ( F i gur e 1 6) I n t hi s i nve s t i ga t i on, onl y f our di ve s a nd t he ba s a l t s s a m pl e d dur i ng t hos e di ve s a r e di s c u s s e d i n de t a i l : 3963 a nd 3974 ( f l ow uni t s ) 3968 ( t w o c ha nne l s ) a nd 3970 ( t w o of f a xi s m ounds ) H i s t or i c a l l y, m os t of t he s ubm e r s i bl e di ve s a nd r oc k s a m pl i ng i n t he 9 10 N E P R r e gi on ha ve be e n c on c e nt r a t e d i n a nd a dj a c e nt t o t he A S C T [ P e r f i t and C hadw i c k 1 998; K ur r as e t al 2000] H ow e ve r di ve s 3963 a nd 3974 be ga n ov e r 2 km of f a xi s ne a r 9 50 N a nd pr oc e e de d t ow a r ds t h e a xi s s a m pl i ng a c ont i nuum o f f l ow uni t s i n or de r t o e va l ua t e i nt r a a nd i nt e r f l ow uni t c he m i c a l a nd p e t r ogr a phi c va r i a bi l i t y of s pa t i a l l y r e l a t e d f l ow s ( F i gur e 1 3) B ot h di v e s s a m pl e d pa r t s of s i x f l ow uni t s t ha t w e r e di s t i ngui s he d by bot h c h a nge s i n m or phol ogy a nd t opogr a phi c r e l i e f a t t he f r ont of e a c h f l ow uni t ( F i gu r e 1 7) T he di vi s i on of f l ow uni t s i s l e s s c l e a r f or di ve 3974 d ue t o a s l i ght l y c ha ot i c t r a c k. T w o c ha nne l s w e r e e xt e n s i ve l y s a m pl e d ne a r 9 29 N dur i ng di v e 3968. L a va s a m pl e s f r om t he c e nt e r t r a ns i t i on z o ne a nd m a r gi n of t he s out he r n c ha nne l w e r e r e c ove r e d f r om onl y one l oc a l i t y ~ 450 m f r om t he a xi s w he r e a s t h e c e nt e r ( a nd m a r gi n i n one c a s e ) of t he m or e nor t he r n c ha nne l w a s s a m pl e d a t 300, 450, a nd 750 m f r om t he a xi s ( F i gur e 1 5) L oba t e f l ow s c om pos i ng t he s ur r oundi ng t e r r a i n w e r e a l s o s a m pl e d. T he f i na l di ve of i nt e r e s t ( 3970) s a m pl e d t w o pi l l ow m ound s l oc a t e d 1. 5 km f r om t he s pr e a di ng a xi s b e t w e e n 9 29 30 N a nd 9 31 N ( F i gur e 1 6) S e ve r a l pi l l ow l a va s f r om e a c h c on s t r uc t i ona l m ound w e r e s a m pl e d i n a ddi t i on t o s e ve r a l f i s s ur e f i l l i ng l a va s a nd s ur r oundi ng ol de r l oba t e a nd ha c kl y l a va f l ow s

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13 F i gu r e 1 6 L oc a t i on of l a va s s a m pl e d dur i ng di ve 3970. A ) M i c r oba t hym e t r y a nd B ) s i de s c a n s ona r m a p s of of f a xi s m ounds s how i n g l oc a t i ons of l a va s a m pl e s c ol l e c t e d by A l vi n dur i ng di ve 3970 ( num be r s ym bol s ) a nd by e a r l i e r r oc k c or e s ( c i r c l e s ) a nd A l vi n di ve s ( s qua r e s ) A s ym b ol s c ol or r e f e r s t o t he w t % M gO f or t he s a m pl e O n t he l e f t s i de of e a c h m a p, t he A S C T i s de not e d by t he ne a r ve r t i c a l t r e nd i n r oc k c or e a nd A l vi n s a m pl i ng. T he nor t he r nm os t pi l l ow m ound c ons i s t s of s a m pl e s 4, 5, a nd 6 w h i l e t he s out he r nm os t pi l l ow m ound c ons i s t s of s a m pl e s 10 a nd 11.

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14 F i gur e 1 7. B a t hym e t r i c pr of i l e f or di ve 3963 E a c h f l ow f r ont s a m pl e d dur i ng di ve 3963 a nd di ve 3974 a r e m a r ke d by not onl y a m or phol o gi c a l c ha nge i n t he l a va s but a l s o a c ha nge i n t opogr a phy. T he a xi a l s um m i t c ol l a ps e t r ough ( A S C T ) i s t o t he w e s t

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15 C H A P T E R 2 B A C K G R O U N D C on s t r u c t i on of O c e an i c C r u s t at t h e F as t S p r e ad i n g E as t P ac i f i c R i s e M a gm a t i c pr oc e s s e s f oc us e d be ne a t h M O R s a r e r e s pons i bl e f or c r e a t i ng a m ul t i l a ye r e d c r us t a l s t r uc t ur e w i t hi n t he f i r s t f e w km of t he s pr e a di ng a xi s T he t hr e e m a i n s e i s m i c a l l y de f i ne d l a ye r s c om pr i s i ng oc e a ni c c r us t i nc l ude s e di m e nt s ( L a ye r 1) c ove r i ng e xt r ude d ba s a l t s ( L a ye r 2A ) a t t he t op, a s he e t e d di ke c om pl e x i n t he m i ddl e ( L a ye r 2B ) a nd ga bbr os w i t h s om e c um ul a t e ul t r a m a f i c r oc ks a t t he bot t om ( L a ye r 3) M uc h of our unde r s t a ndi ng r e ga r di ng t he c om pos i t i on a nd f or m a t i on of t hi s oc e a ni c c r us t a l l i t hol ogy ha s be e n obt a i ne d f r om ge ophys i c a l r e s ul t s r e l a t e d t o ge ol ogi c obs e r va t i ons D e e p dr i l l hol e s ( e g. O D P H ol e 504B ) i nt o oc e a ni c c r us t o phi ol i t e s be l i e ve d t o r e pr e s e nt s l i c e s of oc e a ni c c r us t pr e s e r ve d on l a nd [ e g. C ann 1974; N i c ol as 1989] a nd t e c t oni c w i ndow s a l ong r i f t a nd t r a ns f or m z one s s uc h a s t he H e s s D e e p R i f t a nd B l a n c o T r a ns f or m s c a r p [ K ar s on e t al 2002a 2002b; St e w ar t e t al 2005] pr ovi de u ni que pe r s pe c t i ve s i nt o t he c r os s s e c t i ona l s t r uc t ur e a nd c om pos i t i on of t he uppe r m os t oc e a ni c c r us t T he s e di r e c t obs e r va t i ons c om bi ne d w i t h s e i s m i c s t udi e s [ D e t r i c k e t al 1987; T oom e y e t al 1990 1994; H ar di ng e t al 1993; V e r a and D i e bol d 1993; C hr i s t e ns on e t al 1994 1996; H oof t e t al 1996; C r aw f or d and W e bb 2002] a nd de t a i l e d s a m pl i ng a nd m a ppi ng of l a va s f r om t he E P R [ M ac donal d and F ox 1988; H ay m on e t al 1993; P e r f i t e t al 1994; F or nar i and E m bl e y 1995] ha ve be e n us e d t o be t t e r unde r s t a nd M O R m a gm a t i c a nd vol c a ni c pr oc e s s e s

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16 L a va s f ound a t t he E P R a r e pr i m a r i l y l ow pot a s s i um t hol e i i t i c ba s a l t s ot he r w i s e r e f e r r e d t o a s m i d oc e a n r i dge ba s a l t s ( M O R B ) T he y a r e f e d f r om s om e t ype of m a gm a c ha m be r a t de pt h t o t he s ur f a c e by di ke s M a g m a s f e d by di ke s t ha t do not e r upt ont o t he s e a f l oor a s gl a s s y t o m i c r oc r ys t a l l i ne s he e t l oba t e a nd pi l l ow l a va s s ol i di f y t o f o r m t he s he e t e d ba s a l t i c di ke s t ha t c om pr i s e s e i s m i c L a ye r 2B D i ke s s t e m f r om t he m a gm a c ha m be r a nd t r a ns por t m a gm a s bot h ve r t i c a l l y t o t he A S C T a s w e l l a s l ong di s t a nc e s l a t e r a l l y a l ong t he s pr e a di ng a xi s [ D z i ak e t al 1995; F ox e t al 1995; E m bl e y e t al 2000] A l a r ge f ul l y or m os t l y l i qui d m a gm a c h a m be r t ha t e xt e nds f r om t he bot t om of L a ye r 2 dow n t o t he M oho i s no l onge r be l i e ve d t o e xi s t a t M O R [ e g. C ann 1974] I ns t e a d, s e i s m i c r e f l e c t i on a nd r e f r a c t i on da t a i n di c a t e a 1 2 km w i de t hi n m e l t l e ns ( < 100 m t hi c k) e xi s t s a t de pt hs ~ 1. 5 km a bove a l a r ge r c r ys t a l m us h z one 6 7 km w i de [ D e t r i c k e t al 1987; T oom e y e t al 1990 1994; Si nt on and D e t r i c k 1992; H ar di ng e t al 1993; V e r a and D i e bol d 1993; H oof t e t al 1996; C ar bot t e e t al 1997] S ol i di f i c a t i on of t he c om pos i t e m a gm a c ha m be r on e i t he r s i de of t he a xi s f or m s t he ga bbr oi c r oc ks of L a ye r 3 ( 5 km t hi c k) w i t h c um ul a t e ul t r a m a f i c r oc ks a t i t s b a s e A s e c ond, de e pe r m e l t l e ns l oc a t e d a t t he M oho be l ow t he m us h z one ha s a l s o be e n i de nt i f i e d [ G ar m any 1989 ; D unn and T oom e y 19 97; C r aw f or d e t al 1999] C r aw f or d and W e bb [ 2002] de ve l ope d i nt e r pr e t i ve c r os s s e c t i ons of t he s uba xi a l c r us t f or t hr e e a r e a s of t he E P R ( 9 48 N 9 3 3 N a nd 9 08 N ) ba s e d on p r e vi ous s t udi e s r e ga r di ng t he pl a c e m e nt a nd s i z e of t he a xi a l m e l t l e ns [ K e nt e t al 1993a 1993b] t he di s t r i bu t i on of m e l t i n t he l ow ve l oc i t y c r ys t a l m us h z one [ D unn e t al 2000; T oom e y e t al 1990] a nd t he pr e s e nc e of m e l t n e a r t he M oho [ D unn e t al 2001] ( F i gur e 2 1 ) A t t he no r t he r n e nd of t he 9 10 N s e gm e nt of t he E P R t he m e l t l e ns i s na r r ow

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17 F r om C r aw f or d and W e bb [ 2002] F i gur e 2 1. I nt e r p r e t a t i ve c r os s s e c t i on of t he s ub a xi a l c r us t a l s t r uc t ur e be ne a t h t he E a s t P a c i f i c R i s e N ot e t he c ha ngi ng l oc a t i ons a nd s i z e s of t he a xi a l m a gm a l e ns a bove i t s a s s oc i a t e d m u s h z one ( pa r t i a l m e l t a r e a ) f r om 9 48 08 N T he pr e s e nc e of m e l t s i l l s a t t he M o ho i s i nt e r m i t t e nt a l ong t hi s s e c t i on of t h e r i dge

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18 a nd c e nt e r e d a t t he s pr e a di ng a xi s but be c om e s w i de r a nd of f s e t t o t he w e s t of t he r i dge a xi s c l os e r t o t he ove r l a ppi ng s pr e a di ng c e nt e r ( O S C ) a t 9 03 N P a s t t he O S C t he r i dge a xi s i s onc e a ga i n a l i g ne d w i t h a na r r ow m e l t l e ns R e gi on al G e ol ogy o f T h e 9 10 N s e gm e n t of t h e E as t P ac i f i c R i s e T he f a s t s pr e a di ng ( 110 m m / yr ) E P R be t w e e n 9 10 N i s bounde d by t he C l i ppe r t on T r a ns f or m F a ul t a t 10 10 N a nd a l a r ge ove r l a ppi ng s pr e a di ng c e nt e r ( O S C ) a t 9 03' N ( F i gu r e 1 1 ) A xi a l l i ne a r i t y i s di s r upt e d a t 9 37' N by a s m a l l e r O S C w hi c h Sm i t h e t al [ 2001] i nt e r p r e t t o be bot h a hyd r ot he r m a l a nd vol c a nol ogi c a l di vi de f o r a r e a s nor t h a nd s out h of i t I n a ddi t i on t o de t a i l e d m a ppi ng a nd num e r ous i n s i t u obs e r va t i ons t hi s s e gm e nt of t he E P R r e m a i ns one of t he m os t s ys t e m a t i c a l l y s a m pl e d s e gm e nt s of a l l M O R s [ F or nar i e t al 1990, 1992, 1994, 1998; B at i z a and N i u 1992; H ay m on e t al 1993; P e r f i t e t al 1994 ; F or nar i and E m bl e y 199 5; G r e gg e t al 1995, 1996 ; P e r f i t and C hadw i c k 1998; Sm i t h e t al 2001 ] M or e t ha n 1200 r oc k s a m pl e s ha ve be e n c ol l e c t e d f r om bot h t he r i dge a xi s a nd r i dge f l a nks by m e a ns of r oc k c o r e d r e dge a nd s ubm e r s i bl e H ow e ve r m os t of t he s e s a m pl e s c om e f r om w i t hi n or c l os e t o t h e A S C T w he r e t he t ow e d ve hi c l e A R G O a nd t he s ubm e r s i bl e A L V I N w e r e us e d t o i nve s t i ga t e t he vol c a ni c t e c t oni c a nd hydr ot he r m a l f e a t ur e s a c t i ve w i t hi n t he r i dge a xi s [ H ay m on e t al 1991 1993; W r i ght e t al 1995a 1995b; K ur r as e t al 2 000] T he A S C T be t w e e n 9 22 N a nd 9 51 N i s a s i nuous a nd ne a r l y c ont i nuous f e a t ur e of t he r i dge a xi s w i t h w i dt h t ha t va r i e s f r om ~ 4 0 m t o 100 m w i de B e t w e e n 9 37 N a nd 9 50 N a s m oot h, br oa d, a nd r ounde d b a t hym e t r i c pr of i l e i ndi c a t e s a r obus t m a gm a t i c hi s t or y f or t hi s por t i on of t he r i dge ( F i gur e 2 2 ) [ Sc he i r e r and F ox 1993; P e r f i t and C hadw i c k 1998; F or nar i e t a l 1998; S m i t h e t al 2001] H e r e t he A S C T i s

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19 A 0 500 1000 1500 2000 2500 3000 3500 4000 4500 Distance (m) Axis -2520 -2540 -2560 -2580 -2600 -2620 -2640 -2660 B 0 1000 2000 3000 4000 5000 Distance (m) Axis -2600 -2620 -2640 -2660 -2680 -2700 F i gur e 2 2. C r os s a xi a l pr of i l e s f or t w o s e c t i ons of t he E a s t P a c i f i c R i s e A ) s how s t he s m oot h, br oa d pr o f i l e of t he E P R a t 9 50 N i s a t ypi c a l of m a gm a t i c a l l y r obus t s e c t i on s w hi l e B ) s how s t he c r os s a xi a l pr of i l e a t 9 28 N E P R ha s a gr e a t e r i nf l ue nc e by t e c t oni s m P r of i l e s w e r e c ons t r uc t e d us i ng C oc h r an e t al 1999 m i c r oba t hym e t r y. E a c h pr o f i l e i s s how n w e s t t o e a s t na r r ow e s t ( ~ 40 80 m w i de ) w i t h s t e e pl y s l opi ng ( > 60 ) t o ne a r ve r t i c a l s i de w a l l s l e s s t ha n 15 m hi gh. R e c e nt vol c a ni c a c t i vi t y ( A pr i l t o M a y, 1991) w i t hi n t he A S C T be t w e e n 9 46 N a nd 9 51 N w a s i de nt i f i e d a nd doc um e n t e d by H ay m on e t al [ 1993] a nd da t e d by R ubi n e t al [ 1994] T he s m a l l vol um e e r upt i ons a ppe a r e d t o e m a na t e f r om a n 8. 5 km l ong, 1 4 m w i de f i s s ur e i n t he a xi a l f l oor [ F or nar i e t al 1994; L ut z e t al 1994] S i nc e t he s e r e c e nt vol c a ni c e ve nt s ne w di f f us e a nd hi gh t e m pe r a t ur e hydr ot he r m a l ve nt i ng ha s c om m e nc e d [ V on dam m e t al 1992, 1995 ; H ay m on e t al 1993] T he s t r a t i gr a phi c r e l a t i ons hi ps a nd m or pho l ogi e s of l a va s on t he f l oor a nd s i de s of t he A S C T r e f l e c t m odi f i c a t i on of a nd de pos i t i on by, t he l a s t vo l c a ni c e ve nt ( F i gu r e 2 3 ) B ot h l a va pi l l a r s

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20 A F r om P e r f i t and C hadw i c k [ 1998 ] B C F i gur e 2 3 T ypi c a l l a va m o r phol ogi e s a nd s t r a t i gr a phi c pos i t i ons w i t hi n t he a xi a l s um m i t c ol l a ps e t r ough a t t he E a s t P a c i f i c R i s e A ) A n a r t i s t s r e ndi t i on of t he A S C T a t t he E P R i s b a s e d on A l vi n obs e r va t i ons of B ) c ol l a ps e d f e a t ur e s C ) l a v a pi l l a r s a nd D ) a nd E ) c om pl e x s t r a t i gr a phi c r e l a t i ons w hi c h of t e n r e f l e c t t he m os t r e c e nt e pi s ode of vol c a ni s m

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21 D E F i gur e 2 3 C ont i nue d 5 8 m t a l l a nd l a va t ha t br e a c he d t he r i m o f t he A S C T w e r e us e d t o e s t i m a t e t he t hi c kne s s of t he 1991 f l ow be f or e a l l but 2 3 m of l a va dr a i ne d ba c k i nt o t he f i s s ur e T he r e m a i ni ng l a va w a s dom i na nt l y c om pos e d of c om pl e x s he e t f l ow s ( r opy, pl a t y a nd j um bl e d va r i e t i e s ) e xhi bi t i ng e xt e ns i ve c ol l a ps e f e a t ur e s [ H ay m on e t al 1993; V on D am m e t al 1995; G r e gg e t al 1996] I n c ont r a s t a l e s s r obus t m a gm a t i c por t i on o f t he r i dge be t w e e n 9 30 N a nd 9 35 N i s m a r ke d by a de e pe r a xi a l t r ough m o r e e vol ve d a nd ol de r l ooki ng l a va s a nd t e c t oni c a l l y dom i na t e d a xi a l m or phol ogy ( F i gur e 2 2 ) [ F or nar i e t al 1998 ; P e r f i t and C hadw i c k 1998; Sm i t h e t al 2001 ] T he A S C T i n t hi s a r e a i s up t o 100 m w i de 10 25 m de e p, a nd i s l oc a t e d a s ym m e t r i c a l l y w i t hi n a n a xi a l gr a be n 250 350 m w i de [ F or nar i e t al 1 998] F r a c t ur i ng of t he younge s t f l ow s a t t e s t s t o t he t e c t oni c dom i na nc e i n a n ot he r w i s e m a gm a t i c a l l y r obus t r e gi on T hr e e t ype s of vol c a ni c e m pl a c e m e nt w e r e r e c ogni z e d by K ur r a s e t al [ 2000] be t w e e n 9 49 N a nd 9 52 N of t he E P R a xi a l r i dge : 1) m a i nl y s he e t a nd l oba t e f l ow s e xt e ndi ng < 500 m f r om t he A S C T ; 2) c h a nne l i z e d l a va f l ow s t r a n s por t e d of f a xi s f r om t he s i t e of e r upt i on a t or n e a r t he A S C T ; a nd 3) l a va s e r upt e d of f a xi s ( 0. 5 t o 1. 5 km )

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22 r e s ul t i ng i n t he c ons t r uc t i on of pi l l ow m ounds a nd r i dge s I n ge ne r a l t he s he e t a nd l oba t e f l ow s c l os e t o t he A S C T be t w e e n 9 30 N a nd 9 51 N ( 50 100 m f r om a xi s ) s how e xt e ns i ve c ol l a ps e f e a t ur e s i nt e r m i ngl e d w i t h l a va t ube s a nd c ha nne l s r e s ul t i ng f r om t he e f f us i ve na t ur e of t he s e l a va m or phol ogi e s [ G r i f f i t hs and F i nk 1992; G r e gg and F i nk 1995, E nge l s e t al 2003] T he l a va s a r e a l s o m or e m a f i c a nd ha ve f e w e r phe noc r ys t s t ha n f l ow s f ur t he r a w a y f r om t he A S C T [ K ur r as e t al 2000] T he of f a x i s pi l l ow m ounds ne a r 9 31 N E P R f or m pi l l ow r i dge s 10 15 hi gh a nd 50 m w i de pa r a l l e l i ng t he a xi s [ P e r f i t e t al 1994; Si m s e t al 2003 ] W he n c om pa r e d t o on a xi s l a va s t he of f a xi s f l ow s ne a r 9 31 N a nd 9 50 N a r e c om pos i t i ona l l y m or e di ve r s e w i t h s ubs t a nt i a l i nt e r f l ow ge oc he m i c a l va r i a t i ons ove r s m a l l s pa t i a l s c a l e s of l e s s t ha n 600 m [ P e r f i t e t al 1994] O t he r a r e a s of t he E P R s pe c i f i c a l l y 11 20 N a nd 12 13 N pos s e s s of f a xi s l a va s s how i ng s i m i l a r c om pos i t i ona l c om pl e xi t y a nd di ve r s i t y [ H e k i ni an e t al 1989 ; R e y nol ds e t al 1992] O f f a xi s vol c a ni s m i s t hought t o be a s s oc i a t e d w i t h t he de ve l opm e nt of nor m a l f a ul t i ng a nd t he i ni t i a t i on of f i s s ur i ng pa r a l l e l t o t he s pr e a di ng a xi s [ L uy e ndy k 1970; L ons dal e 1977; M ac donal d 1982; P e r f i t e t a l 1 994; A l e x ande r and M ac donal d 1996; M ac donal d e t al 1996] T he r e gi ona l a nd l oc a l s t r e s s e s r e s pons i bl e f or br i t t l e c r us t a l f a i l ur e a nd t he r e s ul t i ng r ol l i ng t opog r a phy of t he a bys s a l hi l l s 2 6 km f r om t he a xi s a r e r e l a t e d t o a va r i a bl e m a gm a t i c s up pl y [ E dw ar d s e t al 1992] m ove m e nt of t he c r us t a w a y f r om t hi s s uppl y a nd t he a s s o c i a t e d c ool i n g of t he l i t hos phe r e [ L ons dal e 1977; Se ar l e 1984; G of f 1991; G of f e t al 1993] ne a r by di s c ont i nui t i e s i n t he r i dge a xi s [ P ol l ar d and A y di n 1984; Se m pe r e and M ac dona l d 1986] o r be ndi ng a nd s na ppi ng of t he l i t hos phe r e [ Sohn and Si m s 2005] A c c or di n g t o A l e x ande r and M ac donal d [ 1996]

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23 80% of f a ul t s c a r ps t ha t f a c e t he r i dge a xi s ( i nw a r d f a c i ng) s t op l e ngt he ni ng a nd r e a c h f i na l he i ght s of 60 70 m w i t hi n 30 k m of t he a xi s A l t e r na t i ve l y, m or e t ha n 95% of a l l f a ul t s c a r ps f a c i ng a w a y f r om t he r i dge a xi s ( out w a r d f a c i ng) r e a c h a f i na l he i ght of ~ 60 m w i t hi n 5 7 km of t he a xi s S ubm e r s i bl e obs e r va t i ons of t he out w a r d f a c i ng f a ul t s c a r ps r e ve a l s l ope s of ~ 30 a nd e vi de n c e of r e c e nt vol c a ni s m i n t he f or m of dr a pe d l a va f l ow s a nd f i s s ur e f i l l i ng pi l l ow s [ E dw ar ds e t al 1992; M ac donal d e t al 1996; K ur r as e t al 2000] F e e de r di ke s m os t l i ke l y r e pr e s e nt t he s our c e s of t he s e s ynt e c t oni c f e a t ur e s a s i nt e r pr e t e d f r o m na r r ow B ougue r gr a vi t y h i ghs m e a s ur e d a bove pi l l ow r i dge s f or m e d ove r f i s s ur e s [ C oc hr an e t al 1999] I n a ddi t i on, s e ve r a l l a va s c ol l e c t e d f r om s i t e s of of f a xi s m ound c ons t r uc t i on a r e m or e c he m i c a l l y e vol ve d, a l ka l i e nr i c he d t r a ns i t i ona l M O R B ( T M O R B ) w hi c h s ugge s t s t he s our c e of t he di ke s t o be di f f e r e nt t ha n t ha t of t he di ke s a t t he a xi s [ P e r f i t e t al 1994; P e r f i t and C hadw i c k 1998]

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24 C H A P T E R 3 A N A L Y T I C A L M E T H O D S N a t ur a l gl a s s e s f r om t he out e r que nc he d s ur f a c e s of l a va s w e r e s e pa r a t e d dur i ng de s c r i pt i on a nd phot ogr a phi ng f or e a c h of t he s a m pl e s dur i ng t he c r ui s e T he gl a s s c hi ps a nd r e m a i ni ng ha nd s pe c i m e ns w e r e br ought ba c k t o t he U ni ve r s i t y of F l or i da t o pr oc e s s f or m a j or a nd t r a c e e l e m e nt a na l ys i s A ppr oxi m a t e l y 100 m g of t he c l e a ne s t gl a s s f r om e a c h s a m pl e w a s ha ndpi c k e d us i ng a bi noc ul a r m i c r os c ope V e s i c l e s c r i s t ol i t e s c a r bona t e a nd M n c oa t i ngs w e r e a voi de d a s m uc h a s pos s i bl e T o r e m ove a ny r e m a i ni ng s ur f i c i a l c oa t i ngs t he s ub s a m pl e s w e r e e ns oni f i e d t hr e e t i m e s i n 2X H 2 O f o r f i ve m i nut e s e a c h, a n a ddi t i ona l f i ve m i nut e s i n a m i xt ur e c ont a i ni ng 100 m l H 2 O 2 80 m l 2X H 2 O a nd 20 m l 12N H C L a nd f i na l l y t hr e e f i v e m i nut e s e s s i on s i n 4X H 2 O A f t e r c om pl e t i ng t he e ns oni f i c a t i on p r oc e s s s a m pl e s w e r e dr i e d unde r a he a t l a m p. O nc e dr y, s e ve r a l of t he f r e s he s t gl a s s c hi ps f r om e a c h of t he 111 s a m pl e s w e r e m a de i nt o pol i s he d t hi n s e c t i on s a nd s ubs e que nt l y w e r e a na l yz e d f or m a j or e l e m e nt s by I a n R i dl e y a t t he U S G S M i c r obe a m L a bor a t or y us i ng a J E O L 8900 E l e c t r on M i c r opr obe A na l ys i s of s e ve n t o t e n s e p a r a t e poi nt s ( i nc l udi ng s pot s on s e pa r a t e c hi p s of t he s a m e s a m pl e ) w e r e a ve r a ge d f or e a c h s a m pl e a nd t he n c or r e c t e d f or i ns t r um e nt d r i f t by a ppl yi ng a c or r e c t i on f a c t or ba s e d on t he nor m a l i z a t i on of t he m e a s ur e d va l ue s t o e s t a bl i s he d va l ue s f or t he i n hous e s t a nda r ds J dF D 2 a nd 2392 9 ( s e e Sm i t h e t al [ 2001] ) S i O 2 w a s a l s o c or r e c t e d f or e r r o r due t o a ut o f oc us i ng of t he e l e c t r on be a m a c c or di ng t o t he m e t hod s of R e y nol ds and L angm ui r [ 1997] C or r e c t i ons w e r e t ypi c a l l y l e s s t ha n 1. 22 r e l a t i ve % of t he m e a s ur e d va l ue T he 2 s i gm a e r r or s ( pr e c i s i on) c a l c ul a t e d

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25 f r om va r i a t i on i n t he a na l ys e s of 2392 9 dur i ng t he s e a na l yt i c a l r uns a r e a s f ol l ow s : S i O 2 ( 0. 29 w t % o r 0. 57 r e l a t i ve % ) T i O 2 ( 0 07 w t % or 5. 4% ) A l 2 O 3 ( 0 17 w t % o r 1. 1% ) F e O ( 0. 17 w t % o r 1. 8 % ) M nO ( 0. 0 5 w t % or 26 8% ) M gO ( 0 11 w t % or 1 3% ) C a O ( 0. 14 w t % or 1. 2 % ) N a 2 O ( 0. 09 w t % or 3 6% ) K 2 O ( 0. 02 w t % or 18. 8 % ) a nd P 2 O 5 ( 0. 04 w t % or 29. 8% ) A ddi t i ona l s a m pl e s pl i t s ( ~ 40 m g) of t he f r e s he s t gl a s s f r om 48 of t he s a m pl e s ( f r om f our di ve s ; 3963, 3968 3970, 3974 ) w e r e s e l e c t e d f or t r a c e e l e m e nt a na l ys i s b y I C P M S A n i n de pt h de s c r i pt i on of t he di s s ol ut i on a nd a n a l yt i c pr oc e dur e i s pr ovi de d i n A ppe ndi x A S e ve r a l c e r t i f i e d r oc k s t a nda r d s ( A G V 1 a nd B C R 2) a nd t w o i n hous e M O R B s t a nda r ds ( E N D V a nd 2392 9) w e r e us e d t o c a l i br a t e t he m a c hi ne w hi l e r e pe a t e d c he m i c a l a na l y s e s of t he e nr i c he d M O R B E N D V w a s us e d t o e v a l ua t e a nd c or r e c t f or i ns t r um e nt dr i f t P r e c i s i on w a s f ound t o be be t t e r t ha n 1% ( r e l a t i ve t o t he c onc e nt r a t i on) f or L a N d; 2% f o r Z r D y, Y b; 3% f o r C o, G a S m E u, G d; 4% V N i C u, Y N b, C e T b, H o H f T h; 5% S c R b, S r B a P r T m L u P b; 6% f or E r T a ; 7% C r U ; a nd 9% f or Z n A c c ur a c y w a s e va l ua t e d us i ng t he va l ue s w e m e a s ur e d f or B H V O 1. A di s c us s i on of a c c ur a c y a nd pr e c i s i on ( e r r or ) f o r t hi s s t udy i s a va i l a bl e i n A ppe ndi x B T hi r t y f i ve l a va s w e r e s e l e c t e d f or pe t r ogr a phi c a na l ys i s us i ng t hi n s e c t i ons t ha t i nc l ude d t he out e r gl a s s y m a r gi ns a s w e l l a s t he m or e c r ys t a l l i ne por t i ons of e a c h s a m pl e T he y w e r e e xa m i ne d w i t h a pe t r ogr a phi c m i c r os c ope t o de s c r i be t he m i ne r a l ogy a nd c ot e c t i c r e l a t i ons hi ps a s w e l l a s e s t i m a t e t he m od a l a bunda nc e s t e xt ur a l pr opor t i ons a nd ve s i c ul a r i t y ( by poi nt c ount i ng) R e pr e s e nt a t i ve phot om i c r ogr a phs f r om s e ve r a l of t he t hi n s e c t i ons w e r e t a ke n. T he c om pos i t i ons of ol i vi ne pl a gi oc l a s e a nd c l i nopyr oxe ne i n

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26 t w o t hi n s e c t i ons 3970 9 a nd 3974 11 w e r e a na l yz e d by M i c ha e l P e r f i t us i ng t he J E O L 8900 E l e c t r on M i c r opr obe a t t he U S G S i n D e nve r P r e c i s i on w a s e s t i m a t e d f r om m ul t i pl e m a j or e l e m e nt a na l ys e s of gl a s s i n s a m pl e s 3970 9o, 3970 9i a nd 3974 11 F o r t he A n# ( ( C a / ( C a + N a ) ) *100) a nd M g# ( ( M g/ ( M g+ F e ) ) *100) pr e c i s i on w a s 0. 76 a nd 1. 04 r e l a t i ve % r e s pe c t i ve l y. P hot ogr a phs obt a i ne d by t he s ubm e r s i bl e A L V I N a nd t he R a bbi t C a m t ow e d c a m e r a s ys t e m di ve t r a ns c r i pt s A B E m i c r oba t hy m e t r y, a nd D S L 120A s i de s c a n s ona r i m a ge s w e r e us e d i n c onj unc t i on w i t h t he m a j or a n d t r a c e e l e m e nt a na l ys e s t o c a t a l og a nd m a p: 1) t he phys i c a l e xt e nt of l a va f l ow s i nc l udi n g f l ow f r ont s a nd c ha nne l s a nd of f a xi s m ounds ; 2) t he di s t r i but i on of l a va m or phol ogi e s a nd r e l a t e d c ont a c t r e l a t i ons hi ps ; 3) c he m i c a l va r i a t i ons w i t hi n a nd be t w e e n f l ow s i n c l udi ng pos s i bl e ge ne t i c r e l a t i on s hi ps ; a nd 4) r e l a t i ve c r us t a l a ge s ba s e d on s e di m e nt c ove r a nd i nf i l l C om pi l e d i nt o A R C / G I S t hi s i nf or m a t i on w a s us e d t o c ons t r uc t ge ol ogi c m a ps a nd ba t hym e t r i c pr of i l e s f or t he f our di ve s of i nt e r e s t : 3963 a nd 3974 ( f l ow f r ont s ) 3968 ( c ha nne l s ) a nd 3970 ( of f a xi s m ounds )

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27 C H A P T E R 4 P E T R O G R A P H Y P e t r ogr ap h y T he 31 s a m pl e s s e l e c t e d f or pe t r ogr a phi c s t udy w e r e c ol l e c t e d dur i ng A L V I N di ve s t o t he t hr e e m a i n s i t e s of i nt e r e s t a l ong t he 9 10 N s e gm e nt of t he E a s t P a c i f i c R i s e ( E P R ) : a s e r i e s of f l ow f r ont s on e i t he r s i de of A S C T a t 9 50 30 N t w o l a v a c ha nne l s a t ~ 9 29 N E P R a nd a s e r i e s of o f f a xi s pi l l ow m ounds a t 9 30 N T he s a m pl e s r a nge f r om m a f i c t o m ode r a t e l y e vol ve d ( 7. 23 8. 91 w t % M gO ) a nd a l l ha ve de pl e t e d i nc om pa t i bl e e l e m e nt c onc e nt r a t i ons ( N M O R B ) A t ot a l of 35 t hi n s e c t i ons w e r e c ut f r om t he out e r gl a s s y r i nds dow n t hr ough t he m or e c r ys t a l l i ne i nt e r i or s of t he 31 l a va s ( F i gur e 4 1) C om pos e d of s he e t f l ow s l ob a t e f l ow s a nd pi l l ow ba s a l t s t he l a va s w e r e c ol l e c t e d up t o 2. 17 km f r om t he A S C T a nd a r e m a i nl y a phyr i c t o s pa r s e l y pl a gi oc l a s e phyr i c ba s a l t s ( F i gur e 4 2) A c om pr e he ns i ve de s c r i pt i on f or e a c h pe t r ogr a phi c a l l y e xa m i ne d s a m pl e i s p r ovi de d i n T a bl e s 4 1 t o 4 4. F l ow U n i t s Wi t h F r on t s D i ve s 3963 a n d 3974 T he f l ow s s a m pl e d f r om bot h s i de s of t he A S C T a t 9 50 30 N E P R a r e m a i nl y a phyr i c t o s pa r s e l y pl a gi oc l a s e phyr i c pl a gi oc l a s e ba s a l t s ( T a bl e 4 1 a nd 4 2) O nl y t w o pi l l ow ba s a l t s c ol l e c t e d f r om f l ow f r ont s c ont a i n m or e t ha n 1. 5% m i c r ophe noc r ys t s ( 3963 3, 3. 9 % a nd 3963 10, 1. 72 % ) S ubhe d r a l t o e uhe dr a l C a r l s ba d t w i nne d pl a gi oc l a s e l a t hs ( 0. 1 1 m m ) a l ong w i t h m or e a c i c ul a r ( 0 1 0. 4 m m ) f or m s r e pr e s e nt t he m os t a bunda nt m i c r ophe noc r ys t pha s e ( F i gur e 4 2) I n a ddi t i on t he m a j or i t y o f pl a gi oc l a s e

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28 T a bl e 4 1 T hi n S e c t i on P e t r og r a phy of L a va s C ol l e c t e d d ur i ng A l vi n D i ve 3963 N e a r 9 50 N E a s t P a c i f i c R i s e

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32 T a bl e 4 2 T hi n S e c t i on P e t r ogr a phy o f L a va s C ol l e c t e d D ur i ng A L V I N D i ve 3974 N e a r 9 N E a s t P a c i f i c R i s e

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36 36 T a bl e 4 3 T hi n S e c t i on P e t r ogr a phy o f L a va s S a m pl e d D ur i ng A L V I N D i ve 3968 N e a r 9 N E a s t P a c i f i c R i s e

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37 T a bl e 4 4 T hi n S e c t i on P e t r ogr a phy o f L a va s S a m pl e d D ur i ng A L V I N D i ve 3970 N e a r 9 N E a s t P a c i f i c R i s e

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41 41 A B C F i gur e 4 1. P hot om i c r ogr a phs of c om m on t e xt ur e s f ound i n ba s a l t s c ol l e c t e d ne a r 9 30 a nd 9 51 N E P R T he c om m on gr a da t i ona l s e que nc e of t e xt ur e s s t a r t i ng a t t he out e r gl a s s y r i nd a nd c ont i nui ng i nt o t he m or e c r ys t a l l i ne i nt e r i or of t he s l i de i s a s f ol l ow s : A ) vi t r ophyr i c t r a ns i t i ona l i nt o va r i ol i t i c ( 3963 2 ) B ) ba nds of va r i ol i t e s c a n d e ve l op i n s he e t f l ow s ( 3963 6) C ) a gr a da t i on be t w e e n f a s i c ul a r a nd D ) l e s s c om m on s phe r ul i t i c ( 3963 6) a nd f i na l l y E ) m i c r ol i t i c ( 3963 8 ) r e f e r r e d t o a s F ) t r a c hyt i c i f i ndi vi dua l m i c r ol i t e s a r e a l i gne d ( 3974 4) G ) w hi c h c a n be qui t e c om pl e x ( 3974 1) G ) A gl om e r opor phor i t i c t e xt ur e ( 3970 5) i s f ound t hr oughout t hi s s e que nc e T e xt ur e s pi c t ur e d i n F ) a nd H ) a r e s how n i n c r os s e d pol a r s w hi l e a l l ot he r s a r e pi c t ur e d i n pl a ne pol a r i z e d l i ght T he s c a l e i s t he s a m e f or A ) B ) a nd E ) G ) 1 mm 1 mm

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42 42 D E F G H F i gur e 4 1 C ont i nue d 0. 5 mm mm mm 1 mm 1 mm

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43 43 A B C D E F F i gur e 4 2: P hot om i c r og r a phs of ol i vi ne c l i nopyr oxe ne a nd pl a gi oc l a s e c r ys t a l m or phol ogi e s a l l i n c r os s e d pol a r s A ) R a r e ol i vi ne ( 3963 6) a nd B ) c l i nopyr oxe ne ( 3970 1) xe noc r ys t s a r e out num be r e d by s m a l l e r c r ys t a l s ( < 0. 1 m m ) C ) M os t l y i ndi s t i ngui s ha bl e t he y a r e r e f e r r e d t o a s m a f i c m i c r ophe noc r ys t s unl e s s a n ol i vi ne e xhi bi t s a c ha r a c t e r i s t i c hoppe r f or m ( 3974 8t ) P l a gi oc l a s e c r ys t a l s c on s t i t ut e t he m a j or i t y of m i c r ophe noc r ys t s a nd a ppe a r a s s ke l e t a l m i c r ol i t e s f or m i ng pa r t s of t he gr oundm a s s D ) a nd E ) c l us t e r s of C a r l s ba d t w i nne d l a t hs know n a s gl om e r oc r ys t s ( 3963 3 a nd 3963 3) E ) a nd a c i c ul a r f or m s F ) M a ny pl a gi oc l a s e l a t hs a r e z one d ( 3970 11) T he s c a l e i s t he s a m e f or A ) a nd C ) F ) 0. 5 mm 1 mm

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44 44 a r e c l us t e r e d t oge t he r i n c r ys t a l c l ot s or gl om e r o c r ys t s S a m pl e s f r om e a s t of t he a xi s c om m onl y c ont a i n gl om e r oc r ys t s ( 0. 2 1 1 m m i n di a m e t e r ) e xc e pt f or t hr e e s a m pl e s ( 3963 2, 3963 7 a nd 3963 10) w hi c h e a c h c ont a i n a xe noc r ys t a l c l ot a s i ndi c a t e d by l a r ge r s i z e s ( 1 2. 5 m m a c r os s ) a nd c r ys t a l f a c e s w i t h e m ba ym e nt s a nd r ounde d e dge s ; a l l e vi de nc e f or di s e qui l i br i um w i t h t he hos t m e l t ( F i gur e s 4 2 a nd 4 3 ) I n c ont r a s t onl y t w o s a m pl e s ( 3974 4 a nd 3974 10) r e c ove r e d f r o m t he w e s t s i de of t he a xi s c ont a i ne d gl om e r oc r ys t s ( 1 1. 4 m m ) F ur t he r e vi de nc e of c r ys t a l m e l t di s e qui l i br i um i nc l ude s pr e va l e nt s ke l e t a l pl a gi oc l a s e l a t h s a nd m i c r ol i t e s a s w e l l a s t h e pr e s e nc e of z one d pl a gi oc l a s e f r om 3963 2 ( F i gur e 4 2) I n a ddi t i on, t w o r a r e ol i vi ne xe noc r ys t s w e r e f ound i n s a m pl e s 3963 6 ( 0 6 m m ) a nd 3974 6 ( 0. 55 m m ) ( F i gu r e 4 2 ) T he m a j or i t y of e a c h s e c t i on i s que n c he d gr oundm a s s c ont a i ni ng va r i a bl e a m ount s of ( s ke l e t a l ) pl a gi oc l a s e m i c r ol i t e s i nt e r gr ow n w i t h a nhe dr a l t o s ubh e dr a l c l i nopyr oxe ne a nd ol i vi ne ; a l l c om m onl y < 0 1 m m T he s e e qui l i br i um i nt e r gr ow t hs a r e a r r a nge d i n va r i ol i t i c s phe r ul i t i c a nd f a s c i c ul a r m a s s e s ( F i gur e 4 1) c om pos e d us ua l l y of c l i nopyr oxe ne nuc l e a t i ng a r ound a pl a gi oc l a s e m i c r ol i t e I n ove r ha l f t he s e c t i ons m i c r ol i t e s a r e ( s ub ) a l i gne d du e t o s he a r f or c e s c r e a t e d a s t he l a va c ool e d a nd f l ow e d ove r t he s e a f l oor T i ny e uhe dr a l c r ys t a l s of ol i vi n e a nd c l i nopyr oxe ne a l s o a ppe a r i n t he gr oundm a s s but a r e i ndi s t i ngui s ha bl e f r om e a c h ot he r unl e s s t he c ha r a c t e r i s t i c hoppe r s ha pe of ol i vi ne i s obs e r ve d ( F i gu r e 4 2 ) V e s i c l e s c ons t i t ut e 1 2% o f m os t s l i de s a nd a r e 0. 2 0. 4 m m i n di a m e t e r M i c r oba t hym e t r y, s i de s c a n s ona r a nd A l vi n obs e r va t i ons w e r e us e d t o i de nt i f y a nd s a m pl e t hr e e c om pl e t e f l ow uni t s e a c h f r om di ve s 3963 a nd 3974. T he l oba t e a nd s he e t f l ow s c om pr i s i ng t he m a i n bodi e s of t he l a va uni t s w e r e a l s o s a m pl e d i n a ddi t i on t o

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45 45 A B C F i gur e 4 3. P hot o m i c r ogr a phs of xe nogl om e r oc r ys t s f ound i n ba s a l t s c ol l e c t e d dur i ng di ve 3963 ne a r 9 51 N E P R S a m pl e s A ) 3963 2 B ) 3963 7, a nd C ) 3963 10 s how obvi ous di s e qui l i br i um f e a t ur e s i nc l udi ng e m ba ym e nt s a s w e l l a s r ounde d a nd c or r ode d s h a pe s A l l pi c t ur e s a r e s h ow n i n c r os s e d pol a r s T he s c a l e i s t he s a m e f or A ) a nd B ) 1 mm 0. 5 mm

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46 46 t he pi l l ow e d f l ow f r ont s P e t r ogr a phi c e xa m i na t i on of t he s l i de s of l a va s f r om di f f e r e nt pa r t s of e a c h f l ow uni t r e ve a l t ha t t he pi l l ow ba s a l t s c om pr i s i ng t he f l ow f r ont s a r e ~ 1 2. 4 % ( by vol um e ) m or e c r ys t a l l i ne t ha n t he r e s t of t he f l ow ( T a bl e 4 5 ) C h an n e l s D i ve 3968 T he t w o t hi n s e c t i ons one f r om e a c h c h a nne l i nv e s t i ga t e d dur i ng di ve 3968, a r e bot h a phyr i c ba s a l t s ( T a bl e 4 3) T he s e c t i on f r o m t he f i r s t c ha nne l ( 3968 2T ) i s m a i nl y va r i ol i t i c w i t h r a r e ( < 1% ) s ubhe dr a l t o e uhe dr a l pl a gi oc l a s e l a t h s T he e nt i r e l y hya l i ne s e c t i on f r om t he no r t he r n c ha nne l ( 3968 6 T ) a l s o c ont a i ns r a r e p l a gi oc l a s e m i c r ophe noc r ys t s i nc l udi ng a c i c ul a r ( 0. 3 0. 4 m m ) a nd l a t h ( 0 1 0 8 m m ) va r i e t i e s c l us t e r e d t oge t he r i nt o gl om e r oc r ys t s 0. 6 a nd 1. 7 m m a c r os s E a c h s a m pl e a l s o c ont a i ns bot h pl a gi oc l a s e s t ha t e xhi bi t C a r l s ba d t w i nni ng a nd z oni ng, a s w e l l a s s ubhe dr a l t o e uhe dr a l ol i vi ne ( < < 1 m m ) O f f A xi s M ou n d s D i ve 3970 B ot h pi l l ow m ounds a r e c om pos e d of m ode r a t e l y phyr i c pi l l ow ba s a l t s c ont a i ni ng c r ys t a l s of pl a gi oc l a s e ol i vi ne a nd c l i nopyr oxe n e ( T a bl e 4 4 ) H ow e ve r t he s he e t a nd l oba t e f l ow s s ur r oundi ng t he m a r e m a i nl y a phyr i c ba s a l t s A c i c ul a r a nd l a t h f or m s of pl a gi oc l a s e ( bot h 0. 1 1. 4 m m ) e xhi bi t i ng C a r l s ba d t w i ns a r e c om m on t o a l l t he p i l l ow s a m pl e s M i c r ophe noc r ys t s i n r oc ks f r om t he t w o pi l l ow m ounds c ons t i t ut e 5 10% of e a c h t hi n s e c t i on a nd f r e que nt l y f or m gl om e r oc r ys t s c om pos e d of pl a gi oc l a s e i nt e r gr ow n w i t h bot h s ubhe dr a l t o e uh e dr a l ol i vi ne ( 0. 7 m m ) a nd a nhe dr a l t o s ubhe dr a l c l i n opyr oxe ne ( 0. 6 m m ) ( F i gur e 4 3 a nd 4 4) T he s e c r ys t a l c l ot s a r e up t o 5. 8 m m a c r os s a nd f or m a dom i na nt gl om e r opor phyr i t i c t e xt ur e ( F i gur e 4 1) M a ny ol i vi ne a nd pl a gi oc l a s e c r ys t a l s c om pr i s i ng t he c l ot s a r e e m ba ye d i ndi c a t i ng di s e qui l i br i um w i t h t he

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47 T a bl e 4 5 P e t r ol ogi c C ha r a c t e r i s t i c s of F l ow U ni t s F r om A L V I N D i ve s 3963 a nd 3974

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48 48 A B C F i gur e 4 4 P ho t om i c r ogr a phs of ba s a l t i c s a m pl e s r e t r i e ve d d u r i ng A l vi n di ve 3970 t o a s e r i e s of of f a xi s m ounds 1. 23 1. 58 km w e s t of t h e A S T ne a r 9 30 N E P R ; a l l i n c r os s e d pol a r s A ) A r a r e ol i vi ne xe noc r ys t i nt e r gr ow n w i t h C a r l s ba d t w i nne d, a c i c ul a r pl a gi oc l a s e w a s ob s e r ve d i n a l oba t e l a va w e s t of t he nor t he r n m os t pi l l ow m ound ( 3970 1) B ) a nd C ) G l om e r oc r ys t s c om pos e d of e m ba ye d ol i vi ne s ( 3970 10) a c i c ul a r pl a gi o c l a s e a nd c l i nopyr oxe ne m i c r ophe noc r ys t s c om pos e t he gl om e r opor phor i t i c t e xt ur e of pi l l ow ba s a l t s r e c ove r e d f r om bot h pi l l ow m ounds ( 3970 4) T h e s c a l e i s t he s a m e f or bot h ( A ) a nd ( B ) 0. 5 mm 1 mm

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49 49 hos t m e l t ( F i gur e 4 4 ) I n a ddi t i on, z one d pl a gi oc l a s e s w e r e ob s e r ve d i n ha l f t he m ound s a m pl e s ( F i gur e 4 3) S e ve r a l gl om e r oc r ys t s ( 0. 3 0. 75 m m a c r os s ) w e r e obs e r ve d i n r oc ks f r om t he a phyr i c l oba t e a nd pi l l ow l a v a s l oc a t e d t o t he w e s t of t he nor t he r n m os t m ound, a nd onl y s a m pl e 3970 1 c ont a i ne d e uhe dr a l ol i vi ne ( up t o 0. 65 m m ) a nd c l i nopyr oxe ne ( 0. 5m m ) xe noc r ys t s t ha t a r e a s l a r ge a s t he c r ys t a l s i n t he pi l l ow m ounds ( F i gur e 4 2) A not he r r a r e ol i vi ne xe noc r ys t ( 0 7 m m ) w a s f ound i n s a m pl e 3970 9; a pi l l ow ba s a l t f r om of a hor ni t o j us t no r t h o f t he s e c ond, m or e s out he r n pi l l ow m ound. T he que nc he d gr oundm a s s f or t he s e t hi n s e c t i ons h a s a s i m i l a r c om pos i t i on t o t ha t of t he r oc ks c ol l e c t e d dur i ng di ve s t o t he f l ow u ni t s ( 3963 a nd 3974) P l a gi oc l a s e m i c r ol i t e s a c t a s nuc l e a t i on s i t e s f or c l i nopyr oxe ne s w hi c h t he n f or m s a dom i na nt l y gr a da t i ona l m os a i c of va r i ol i t i c s phe r ul i t i c a nd/ or f a s c i c ul a r m a s s e s ( F i gur e 4 1) i nt e r s pe r s e d w i t h va r i a bl e a m ount s of s ubhe dr a l t o e uhe dr a l ol i vi ne m i c r ophe noc r ys t s ( < 1 m m ) S ub a l i gnm e nt of m i c r ol i t e s w a s not e d i n s e ve r a l s l i de s V e s i c l e s c ons t i t ut e ~ 1% of m os t s l i de s a nd a r e c om m onl y 0 1 0 4 m m i n d i a m e t e r P h as e C h e m i s t r y T he c om pos i t i ons of ol i vi ne pl a gi oc l a s e a nd c l i nopyr oxe n e w e r e m e a s ur e d i n t w o r e pr e s e nt a t i ve t hi n s e c t i ons : 3970 9 a nd 3974 11t ( ol i vi ne onl y) ( T a bl e s 4 6 t o 4 8 ) T he a phyr i c 3974 11t c ont a i ne d onl y m i nut e c r ys t a l s of gr oundm a s s non z one d ol i vi ne s w i t h F o 8 7 ( one F o 8 6 ) ( F i gur e 4 5 a nd 4 6; T a bl e 4 6) O n t h e ot he r ha nd t he s pa r s e l y phyr i c 3970 9 c ont a i ne d ol i vi ne ( F o 8 5 8 6 ) pl a gi oc l a s e ( A n 6 6 3 7 3 1 ) a nd c l i nopyr oxe ne ( M g# 80 87) c r ys t a l s of t e n i nt e r gr ow n i n l a r ge c r ys t a l c l ot s ( F i gur e 4 5; T a bl e s 4 6 t o 4 8) T he m a j o r i t y o f ol i vi ne s w he t he r a pa r t o f t h e gr oundm a s s or l a r ge phe noc r ys t s w e r e F o 8 6 ( 5 of 6 c r ys t a l s m e a s ur e d a nd 19 of 25 c r ys t a l s m e a s ur e d, r e s pe c t i ve l y) w hi l e

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50 T a bl e 4 6 O l i vi ne C he m i c a l C om pos t i ons by E l e c t r on M i c r opr obe f or 3970 9 a nd 3974 11t

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51

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52 T a b l e 4 7 P l a gi oc l a s e C he m i c a l C om pos i t i ons by E l e c t r on M i c r opr obe f or 3970 9

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53 T a bl e 4 8 C l i nopyr oxe ne C he m i c a l C om pos i t i ons by E l e c t r on M i c r opr obe f or 3970 9

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54 F i gur e 4 5 O l i v i ne C om pos i t i ons C om pos i t i ons of ol i vi ne i n l a va s a m pl e s A ) 3974 11 a nd B ) 3970 9 C om pos i t i ons of C ) p l a gi oc l a s e a nd D ) c l i nopy r oxe ne i n l a va s a m pl e 3970 9.

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55 55 F i gur e 4 5 C ont i nue d

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56 56 F i gur e 4 6 Z oni ng i n ol i vi ne pl a gi oc l a s e a nd c l i n opyr oxe ne A ) T he r e i s no obvi ous z oni ng i n ol i vi ne ( e r r or i s 01. 04 r e l a t i ve % ) B ) V a r i ous t ype s of z oni ng a r e e v i de nt i n pl a gi oc l a s e ( e r r o r i s 0 76 r e l a t i ve % ) w hi l e C ) a c l i nopy r oxe ne e xhi bi t s r e ve r s e z oni ng ( e r r or i s 1. 04 r e l a t i ve )

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57 57 F i gur e 4 6 C ont i nue d t he r e m a i ni ng c r ys t a l s w e r e F o 8 5 A l a r ge r c om pos i t i ona l r a nge w a s ob s e r ve d f or bot h pl a gi oc l a s e a nd c l i n opyr oxe ne e s pe c i a l l y i n t he c l ot f or m i ng c r ys t a l s T he gr oundm a s s pl a gi oc l a s e i nc l udi ng t he c or e s of l a r ge r c r ys t a l s w e r e A n 6 9 7 1 w i t h m os t a t A n 6 9 ( 7 of 11 m e a s ur e m e nt s ) w hi l e t he c l ot f or m i ng pl a gi oc l a s e s i nc l udi ng t he r i m s of l a r ge r c r ys t a l s w e r e A n 6 6 7 3 w i t h m os t a t A n 6 8 ( 9 of 20 m e a s ur e m e nt s ) F i ve pl a gi oc l a s e c r ys t a l s ( l e t t e r e d A t o E ) e xhi bi t e d va r i ous t ype s of z oni ng i nc l udi ng one w i t h w e a k nor m a l z oni ng ( P l a gi oc l a s e E : c or e A n 7 0 0 r i m A n 6 8 8 ) one w i t h os c i l l a t or y z oni ng ( P l a gi oc l a s e D : c o r e A n 7 0 9 c e nt e r A n 7 3 1 r i m A n 7 0 7 ) a nd t h r e e w i t h r e ve r s e d z oni ng ( P l a gi oc l a s e A : c or e A n 6 6 6 r i m A n 6 9 1 ; P l a gi oc l a s e B : c or e A n 6 8 3 r i m A n 7 0 1 ; P l a gi oc l a s e C : c or e A n 6 7 5 r i m A n 7 0 4 ) ( F i gu r e 4 6 ) M os t c l i nopyr oxe ne s w he t he r f or m i ng t he gr oundm a s s or a pa r t of c r ys t a l c l ot s w e r e M g# 8 4 86, how e ve r t he c om pos i t i ons of t he

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58 58 c l ot f or m i ng c r ys t a l s va r i e d be t w e e n M g# 80 a nd 87. O ne c l i nopyr oxe ne c r ys t a l di s pl a ye d r e ve r s e d z oni ng w i t h a c o r e M g# 84 a nd r i m M g # 86 ( F i gu r e 4 6 ) S u m m ar y O ve r a l l t he pe t r ogr a phi c obs e r va t i ons i ndi c a t e onl y m i ni m a l a m ount s of c r ys t a l l i z a t i on ( < 4% ) i n t he f l ow c ha nne l s a nd un i t s but t he r e i s s om e i ndi c a t i on t ha t t he gr e a t e s t a m ount of c r ys t a l l i z a t i on ha s oc c ur r e d i n t he di s t a l pa r t s of t he f l ow i n t he pi l l ow e d f l ow f r ont s I n c ont r a s t t he pi l l ow m ounds a r e c om pr i s e d of l a v a s w i t h s i gni f i c a nt l y gr e a t e r c r ys t a l l i ni t y ( up t o 10% ) M i ne r a l t e xt ur e s a nd c om pos i t i ons s ugge s t s om e of t he l a r ge r c r ys t a l s a nd c l ot s a r e xe no c r ys t a l T he r e c om pos i t i ons a r e not e xt r e m e l y out of e q ui l i br i um w i t h t he i r hos t s but gr e a t e nough t o i ndi c a t e t he y c r ys t a l l i z e d f r om a m a gm a w i t h s l i ght l y di f f e r e nt c om pos i t i on be f or e be i ng e nt r a i ne d i n t he i r pr e s e nt hos t l a va

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59 C H A P T E R 5 M A J O R A N D T R A C E E L E M E N T C H E M I S T R Y M aj or E l e m e n t D a t a T he ba s a l t i c l a va s a na l y z e d i n t hi s s t udy w e r e r e c ove r e d up t o s e ve r a l km f r om t he A S C T i n one of t hr e e m a i n a r e a s a l ong t he 9 10 N s e gm e nt of t he E P R : 9 50 N 9 43 N a nd 9 28 31 N ( F i g ur e 1 2 ) G l a s s c hi ps f r om 107 of t he s a m pl e d s h e e t l oba t e a nd pi l l ow l a va s w e r e a na l yz e d f or m a j o r e l e m e nt c onc e nt r a t i ons ( T a bl e 5 1 ) A na l yt i c a l unc e r t a i nt y i s di s c u s s e d i n A pp e ndi x B a nd t he 2 s i gm a e r r or s a r e pr e s e nt e d i n t he M e t hods s e c t i on. O ve r a l l t he M gO c ont e nt ( i n w t % ) of t he s e s l a va s i s r e l a t i ve l y l i m i t e d a nd r a nge s be t w e e n 7. 1 a nd 9 0% M a j or e l e m e nt va r i a t i ons of t he ba s a l t s a na l yz e d i n t hi s s t udy m i m i c t he a ppa r e nt f r a c t i on a l c r ys t a l l i z a t i on t r e nds doc um e nt e d i n ot he r ba s a l t i c s ui t e s f r om t he 9 10 N s e gm e nt [ e g. Sm i t h e t al 2001 ] ( F i gu r e 5 1 ) A s M gO de c r e a s e s T i O 2 F e O N a 2 O P 2 O 5 M nO S i O 2 a nd K 2 O c ont e nt s i nc r e a s e w he r e a s A l 2 O 3 a nd C a O de c r e a s e T he m os t pr i m i t i ve ba s a l t s w e r e r e c ove r e d f r om ne a r t he a xi s a t 9 50 N ( 3963 9, 10, 11) a nd ha ve M gO c ont e nt s be t w e e n 8. 9 a nd 9. 0 w t % w hi l e t he m os t e vol ve d s a m pl e s w e r e f ound f a r t o e i t he r s i de of t he a xi s a t 9 28 30 N T w o ha c kl y s he e t f l ow s ( 3975 2, 2 i ) a nd one l oba t e f l ow ( 3975 1i ) w e s t of t he A S C T ha ve M gO c ont e nt s of 7 0 7. 1 w t % T o t he e a s t of t he A S C T one pi l l ow f l o w ( 3970 5) a nd one ha c kl y f l ow ( 3970 12) c ont a i n 7 2 w t % M gO A l l s a m pl e s a r e l o w K t hol e i i t i c ba s a l t s t ha t a s a gr oup e xhi bi t t he c l a s s i c i r on e nr i c hm e nt t r e nd o f t hol e i i t i c s ui t e s a nd ha ve l e s s t ha n 0. 27 w t % K 2 O H ow e ve r a f e w s a m pl e s ( 3965 7, 3966 3, 3966 3s 3966 5a 3966 5b 3966 5b 2)

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60 T a bl e 5 1: M a j or E l e m e nt C he m i s t r y of L a va s C ol l e c t e d F r om 9 10N E a s t P a c i f i c R i s e D ur i ng A T 11 7 C r ui s e

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66 66 F i gur e 5 1 M a j or e l e m e nt oxi de va r i a t i on di a gr a m s f or l a va s s a m pl e d dur i ng di ve s 3963, 3968, 3970 a nd 3974 E r r or i s i ndi c a t e d by c r os s ha i r s

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67 67 F i gur e 5 1 C ont i nue d ha ve s l i ght l y hi gh K 2 O / T i O 2 r a t i os ( > 11) ( T a bl e 5 1) w hi c h c l a s s i f i e s t he m a s e nr i c he d m i d oc e a n r i dge ba s a l t s ( E M O R B ) r e l a t i ve t o a l l ot he r s a m pl e s w hi c h a r e no r m a l m i d oc e a n r i dge ba s a l t s ( N M O R B ) [ R e y nol ds e t a l 1 992; P e r f i t e t al 1994 ] I n ge ne r a l t he l a va s a r e m or e m a gne s i a n t ow a r ds t he nor t h i n t he s t udy a r e a ( F i gur e s 5 2 a nd 5 3 ) T he r e i s l i t t l e e vi de nc e t o s ugge s t t ha t t he c om po s i t i ons of t he l a va s a r e r e l a t e d t o t he i r m or phol ogy. T he c om pos i t i ona l di s t r i but i ons of s he e t s l oba t e s a nd pi l l ow s a r e r e m a r ka bl y s i m i l a r bot h ove r a l l a n d a t e a c h m a j or s a m pl i ng s i t e a l ong t he E P R r i dge ( F i gu r e 5 3 )

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68 6 8 F i gur e 5 2 M gO c ont e nt of l a va s s a m pl e d dur i ng di ve s 3963, 3968 3970 a nd 3974 a s a f unc t i on of t he i r l a t i t ude

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69 69 A B C D F i gur e 5 3. C om pos i t i ona l di s t r i but i ons f or s he e t l oba t e a nd pi l l ow l a va s A ) A l l l a va s be t w e e n 9 28 N a nd 9 50 N B ) A l l l a va s a t 9 50 N C ) A l l l a va s a t 9 44 N D ) A l l l a va s be t w e e n 9 28 a nd 9 30 N F l ow U n i t s Wi t h F r on t s D i ve s 39 63 a n d 3974 T w o c om pl i m e nt a r y di ve s on t he e a s t a nd w e s t s i d e s of t he E P R a xi s ne a r 9 50 N s a m pl e d l a va s w i t h s i gni f i c a nt m a j or e l e m e nt di f f e r e nc e s be t w e e n f l ow s pr oxi m a l t o t he A S C T a nd t hos e f a r t he r a w a y. I n e a c h c a s e t he m os t pr i m i t i ve s a m pl e s a r e t ho s e f r om

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70 70 t he f i r s t f e w f l ow uni t s e m a na t i ng f r om t he A S C T w hi l e t hos e now l oc a t e d up t o s e ve r a l ki l om e t e r s a w a y f r om t he a xi s w e r e s i gni f i c a nt l y m or e e vol ve d ( F i gur e 5 1 ) D i ve 3963 s a m pl e d t hr e e f l ow uni t s i nc l udi ng a s i ngl e f l ow f r ont c l os e t o t he e a s t e r n e dge of t he A S C T T w o a na l yt i c a l l y s e pa r a t e gr oups a r e pr e s e nt w i t hi n t hi s s ui t e of ba s a l t s ( F i gur e s 5 1 a nd 5 2 ) T he f our s a m pl e s c l os e s t t o t he A S C T ( 3963 7, 9, 10, 11 ) ha ve hi ghe r M gO ( 8. 7 9. 0% ) t ha n t he t w o f l ow un i t s ( 3963 3, 4 5, 6 8) l oc a t e d f ur t h e r of f a xi s ( 7. 7 8 0% ) S a m pl e s c om pr i s i ng t he f i r s t t w o f l ow uni t s e xhi bi t onl y m i nor c om pos i t i ona l di f f e r e nc e s ( onl y M gO c ont e nt s a r e out s i de of a na l yt i c a l unc e r t a i nt y) a nd t he s e f ol l ow r e gi ona l f r a c t i ona l c r ys t a l l i z a t i on t r e nds w hi l e l a va s f r om t he t hi r d f l ow uni t a r e a na l yt i c a l l y i de nt i c a l O n t he ot he r s i de of t he a xi s di ve 3974 a l s o s a m pl e d t hr e e f l ow uni t s but t he c he m i s t r i e s of t he s e ba s a l t s a r e m uc h m or e hom oge nous t ha n t hos e f r om di ve 3963 ( F i gur e 5 1 ) T he s a m pl e s c om pr i s i ng t he t hi r d f l o w uni t ( 3974 2, 3 4, 7 8, 8 2) ha ve l ow e r M gO c ont e nt s ( 7. 6 8. 1% ) t ha n t hos e of t he f i r s t f l ow uni t ( 3974 10, 11) w hi c h i s c l os e s t t o t he a xi s ( 8. 3 8 7% ) T he l e s s m or phol ogi c a l l y di s t i nc t m i ddl e f l ow uni t ( 3974 5 6, 9 ) ha s M gO va l ue s be t w e e n t he s e e xt r e m e s ( 7. 7 8. 4% ) F or a l l f l ow uni t s t he oxi de di f f e r e nt i a t i on s e que nc e s pa r a l l e l r e gi ona l t r e nds w i t h onl y m i nor c om pos i t i ona l di f f e r e nc e s ( a ga i n, onl y M gO c ont e nt s a r e out s i de of a na l yt i c a l unc e r t a i nt y) be t w e e n t he s a m pl e s of e a c h f l ow uni t C h an n e l s D i ve 39 68 B ot h s he e t f l ow s t ha t c om pr i s e c ha nne l s l oc a t e d w e s t of t he a xi s a t 9 29 N ha ve M gO c ont e nt s of 7. 5% w hi l e t he f l ow s t ha t s ur r ound t he m di s pl a y s l i ght l y m or e va r i a t i on ( 7. 4 7 6% ) ( T a bl e 5 1 ) T he m a j or e l e m e nt c he m i s t r i e s of ba s a l t s f r om t hi s di ve a r e a na l yt i c a l l y i de nt i c a l a nd pl ot m a i nl y w i t h t he a ve r a ge di f f e r e nt i a t i on t r e nd f or l a va s f r om

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71 71 t he 9 10 N s e gm e nt ( F i gur e 5 1 ) but s om e s pr e a d i s not i c e a bl e f or S i O 2 M nO a nd P 2 O 5 due t o t he l a r ge r e r r or a s s oc i a t e d w i t h t he s e e l e m e nt s W hi l e a l l N a 2 O c ont e nt s a r e l ow s a m pl e s 3968 4 s o, 3968 5, 3968 6, a nd 3968 7 a r e e xc e pt i ona l l y l ow a nd r e pr e s e nt t he l ow e r e xt r e m e l i m i t f or t hi s oxi de O f f A xi s M ou n d s D i ve 3970 T he s out he r n of f a xi s pi l l ow m ound a t 9 30 N ( 3970 9, 10 11) s how s l i t t l e va r i a t i on i n M gO c ont e nt s ( 7. 6 7. 8% ) c om pa r e d t o t he nor t he r n pi l l ow m ound ne a r 9 31 N ( 3970 4, 5 6) w hi c h e xhi bi t s m os t of t he M gO v a r i a t i on out s i de of a na l yt i c a l unc e r t a i nt y s e e n i n s a m pl e s f r om t hi s di ve ( 7. 2 7. 8% ) ( T a bl e 5 1 ) L a va s f r om t he s ur r oundi ng f l ow s ( 3970 1 2, 3 7, 8 12) ha ve M gO c ont e nt s of 7. 4 7. 6% A good c onc or da nc e e xi s t s be t w e e n m o s t a ve r a ge r e gi ona l m a j or oxi de t r e nds a nd ba s a l t s f r om t hi s a r e a e xc e pt va l ue s f or bot h M nO a nd P 2 O 5 s how m or e s c a t t e r a m ongs t t he s a m pl e s t ha n a l l ot he r oxi de s ( F i gu r e 5 1 ) T r ac e E l e m e n t D at a O nl y l a va s c ol l e c t e d dur i ng di ve s 3963, 3968 39 70, a nd 3974 w e r e a na l yz e d f or t r a c e e l e m e nt c ont e nt ( T a bl e s 5 2 a nd 5 3 ) T he s e l a va s a r e N M O R B a s r e f l e c t e d i n t he i r l ow K / T i r a t i os ( 6 87 t o 9. 99) a nd de pl e t e d i nc om pa t i bl e e l e m e nt s nor m a l i z e d t o c hondr i t i c a nd pr i m i t i ve m a nt l e va l ue s ( T a bl e s 5 2 a nd 5 3 a nd F i gu r e s 5 4 t o 5 7 ) [ Sun and M c D onough 1989] A s a w hol e t he i nc om pa t i bl e e l e m e nt c ont e nt s of t he s e s a m pl e s ( S c V Z n, G a Y Z r N b, H f S r R b, U T h, a nd t he r a r e e a r t h e l e m e nt s [ R E E ] ) i nc r e a s e a nd t he c om pa t i bl e e l e m e nt s de c r e a s e ( N i a nd C r ) w i t h de c r e a s i ng M gO c ont e nt w hi l e s e ve r a l t r a c e e l e m e nt c ont e nt s a pp e a r t o c ha nge ve r y l i t t l e ( F i gur e 5 8 )

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72 T a bl e 5 2 S e l e c t T r a c e e l e m e nt s F o r L a va s C ol l e c t e d D ur i ng D i ve s 3963 3968 3970 a nd 3974

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73

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74 T a bl e 5 3 R a r e E a r t h E l e m e nt C onc e nt r a t i ons f or D i ve s 3963, 3968, 3970, a nd 3974

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75

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76 76 F i gur e 5 4. S pi de r di a g r a m a nd R E E pl o t f or l a va s f r om di ve 3963. T r a c e e l e m e nt s w e r e nor m a l i z e d t o p r i m i t i ve m a nt l e a nd c hondr i t e va l ue s f r om Sun and M c D onough [ 1989]

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77 77 F i gur e 5 5. S pi de r di a g r a m a nd R E E pl o t f or l a va s f r om di ve 3974. T r a c e e l e m e nt s w e r e nor m a l i z e d t o p r i m i t i ve m a nt l e a nd c hondr i t e va l ue s f r om Sun and M c D onough [ 1989]

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78 78 F i gur e 5 6 S pi de r di a gr a m a nd R E E pl ot f or l a va s f r om di ve 3968. T r a c e e l e m e nt s w e r e nor m a l i z e d t o p r i m i t i ve m a nt l e a nd c hondr i t e va l ue s f r om Sun and M c D onough [ 1989]

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79 79 F i gur e 5 7 S pi de r di a gr a m a nd R E E pl ot f or l a va s f r om di ve 3970. T r a c e e l e m e nt s w e r e nor m a l i z e d t o p r i m i t i ve m a nt l e a nd c hondr i t e va l ue s f r om Sun and M c D onough [ 1989]

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80 80 F i gur e 5 8. T r a c e e l e m e nt va r i a t i on di a gr a m s f or l a va s f r om di ve s 3963, 3968, 3970, a nd 3974. E r r or i s r e pr e s e nt e d by c r os s ha i r s

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81 81 F i gur e 5 8 C ont i nue d

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82 82 F i gur e 5 8 C ont i nue d

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83 83 F i gur e 5 8 C ont i nue d

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84 84 F i gur e 5 8 C ont i nue d

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85 85 F l ow U n i t s Wi t h F r on t s D i ve s 3963 a n d 3974 T he s e c ond a nd t hi r d f l ow uni t s f r om di ve 3963 ( 3963 3, 6 a nd 3963 4, 5 8) ha ve gr e a t e r a bunda nc e s i n i nc om pa t i bl e e l e m e nt s t ha n t he f i r s t f l ow uni t ( 3963 7, 9 ) a nd f l ow f r ont ( 3963 10, 11 ) c l os e s t t o t he r i dge a xi s ( F i gu r e s 5 4 a nd 5 8 ) M os t t r a c e e l e m e nt c onc e nt r a t i ons f r om e a c h f l ow uni t a nd t he f l o w f r ont a r e e a c h a na l yt i c a l l y i de nt i c a l H ow e ve r unl i ke w i t h t he m a j or e l e m e nt s w he r e onl y M gO c ont e nt s f or t he f i r s t a nd s e c ond f l ow uni t s v a r i e d out s i de of a n a l yt i c a l unc e r t a i nt y, s e ve r a l t r a c e e l e m e nt c onc e nt r a t i ons f r om e a c h f l ow uni t l i ke w i s e va r y. T hos e t ha t f ol l ow t r e nds pr e s um a bl y r e l a t e d t o f r a c t i ona l c r ys t a l l i z a t i on i nc l ude L a a nd N d f or t he f l ow f r ont N i L a a nd T h f or t he s e c ond f l ow uni t a nd L a N d, a nd T h f or t he t hi r d f l ow uni t A l s o, s a m pl e 3974 4i f r om t he s e c ond f l ow uni t a ppe a r s t o ha ve l ow e r N d, D y Y b, a nd Z r a bunda nc e s t ha n t he r e s t of t he l a va s i n t ha t f l ow uni t O ve r a l l s a m pl e s f r om di ve 3974 va r y be t w e e n t he m os t pr i m i t i ve s a m pl e s f r om di ve 3963 a nd t he m or e e vol ve d s a m pl e s f r om di ve s 3968 a nd 3970, c r e a t i ng a ge ne r a l t r e nd of i nc r e a s i ng i nc om pa t i bl e e l e m e nt c onc e nt r a t i ons i n s a m pl e s w i t h l ow e r M gO c ont e nt s W i t hi n e a c h f l ow uni t va r i a t i on i n m o s t t r a c e e l e m e nt a bunda nc e s i s w i t hi n a na l yt i c a l unc e r t a i nt y. A m a j or e xc e pt i on i nvol ve s s a m pl e 3974 5 f r om t he m i ddl e f l ow uni t w hi c h ha s hi ghe r a bunda nc e s of s om e t r a c e e l e m e nt s t ha t e i t he r f o l l ow e xpe c t e d f r a c t i ona t i on t r e nds ( S c V G a Y ) o r do not ( C o, R b, S r N b, T h, a nd U ) i n r e l a t i on t o t he ot he r l a va s f r om t hi s f l ow uni t ( 3974 6 a nd 3974 9) ( F i gur e s 5 5 a nd 5 8 ) I n c om bi na t i on w i t h l a va 3974 6, bot h e xhi bi t l ow e r c onc e nt r a t i ons of N i a nd hi ghe r c onc e nt r a t i ons of Z r a nd t he l i ght R E E r e l a t i ve t o 3974 9 ( F i gur e s 5 5 a nd 5 8 )

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86 86 C h an n e l s D i ve 3968 M os t of t he t r a c e e l e m e nt c onc e nt r a t i ons of t h e s he e t a nd l oba t e f l ow s t ha t c om pr i s e t he t w o c ha nne l s a nd s ur r oundi ng vol c a ni c t e r r a i n a r e a na l yt i c a l l y i de nt i c a l O f t he l a va s s a m pl e d w i t hi n a nd ne a r t he s out he r n c ha nne l ( 3968 1 t o 5) a l l bu t 3968 5 ( s a m pl e d f r om t he m a r gi n) e xhi bi t s l i ght l y l ow e r a bunda nc e s out s i de of a na l yt i c a l l y unc e r t a i nt y of C o, Y U a nd m os t R E E r e l a t i ve t o t he r e s t of t he s a m pl e s f r om di ve 3968 ( F i gur e s 5 6 a nd 5 8 ) H ow e ve r t he s e di f f e r e nc e s m a y be e xpl a i ne d by s l i ght di f f e r e nc e s i n i ns t r um e nt p r e c i s i on a s a r e s ul t of t he s e s a m pl e s be i ng r un a t di f f e r e nt t i m e s O f f A xi s M ou n d s D i ve 3970 T he nor t he r n pi l l ow m ound ( 3970 4, 5 6) ha s hi ghe r a bunda nc e s of i nc om pa t i bl e e l e m e nt s r e l a t i ve t o t he s out he r n pi l l ow m ound ( 3970 9, 10, 11 ) w i t h t he l a va s c om pr i s i ng t he s ur r oundi ng t e r r a i n l yi ng be t w e e n t he s e t w o e xt r e m e s ( 3970 1, 2, 3 7, 8 ) ( F i gur e s 5 7 a nd 5 8 ) A s w i t h t he m a j or e l e m e nt c on c e nt r a t i ons of t he s out he r n pi l l ow m ound, t he t r a c e e l e m e nt c onc e nt r a t i ons a r e a na l yt i c a l l y i de nt i c a l f or m os t t r a c e e l e m e nt s i f s a m pl e 3970 10 2 i s e xc l ude d. O n t he ot he r ha nd, 39 70 4 f r om t he nor t he r n pi l l ow m ound c ons i s t e nt l y ha s hi ghe r i nc om pa t i bl e e l e m e nt a l a bunda nc e s out s i de of a na l yt i c a l unc e r t a i nt y r e l a t i ve t o t he ot he r pi l l ow l a va s f r om t hi s m ound. S u m m ar y O ve r a l l t he f l ow f r ont s ne a r 9 50 N di s pl a y t he gr e a t e s t va r i a t i on i n bot h m a j or a nd t r a c e e l e m e nt c onc e nt r a t i ons r e l a t i ve t o t h e ot he r s a m pl e s a na l yz e d. O n t he ot he r ha nd, t he of f a xi s m ounds a nd c ha nne l s ne a r 9 30 N ha ve i n ge ne r a l m or e e vol ve d c om pos i t i ons a nd l ow e r a bunda n c e s of t he m os t i nc om pa t i bl e e l e m e nt s r e l a t i ve t o t he l e a s t i nc om pa t i bl e e l e m e nt s A s a w hol e bot h t he m a j or a nd t r a c e e l e m e nt va r i a t i ons f or e a c h di ve m i m i c a pp a r e nt f r a c t i ona l c r ys t a l l i z a t i o n t r e nds W hi l e onl y M gO c ont e nt s f or

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87 87 s om e of t he f l ow uni t s a nd pi l l ow m ounds v a r y out s i de of a na l yt i c a l unc e r t a i nt y, va r i a t i ons i n t he m or e a na l yt i c a l l y pr e c i s e t r a c e e l e m e nt c onc e nt r a t i ons o c c ur i n a l l c a s e s f or a t l e a s t of f e w e l e m e nt s

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88 C H A P T E R 6 P E T R O G E N E S I S T he pe t r oge ne t i c hi s t or y of l a va s e r upt e d f r om t h e 9 10 N s e gm e nt of t he E P R i s c ont r ol l e d by va r i ous f a c t or s i nc l udi ng t he c om pos i t i on of t he m a nt l e s our c e s t yl e a nd e xt e nt of m e l t i ng a t t he s our c e m i xi ng w i t h ot he r m e l t s a nd t he m a nne r of c r ys t a l l i z a t i on [ L angm ui r e t al 1992] M os t c he m i c a l va r i a t i on obs e r ve d i n a xi a l M O R B c a n be e xpl a i ne d by f r a c t i ona l c r ys t a l l i z a t i on a nd m i x i ng of m a gm a s i n t he a xi a l m a gm a c ha m be r [ B at i z a and N i u 1992; L angm ui r e t al 1992] T he m a j or a nd t r a c e e l e m e nt c onc e nt r a t i ons of t he f l ow uni t s a nd of f a xi s m ou nds w e r e m ode l e d t o be t t e r unde r s t a nd a nd di s t i ngui s h be t w e e n t he s e pr oc e s s e s M aj or E l e m e n t M od e l s P E T R O L O G 2. 1 w a s us e d t o m ode l t he f r a c t i on a l c r ys t a l l i z a t i on of a m e l t by ut i l i z i ng ps e udo l i qui dus t e m pe r a t ur e s [ N at han an d V ank i r k 1978; N i e l s e n and D ungan 1983; A r i s k i n e t al 1986 ] T hi s t e c hni que a l l ow s c a l c ul a t i ons of l i qui dus t e m pe r a t ur e s f or e a c h pha s e of i nt e r e s t i n a r a nge of m e l t c om pos i t i ons us i ng m i ne r a l m e l t e qui l i br i a m od e l s I f t he pha s e i s m e t a s t a bl e i n a gi ve n m e l t c om pos i t i on, t he n a ps e udo l i qui dus t e m pe r a t ur e i s c a l c ul a t e d. O nc e ps e udo l i qui dus t e m pe r a t ur e s ha ve be e n c a l c ul a t e d f or s e l e c t e d m i ne r a l s c a pa bl e of c r ys t a l l i z i ng f r om a us e r pr ovi de d m e l t c om pos i t i on, t h e t e m pe r a t ur e s a r e c om pa r e d. T he m i ne r a l f i r s t on t he l i qui dus i s t he one w i t h t he hi ghe s t c a l c ul a t e d t e m pe r a t ur e T hi s m i ne r a l i s r e m ove d f r om t he m e l t a nd t he pr oc e s s c ont i nue s a s a ddi t i ona l pha s e s a r e a dde d t o t he f r a c t i ona t i ng a s s e m bl a ge unt i l a us e r de f i ne d a m ount of c r ys t a l l i z a t i on i s a t t a i ne d. I nc r e m e nt a l f r a c t i ona l c r ys t a l l i z a t i on i n 1 w t %

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89 89 i nc r e m e nt s r e s ul t s i n a s e r i e s of m e l t c om pos i t i ons t ha t f or m a l i qui d l i ne of de s c e nt ( L L D ) t he pa t hw a y t a ke n a s t he m e l t e vol ve s due t o t he s e pa r a t i on of m i ne r a l s f r om t he m e l t i n w hi c h t he y f o r m e d. B e f or e s i m ul a t i ng t he c r ys t a l l i z a t i on of a m e l t t he us e r m us t i nput da t a a nd s e l e c t c a l c ul a t i on pa r a m e t e r s i nc l udi ng t he m a j or e l e m e nt c om pos i t i on of a pa r e nt a l m e l t P a r e nt a l c om pos i t i ons w e r e e s t i m a t e d f r om t he m os t pr i m i t i ve ( hi ghe s t w t % M gO ) ba s a l t i c s a m pl e s c ol l e c t e d f r om t he f l ow uni t s a n d pi l l ow m ounds ( T a bl e s 6 1 t o 6 3 ) T he c ha nne l s i nve s t i ga t e d dur i ng di ve 3968 w e r e not e va l ua t e d f or t he i r pe t r oge ne t i c e vol ut i on due t o t he i r l a c k o f c he m i c a l v a r i a t i on, w hi c h s ugge s t s t ha t t he y di d not e xpe r i e nc e a ny c r ys t a l l i z a t i on. I n t he c a l c ul a t i ons ol i vi ne pl a gi oc l a s e a nd c l i nopyr oxe ne w e r e s e l e c t e d a s pot e nt i a l c r ys t a l l i z i ng pha s e s be c a us e t he s e m i ne r a l s a r e pr e s e nt i n t he l a va s t he m s e l ve s O t he r i nva r i a nt c a l c ul a t i on pa r a m e t e r s i nc l ude s e t t i ng t he oxi da t i on s t a t e a t t he Q F M f O 2 buf f e r [ B or i s ov and Shapk i n 1990] m a ki ng c r ys t a l l i qui d e qui l i br i um c a l c ul a t i ons a f t e r e a c h 0. 01 w t % of t he m i ne r a l s a r e s ubt r a c t e d f r om t he m e l t a nd t e r m i na t i ng c a l c ul a t i o ns onc e 50 % c r ys t a l l i z a t i on ha s be e n c om pl e t e d. W a t e r c ont e nt ( 0. 0 a nd 0. 2 w t % ) a nd pr e s s ur e ( 0. 5 a nd 1 kba r ) w e r e va r i e d t o be t t e r a ppr oxi m a t e t he r a nge of c ondi t i ons unde r w hi c h t he m a gm a s c r ys t a l l i z e d. O nl y a f e w e m pi r i c a l m ode l s a c c ount f or w a t e r i n t he ba s a l t s y s t e m : a n ol i vi ne m e l t m ode l de ve l ope d by F al l oon and D any us he v s k y [ 2000] a nd pl a gi oc l a s e a nd c l i nopyr oxe ne m e l t m ode l s by D any us he v s k y [ 2001] F l ow U n i t s D i ve s 3963 a n d 3974 T he r a nge of c om pos i t i ons of s a m pl e s c ol l e c t e d f r om t he ove r l a p pi ng f l ow uni t s a r e w e l l c ons t r a i ne d by num e r ous L L D c a l c ul a t i ons ge ne r a t e d us i ng s a m pl e s 3963 9, 3963 11, 3974 11, a nd 3973 2 a s pa r e nt a l m a g m a c om pos i t i ons ( F i gur e 6 1 ) T he

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90 T a bl e 6 1 P e t r oge ne t i c P a r a m e t e r s a nd C ondi t i ons U s e d i n F l ow U ni t ( D i ve 3963 ) C a l c ul a t i ons

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91 T a bl e 6 2 P e t r oge ne t i c P a r a m e t e r s a nd C ondi t i ons U s e d i n F l ow U ni t ( D i ve 3974) C a l c ul a t i ons

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92 T a bl e 6 3 P e t r oge ne t i c P a r a m e t e r s a nd C ondi t i ons U s e d i n O f f A xi s M ounds ( D i ve 3970) C a l c ul a t i o ns

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93 93 F i gur e 6 1 L i qui d l i ne s of de s c e nt c a l c ul a t e d f or f l ow uni t s s a m pl e d dur i ng di ve s 3963 a nd 3974 unde r bot h a nhydr ous a nd hydr ous c ondi t i ons S e ve r a l s t a r t i ng c om pos i t i ons ( S C ) r e pr e s e nt i ng t he m os t pr i m i t i ve s a m pl e s f r om t he f l ow uni t s w e r e us e d.

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94 94 F i gur e 6 1 C ont i nue d

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95 95 F i gur e 6 1 C ont i nue d

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96 96 F i gur e 6 1 C ont i nue d pr e s s ur e w a s s e t a t 0. 5 kba r f or a l l c a l c ul a t i ons i n or de r t o a ppr oxi m a t e t he de pt h of t he a xi a l m a gm a l e ns ( ~ 1. 5 km be l ow t he s e a f l o or ) A l t hough t he L L D c a l c ul a t i ons a ppr oxi m a t e t he ge ne r a l m a j or e l e m e nt t r e nds t he da t a ( F i gur e 6 1) s ugge s t m i xi ng be t w e e n di f f e r e nt pa r e nt a l m e l t s s om e w i t h w a t e r c ont e nt s up t o 0. 2 w t % i s r e qui r e d t o e xpl a i n t he s pr e a d of da t a I n or de r t o e xpl a i n t he pos i t i ons of l a va s 3974 1, 3974 2 3974 5, a nd 3974 6 on bot h t he C a O vs M gO a nd C a O / A l 2 O 3 vs M gO pl ot s m i xi ng be t w e e n e vol ve d a nd pr i m i t i ve m e l t s i s ne e de d, or t he s e s a m pl e s a r e f r om a di f f e r e nt s our c e t ha n t he ot he r s a m pl e s i n e a c h di ve T he a m ount of c r ys t a l l i z a t i on c a l c ul a t e d f o r t he f l ow uni t s i s pr e s e nt e d i n T a bl e s 6 1 a nd 6 2 O f t he t hr e e f l ow uni t s obs e r ve d a n d s a m pl e d dur i ng di ve 3963 ( s e e T a bl e 4 5) t he pr e di c t e d c ha nge s i n c r ys t a l l i ni t y ( by v ol um e ) f r om t he l e a s t t o m os t e vol ve d r a nge d f r om 0. 85 a nd 2. 01 % f or t he f i r s t f l ow uni t 3. 01 a nd 6. 04 % f or t he s e c ond f l ow uni t a nd 1 01 a nd 4 02% f or t he t hi r d f l ow un i t R e s ul t s f or t he t h r e e f l ow uni t s f r om di ve 3974 w e r e 1. 0 8 03% 7. 53 14 33% a nd 7. 2 9. 05% r e s pe c t i ve l y. C om pos i t i ons of ol i vi ne pl a gi oc l a s e a nd c l i nopyr oxe ne i n e qui l i b r i um w i t h a m e l t c ont a i ni ng 8. 7 w t %

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97 97 M gO ( f r om 3974 11 ) w e r e F o 8 5 2 F o 8 6 5 A n 7 3 9 A n 7 6 7 2 a nd M g# 83. 1 84 4 r e s pe c t i ve l y. T he i ni t i a l t e m pe r a t ur e s of c r ys t a l l i z a t i on of ol i vi ne pl a gi oc l a s e a nd c l i nopyr oxe ne a r e a l s o i nc l u de d i n T a bl e s 6 1 a nd 6 2. M ode l s t ha t i nc l ude d w a t e r yi e l de d s i m i l a r m i ne r a l c om pos i t i ons but t e m pe r a t ur e s of c r ys t a l l i z a t i on w e r e l ow e r O f f A xi s M ou n d s D i ve 3970 P a r e nt a l m a gm a s w i t h c om pos i t i ons of s a m pl e s 3 970 10 a nd 3970 11 ge ne r a t e d a nhydr ous L L D t ha t f i t t he of f a xi s m ound da t a w e l l A nhydr ous a nd hydr ous L L D ge ne r a t e d a t a hi ghe r c r ys t a l l i z a t i on pr e s s ur e of 1 kba r a r e s how n on t he m a j or e l e m e nt pl ot s f or c om pa r a t i ve pur pos e s ( F i gur e 6 2) M i xi ng of m or e pr i m i t i ve m e l t s ( s i m i l a r i n c om pos i t i on t o t he s out he r n m ound) w i t h m o r e e v ol ve d m e l t s ( s i m i l a r i n c om pos i t i on t o t he nor t he r n m ound) ge ne r a t e s m e l t s w i t h c om pos i t i ons t ha t c a n e xpl a i n t he c om pos i t i ona l t r e nds of l a va f l ow s s ur r oundi ng t he m ounds ( 3970 1, 2, 3, 4 7, 8 12) a s s e e n on t he C a O vs M gO pl o t T he c om pos i t i ons of ol i vi ne pl a gi oc l a s e a nd c l i nopyr oxe ne a s c a l c ul a t e d by P E T R O L O G va r y l i t t l e be t w e e n t he f our m ode l s ( T a bl e 6 3) T he c om pos i t i ons of t he s e m i ne r a l s i n e qui l i br i um w i t h a m e l t c ont a i ni ng 7. 65 w t % M gO ( f r om 3970 9) a r e l i s t e d f or e a c h m ode l i n T a bl e 6 3 O l i vi ne ( F o 8 1 8 F o 8 2 3 ) i s p r e s e nt i n e a c h m ode l w hi l e pl a gi oc l a s e ( A n 6 8 1 A n 6 8 4 ) i s i n e qui l i br i um f or onl y t he a nhydr ous c ondi t i ons a nd c l i nopyr oxe ne ( M g# 83. 1) i s i n e qui l i br i um f o r onl y t he hydr ous m ode l us i ng s t a r t i ng c om pos i t i on s i x ( 3970 10) T he i ni t i a l c r ys t a l l i z a t i on t e m pe r a t ur e s a nd c om pos i t i ons of t he s e m i ne r a l s a r e a l s o pr e s e nt e d i n T a bl e 6 3 U nde r a nhydr ous c ondi t i ons pl a gi oc l a s e ha s a hi ghe r l i qui dus t e m pe r a t ur e ( 1192 a nd 1194 C ) t ha n ol i vi ne ( 1193 a nd 1192 C ) a nd c l i nopyr oxe ne ( 1186 a nd 1185 C ) O n t he ot he r h a nd, c a l c ul a t i ons us i ng s t a r t i ng

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98 98 F i gur e 6 2. L i qu i d l i ne s of de s c e nt c a l c ul a t e d f or of f a xi s pi l l ow m ounds s a m pl e d dur i ng di ve 3970 unde r bot h a nhydr ou s a nd hydr ou s c ondi t i ons S e ve r a l s t a r t i ng c om pos i t i ons ( S C ) r e pr e s e nt i ng t he m os t pr i m i t i ve s a m pl e s f r om t he pi l l ow m ounds w e r e us e d

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99 99 F i gur e 6 2 C ont i nue d c om pos i t i on f i ve ( S C 5 i n T a bl e 6 3) unde r hydr o us c ondi t i ons pr oduc e s a c r ys t a l l i z a t i on s e que nc e s i m i l a r t o t he l ow e r pr e s s ur e m ode l s f o r t he f l ow un i t s w i t h ol i vi ne ( 1140 C ) f ol l ow e d by pl a gi oc l a s e ( 1139 C ) a nd c l i no pyr oxe ne ( 1138 C ) a t l ow e r i ni t i a l c r ys t a l l i z a t i on t e m pe r a t ur e s t ha n t he a nhydr ous c ondi t i ons M ode l s ba s e d on s t a r t i ng c om pos i t i on s i x ( S C 6 i n T a bl e 6 3) unde r h ydr ous c ondi t i ons ha ve ol i vi ne a nd c l i nopyr oxe ne s ha r i ng t he l i qui dus a t 1139 C f ol l o w e d by pl a gi oc l a s e a t 1139 C T r ac e E l e m e n t M od e l s I n or de r t o be t t e r c ons t r a i n t he r e l a t i ve e f f e c t s o f f r a c t i ona l c r ys t a l l i z a t i on a nd m a gm a m i xi ng on M O R B ge o c he m i s t r y, t he va r i a t i ons of s e ve r a l t r a c e e l e m e nt s ( Z r C r Y S r ) w a s m ode l e d. T he c onc e nt r a t i on o f a t r a c e e l e m e nt i n a hypot he t i c a l r e s i dua l m e l t

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100 100 ( C 1 ) w a s c a l c ul a t e d a s s um i ng t e n s u c c e s s i ve i nc r e m e nt s of 5% c r ys t a l l i z a t i on e a c h ( a t ot a l of 50% c r ys t a l l i z a t i on) us i ng t he R a yl e i gh f r a c t i ona t i on e qua t i on ( C 1 = C O *F D 1 ) s t a r t i ng c om pos i t i ons ( C O ) f r om t he s a m e s a m pl e s us e d dur i ng P E T R O L O G m ode l i ng of t he m a j or e l e m e nt s ( 3963 9 3963 11, 3974 11, 3 970 10, 3970 11) a nd t he p r opor t i ons of ol i vi ne pl a gi oc l a s e a nd c l i nopyr oxe ne pr e s e n t dur i ng e a c h s t e p of t he P E T R O L O G c a l c ul a t i ons T he f r a c t i on of r e m a i ni ng m e l t f or e a c h s t e p ( F ) w a s a l w a ys 0. 95. H ow e ve r t he bul k D va r i e d w i t h t he m i ne r a l pr op or t i ons ( e g. D = [ % O l i vi ne *K d Z r i n O l i v i n e ] + [ % P l a gi oc l a s e *K d Z r i n P l a g i o c l a s e ] + [ % C l i nopy r oxe ne *K d Z r i n Cl i n o p y ro x e n e ] ) T he K ds us e d t o m ode l t he be ha vi or of Z r C r Y a nd S r a r e pr ovi de d i n T a bl e 6 4 B ot h Z r a nd Y a r e i nc om pa t i bl e e l e m e nt s a nd t hus i nc r e a s e i n c onc e nt r a t i on a s a m e l t e vol v e s t ow a r d s l ow e r M gO c ont e nt s O n t he ot he r ha nd, C r i s c om pa t i bl e i n c l i nopyr oxe ne a nd S r i s c om pa t i bl e i n pl a gi oc l a s e r e s ul t i ng i n de c r e a s i ng c onc e nt r a t i ons of t he s e e l e m e nt s w h e n t he pr opor t i ons o f c l i nopy r oxe ne a nd/ or pl a gi oc l a s e be c om e s i gni f i c a nt F l ow U n i t s D i ve s 3963 a n d 3974 T he L L D f or Z r C r Y a nd S r s how n i n F i gur e 6 3 r e ve a l s e ve r a l pa r e nt s of s i m i l a r c om pos i t i on a r e ne e de d t o a de qua t e l y m o de l t he obs e r ve d c onc e nt r a t i ons i n t he l a va s f r om bot h di ve s Z r a nd S r e nr i c hm e nt s a r e obs e r ve d i n 3974 5 ( 118 a nd163 pp m ) a n d 3974 6 ( 109 a nd 141 ppm ) r e l a t i ve t o t he r e s t of t he f l ow uni t s a m pl e s W hi l e c ons i de r a bl e s pr e a d i s ob s e r ve d i n S r c onc e nt r a t i ons f or ot he r l a va s f r om di ve 3974, t he e r r or i s a l s o l a r ge f or t hi s e l e m e nt ( 7. 46 ppm ) T he c ha nge i n C r a s pr e di c t e d by t h e f r a c t i ona t i on c a l c ul a t i ons doe s not f i t t he obs e r v e d va r i a t i ons i n t he l a va s how e ve r i f s m a l l a m ount s a s pi ne l w e r e i nvol ve d i n t he i ni t i a l c r ys t a l l i z a t i on hi s t or y ( a l i ke l y s c e na r i o) t he i nf l e c t i on poi n t i n e a c h L L D w oul d oc c ur s oone r a nd t he c a l c ul a t e d c ur ve w oul d pr oba bl y f i t t he da t a be t t e r

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101 T a bl e 6 4 P a r t i t i on C oe f f i c i e nt s U s e d i n M ode l i ng of T r a c e E l e m e nt

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102 102 F i gur e 6 3. L i qui d l i ne s of de s c e nt f or s e l e c t t r a c e e l e m e nt s c a l c ul a t e d f or t he f l ow uni t s s a m pl e d dur i ng di ve s 3963 a nd 3974 u nde r a nhy dr ous ( A ) a nd hyd r ous ( H ) c ondi t i ons S e ve r a l s t a r t i ng c om pos i t i ons ( S C ) r e pr e s e nt i ng t he m os t pr i m i t i ve s a m pl e s f r om t he f l ow uni t s w e r e us e d. O f f A xi s M ou n d s D i ve 3970 W hi l e L L D c a l c ul a t e d f r om t w o p a r e nt s f r om t he s out he r n pi l l ow m ound ( 3 970 10 a nd 3970 11) m a t c he d t he m a j or e l e m e nt da t a t he s e s a m e pa r e nt s c a nnot e xpl a i n t he t r a c e e l e m e nt va r i a t i ons obs e r ve d i n l a va s f r om di ve 3970. T he L L D f or Z r C r Y a nd S r ( F i gur e 6 4) s how t ha t a m or e i nc om pa t i bl e e l e m e nt e nr i c he d pa r e nt i s ne e d t o m ode l t he nor t he r n pi l l ow m ound w hi l e a n i nt e r m e di a t e c om pos i t i on c a n m ode l t he s ur r oundi ng f l ow s

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103 1 03 F i gur e 6 4 L i qui d l i ne s of de s c e nt f o r s e l e c t t r a c e e l e m e nt s c a l c ul a t e d f or t he of f a xi s pi l l ow m ounds s a m pl e d dur i ng di ve 3970 unde r a nhy dr ous ( A ) a nd hydr ous ( H ) c ondi t i ons S e ve r a l s t a r t i ng c om pos i t i ons ( S C ) r e pr e s e nt i ng t he m os t pr i m i t i ve s a m pl e s f r om t he p i l l ow m ounds w e r e us e d. M agm a M i xi n g a n d S ou r c e C h ar ac t e r i s t i c s I f a s e r i e s of l a va s a r e r e l a t e d by f r a c t i ona l c r ys t a l l i z a t i on of a s i ngl e pa r e nt t he n t he c or r e l a t i on f o r pa i r s of e l e m e nt s w i t h l ow pa r t i t i on c oe f f i c i e nt s ( e g Z r N b, C e R b, K R E E ) w i l l be hi gh a nd i nc om pa t i bl e e l e m e nt r a t i os w i l l be ne a r l y c ons t a nt ( e g. Z r / Y Z r / N b, R b/ K L a / L u L a / S m ) A hi gh c or r e l a t i on c oe f f i c i e nt ( r 2 ) i s obs e r ve d be t w e e n t he Z r a nd Y c onc e nt r a t i ons o f l a va s s a m pl e d dur i ng di ve s 3963 ( 0. 905) 3968 ( 0. 981 ) a nd 3970 ( 0. 976) i ndi c a t i ng t he s a m pl e s f r om e a c h di ve c oul d ha ve be e n de r i ve d f r om c om pos i t i ona l l y s i m i l a r pa r e nt s ( F i gur e 6 5 ) F o r d i ve 3974, t he c or r e l a t i on i s poor

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104 104 A B F i gur e 6 5. Z r v. Y f or t he f l ow uni t s ( di ve 3963 a nd 3974) c ha nne l s ( di ve 3968) a nd of f a xi s m ounds ( di ve 3970) A ) C or r e l a t i on c oe f f i c i e nt s c a l c ul a t e d f or l a va s f r om e a c h di ve B ) A nhydr ous L i qui d l i ne s of de s c e nt c a l c ul a t e d f or s e ve r a l s t a r t i ng c om pos i t i ons ( S C ) r e pr e s e nt i ng t he m os t pr i m i t i ve s a m pl e s f r om e a c h di ve a r e pl ot t e d a l ong w i t h t he Z r a nd Y c onc e nt r a t i ons f or t he l a va s of e a c h di ve

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105 105 ( 0. 333) S a m pl e s 3974 1 3974 2 3974 4, 3974 10, a nd 3974 11 p l ot w i t h t he ot he r di ve s a nd w e r e w e l l c or r e l a t e d ( 0. 922) w hi l e s a m pl e s 3974 5, 3974 6, a nd 3974 7 pr oduc e d a c or r e l a t i on of 0. 964 a nd a r e c l e a r l y f or m e d f r om a di f f e r e nt pa r e nt t ha n t he ot he r l a va s S a m pl e s 3974 3, 3974 8, a nd 3974 9 p l ot be t w e e n t he s e t w o e xt r e m e s T he c ha nne l s of f a xi s m ounds a nd t he f l ow s ne a r by t he s e f e a t ur e s a ppe a r t o ha ve f r a c t i ona t e d f r om s i m i l a r l y de pl e t e d pa r e nt s a s e vi de nc e d by t he i r Z r / Y r a t i os ( 2 93 3 0 a nd 2. 82 3. 03 r e s pe c t i ve l y) ( F i gur e 6 6 ) O n t he ot he r ha nd, t he f l ow uni t s f r om di ve 3963 a ppe a r t o ha ve f r a c t i ona t e d f r om s i m i l a r pa r e nt a l m e l t s t ha t ha d hi ghe r i nc om pa t i bl e e l e m e nt a bunda nc e s r e l a t i ve t o t he c ha nne l s a nd o f f a xi s m ounds pa r e nt a l m e l t s ( Z r / Y = 2. 91 3. 32 ) ( F i gur e 6 6) T hi s i s not t he c a s e f or t h e l a va s f r om d i ve 3974. T he da t a s ugge s t t ha t t he m i ddl e s e c t i on of s a m pl e s f r om di ve 3974 ( 3974 3, 5, 6 7, 8 9) a r e f r om a s our c e w i t h hi ghe r a bund a nc e s of hi ghl y i n c om pa t i bl e e l e m e nt s ( Z r / Y = 3. 18 3. 53 ) r e l a t i ve t o t he l a va s bot h f a r t he r a nd c l os e r t o t he a xi s ( Z r / Y = 2 95 3 12) ( F i gur e 6 5 a nd 6 6) T hi s r e l a t i ons hi p i s a l s o no t i c e a bl e i n ot he r i nc om pa t i bl e e l e m e nt r a t i os ( e g C e / Y ; F i gur e 6 7) M i xi ng be t w e e n m e l t s de r i ve d f r o m a m o r e e nr i c he d a nd m or e de pl e t e d s our c e c oul d e xpl a i n t he s pr e a d i n t hi s da t a a s c oul d m ul t i pl e unr e l a t e d f l ow s S u m m ar y T he ge oc he m i s t r y of t he l a va s c ol l e c t e d dur i ng d i ve s 3963, 3974, a nd 3970 a r e c ons i s t e nt w i t h l a r ge e xt e nt s of m e l t i ng of t he m a nt l e be ne a t h t he E P R w hi c h pr oduc e d m ul t i pl e m e l t s of s l i ght l y di f f e r e nt c om pos i t i ons ye t a l l de pl e t e d i n t he m os t i nc om pa t i bl e e l e m e nt s [ L angm ui r e t al 1992] F r a c t i ona t i on of ol i vi ne + pl a gi oc l a s e c l i nopyr oxe ne dur i ng c ool i ng o f t he s e m e l t s oc c ur r e d w i t hi n t he uppe r por t i ons of t he m a gm a c ha m be r ( ~ 1. 5 3 km ) W hi l e t he of f a xi s m ounds r e qui r e a s l i ght l y hi ghe r

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106 106 c r ys t a l l i z a t i on pr e s s ur e ( 1 kba r ) t ha n t he f l o w f r ont s ( 0 5 kba r ) t o e xpl a i n t he c om pos i t i ona l da t a bot h pr e s s ur e s a ppr oxi m a t e t he de pt h of t he m a gm a l e ns a s s oc i a t e d w i t h t he i r s i t e of e r upt i on w hi c h i s or i gi na l l y a t t he a xi s f or e a c h f l ow uni t a nd of f a xi s f or t he m ounds S m a l l a ppa r e nt c ha nge s i n M gO c ont e nt f or e a c h f l ow uni t ( 0. 12 0. 8 w t % ) a nd pi l l ow m ound ( 0. 12 a nd 0. 57 w t % ) i n a ddi t i on t o a na l yt i c a l l y i de nt i c a l t r a c e e l e m e nt c onc e nt r a t i ons s how l i t t l e c r ys t a l f r a c t i ona t i on a c t ua l l y oc c ur r e d onc e t he l a va s w e r e e r upt e d ont o t he s e a f l oor T hi s i s c ons i s t e nt w i t h t he obs e r ve d l a va c r ys t a l c ont e nt s M i xi ng of m e l t s be f or e e r upt i on ont o t he s e a f l oor of t e n be t w e e n r e l a t i ve l y m or e de pl e t e d a nd l e s s de pl e t e d m e l t s a r e a l s o ne c e s s a r y t o de s c r i be t he a nom a l ous m a j or a nd t r a c e e l e m e nt da t a t r e nds w i t hi n f l ow un i t s a nd pi l l ow m ounds

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107 107 F i gur e 6 6. Z r / Y v. Y f or t he f l ow uni t s ( di ve 3963 a nd 3974) c ha nne l s ( di ve 3968) a nd of f a xi s m ounds ( di ve 3970) A nhydr ous a nd H yd r ous L i qui d l i ne s of de s c e nt w e r e c a l c ul a t e d f or s e ve r a l s t a r t i ng c om pos i t i ons ( S C ) r e pr e s e nt i ng t he m os t pr i m i t i ve s a m pl e s f r om e a c h di ve

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108 108 F i gur e 6 7. C e / Y b v. C e f or t he f l ow uni t s ( di ve 3963 a nd 3974) c ha nne l s ( di ve 3968) a nd of f a xi s m ounds ( di ve 3970 )

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109 C H A P T E R 7 D I S C U S S I O N C om pa r e d t o t he hi gh s a m pl i ng de n s i t y a t m a ny w e l l know n r i dge s ( e g. G or da R i dge J ua n de F uc a R i dge M i d A t l a nt i c R i dge E a s t P a c i f i c R i s e e t c ) f e w f l ow s f r om t he e xt e ns i ve M O R s ys t e m ha ve be e n une qui voc a l l y i de nt i f i e d, m a ppe d, a nd s a m pl e d. I n or de r t o m a ke qua nt i t a t i ve s t a t e m e nt s r e ga r di ng t he m a gm a t i c a nd vol c a ni c s y s t e m s a c t i ve a t a r i dge t he r e l a t i ve ge ol ogi c c ont e xt of s a m pl e s m us t be de t e r m i ne d. O nl y t he n c a n t he de gr e e of c h e m i c a l he t e r oge ne i t y w i t hi n s i ngl e f l ow s be e va l ua t e d a nd r e l a t e d t o t he f a c t or s c ont r ol l i ng t he c om pos i t i ons o f l a va s e r upt e d ont o t he s e a f l oor C om b i n i n g an d E val u a t i n g P e t r ogr ap h i c an d G e oc h e m i c al R e s u l t s L a va s f r om t he ne ovol c a ni c z one be t w e e n 9 28 N a nd 9 50 N E P R a r e dom i na nt l y c om pos e d of ho m oge ne ous i nc om pa t i bl e t r a c e e l e m e nt de pl e t e d t hol e i i t i c ba s a l t s ( N M O R B ) T he m a j o r i t y of t he s e l a va s f or m ove r l a ppi ng f l ow uni t s e xt e ndi ng a w a y f r om t he a xi s but c ha nne l s a nd of f a xi s m ounds a r e a l s o pr om i ne nt f e a t ur e s of t he vol c a ni c l a nds c a pe T he pe t r oge ne t i c hi s t or y of t he s e l a va s i s i ni t i a l l y c ont r ol l e d by l a r ge e xt e nt s of m e l t i ng i n t he m a nt l e be ne a t h t he E P R t ha t pr oduc e pr i m a r y pa r e nt a l m e l t s [ L angm ui r e t al 1992] D ur i ng a s c e ns i on t o, a n d s t or a ge w i t hi n t he c r us t t he s e m e l t s a r e m odi f i e d f r om t he i r or i gi na l m or e pr i m i t i ve c om pos i t i ons by c r ys t a l f r a c t i ona t i on of ol i vi ne + pl a gi oc l a s e c l i nopyr oxe ne a nd m i xi ng of m a gm a s w i t h va r i a bl e c om pos i t i ons P e t r ogr a phi c e xa m i na t i on a nd m a j or e l e m e nt m o de l i ng s ugge s t t ha t t he of f a xi s m ounds a nd t he f l ow uni t s w e r e f e d by di f f e r e nt pa r t s of t he m a gm a l e n s c e nt e r e d be ne a t h t he a xi s of t he E P R R e l a t i ve t o t he f l ow uni t s hi ghe r c r ys t a l l i z a t i on pr e s s ur e s ( 1

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110 110 kba r ) a c t i ng on m e l t s w i t h l ow e r a bunda nc e s of i nc om pa t i bl e e l e m e nt s i s r e qui r e d t o m ode l ove r a l l obs e r ve d c he m i c a l t r e nds w i t hi n t he r e l a t i ve l y m o r e e vol ve d pi l l ow m ounds S i nc e t he pi l l ow m ounds a r e l oc a t e d ~ 1 km e a s t of t he a xi s a t 9 30 N i t i s pos s i bl e t ha t t he s our c e of t he s e r e c e nt vo l c a ni c c ons t r uc t s i s t he e a s t e r n e dge a nd c ool e r por t i on o f t he m e l t l e ns T he pr e s e nc e of up t o 10% c r ys t a l s c om pos e d of ol i vi ne pl a gi oc l a s e a nd c l i nopyr oxe ne w i t hi n e a c h pi l l ow m ound i s a t odds w i t h c a l c ul a t e d e qui l i br i um m i ne r a l ogy of t he m ound l a va s ( ~ 7. 7 w t % M gO ) T he obs e r ve d c om pos i t i ons f or ol i vi ne p l a gi oc l a s e a nd c l i nopyr oxe ne a r e F o 8 5 8 6 A n 6 6 7 3 ( m a i nl y A n 6 8 6 9 ) a nd M g# 80 87 ( m a i nl y M g# 84 86 ) r e s pe c t i ve l y, w hi l e ove r a l l c a l c ul a t e d e qui l i br i um c om pos i t i ons dur i ng c r ys t a l l i z a t i on a r e F o 8 2 A n 6 8 a nd M g# 83 ( T a bl e 6 3) H ow e ve r a l l t hr e e m i ne r a l s a r e not f ound on t he l i qui dus a t or unde r e qui l i br i um c ondi t i ons s pe c i f i e d by t he of f a xi s m ounds C l i nopyr oxe ne i s not pr e di c t e d t o c r ys t a l l i z e w i t h ol i vi ne a nd pl a gi oc l a s e unt i l s i gni f i c a nt c ool i ng a nd c r ys t a l l i z a t i on oc c ur s a t l ow pr e s s ur e s ( < a f e w K b) H i ghe r c r ys t a l l i z a t i on pr e s s ur e s w oul d br i ng c l i nopyr oxe ne c l os e r t o t he l i qui dus but t hi s w oul d not e xpl a i n t h e m or e pr i m i t i ve ol i vi ne c om pos i t i ons or t he s pr e a d i n pl a gi oc l a s e a nd c l i nopyr oxe ne c om pos i t i ons out s i de of pr e di c t e d e qui l i br i um va l ue s ( F i gur e 4 5 ) M i xi ng of m or e p r i m i t i ve m a gm a s w i t h m or e e vol ve d, hi ghe r c r ys t a l l i ni t y m a gm a s c oul d e xpl a i n t he s e va r i a t i ons i n pha s e c he m i s t r y a s w e l l a s t he r e ve r s e d z oni ng obs e r ve d i n pl a gi oc l a s e a nd c l i nopyr oxe ne c r ys t a l s ( F i gur e 4 6) A ddi t i ona l e vi d e nc e f or di s e qui l i br i um be t w e e n phe noc r ys t s a nd t he i r hos t m e l t i nc l ude d e m ba ye d c r ys t a l s w i t h r ounde d e dge s obs e r ve d i n l a va s f r o m bot h pi l l ow m ounds

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111 111 R e l a t i ve t o t he of f a xi s m ound s c a l c ul a t i ons i ndi c a t e l a va s t ha t c om pr i s e t he f l ow uni t s e r upt e d f r om t he a xi s ne a r 9 50 N c r ys t a l l i z e d f r om s l i ght l y di f f e r e nt pa r e nt m e l t s a t hi ghe r t e m pe r a t ur e s a nd l ow e r pr e s s ur e ( 0. 5 kba r ) L e s s t ha n 4 w t % ol i vi ne a nd pl a gi oc l a s e m i c r ophe noc r ys t s a r e r e qui r e d t o pr od uc e t he va r i a t i ons i n e l e m e nt s obs e r ve d i n t he l a va f l ow uni t s T he c a l c ul a t i ons a r e c ons i s t e nt w i t h t he ne a r a xi s f l ow s be i ng f e d by a s ha l l ow e r ho t t e r po r t i on o f t he m a gm a l e ns I ndi vi dua l l a va s w i t hi n e a c h f l ow uni t ha ve ve r y s i m i l a r t o ne a r l y i de nt i c a l m a j o r a nd t r a c e e l e m e nt c om pos i t i ons I n a ddi t i on t he r e l a t i ve c ha nge s i n c r ys t a l l i ni t y obs e r ve d be t w e e n t he body of a n i ndi vi dua l f l ow uni t a nd t he f l ow f r ont a r e < 1 2. 4% ( by vol um e ) ( T a bl e 4 5) T he s e obs e r va t i on s s ugg e s t t ha t f r a c t i ona l c r ys t a l l i z a t i on doe s not s i gni f i c a nt l y a f f e c t t he c om po s i t i on o f a f l ow onc e i t i s e r upt e d ont o t he s e a f l oor H ow e ve r t he c h a nge s i n c r ys t a l l i ni t y a s pr e di c t e d by m a j or e l e m e nt m ode l s do no t c ons i s t e nt l y m a t c h t he c r ys t a l l i ni t y c ha nge s ob s e r ve d i n t he f l ow uni t s f r om e a c h di ve s ugge s t i ng i nde pe nde nt f or m a t i on of s uc c e s s i ve f l ow uni t s ( T a bl e s 6 1 a nd 6 2 ) D e s pi t e t he c he m i c a l hom oge ne i t y of e a c h f l ow uni t l a r ge ol i vi ne xe noc r ys t s ( 0. 55 0. 6 m m ) a nd xe noc r ys t a l pl a gi oc l a s e c l ot s ( 1 2. 5 m m ) pr ovi d e e vi de nc e f or t he m i xi ng of m a gm a s pr i or t o e r upt i on W i t hi n t he s e c ond a nd t hi r d f l ow uni t s f r o m di v e 3974, s e ve r a l l a va s a ppe a r t o ha ve va r i a bl y gr e a t e r i nc om pa t i bl e e l e m e nt a b unda nc e s H ow e ve r onl y 3974 5 i s a na l yt i c a l l y di f f e r e nt f r om t he ot he r l a va s c om pr i s i ng t he m i ddl e f l ow uni t D ue t o t he e r r a t i c di ve t r a ve r s e f ol l ow e d ( F i gur e 1 3 ) t he c ont a c t r e l a t i ons hi ps be t w e e n t he f i r s t a nd s e c ond f l ow uni t bounda r i e s a r e l e s s w e l l c on s t r a i ne d. I f t he f l ow c ont a c t s a r e a s t he y ha ve be e n de f i ne d i n t he I nt r oduc t i on t he n t hi s s ugge s t s e i t he r m i xi ng be t w e e n m or e

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112 112 e nr i c he d a nd m o r e de pl e t e d m a gm a s ha s oc c ur r e d or t he s t r a t i gr a phi c r e l a t i ons a r e m or e c om pl e x f or t he l a va s of f a xi s O t he r l a va s f r om t he E P R a l s o c ont a i n a bunda n t e vi de nc e f or m a gm a m i xi ng [ H e k i ni an e t al 1989; Si nt on and D e t r i c k 1992; P an and B at i z a 2002, 2003] P an and B at i z a [ 2003] r e c or de d u p t o s i x c he m i c a l l y a nd s t a t i s t i c a l l y di s t i nc t pl a gi oc l a s e popul a t i ons w i t hi n l a va s f r om 9 30 N 10 30 N a nd 11 20 N O n t he ot he r ha nd, t he c om pos i t i ons of ol i vi ne w e r e m or e hom oge nous ( up t o t hr e e popul a t i ons ) p r e s um a bl y due t o a hi ghe r r a t e of c he m i c a l di f f us i on a nd be c a us e pl a gi oc l a s e qui c kl y j oi ns ol i vi ne on t he l i qui dus but s t a y s on l onge r [ P an and B at i z a 2003] T he m i xi ng of m a gm a s t o f or m t he s e l a va s pr oba bl y oc c ur s a s a m e l t pa s s e s t hr ough t he l a r ge m us hy z on e be ne a t h t he m a gm a l e ns w he r e i t j oi ns ot he r m e l t s a nd a d ds s i ngl e c r ys t a l s c r ys t a l c l us t e r s a nd c r ys t a l a ggr e ga t e s t o i t s m a s s [ P an and B at i z a 2 00 3] I t i s unc l e a r w he t he r t he c r ys t a l ne t w or ks i n t he m us h z one a nd t he i r s ubs e que nt c om pa c t i on pr oduc e t he m e l t l e ns [ H us s e noe de r e t al 1996; N at l and a nd D i c k 1996; P hi l pot t s e t al 1996; P an and B at i z a 2003 ] or i f t he m e l t l e ns r e pr e s e nt s ne w l y i nj e c t e d m a gm a f r e s h f r om a m a nt l e s our c e [ H oof t e t al 1996; H us s e noe de r e t al 1996; C ar bot t e e t al 2000] I f t he f i r s t hypot he s i s i s t r ue t he n t he m a gm a i n t he l e ns m i ght c ont a i n m e l t s of e vol ve d c om pos i t i on [ N at l and a nd D i c k 1996; P an and B at i z a 2003] w he r e a s t he s e c ond m ode l s ugge s t s i t i s c om pr i s e d of r e l a t i ve l y pr i m i t i ve m a gm a s [ Si ngh e t al 19 98] W hi l e t he f l ow f r ont s c ha nne l s a nd of f a xi s m ounds a l l ha ve N M O R B c om pos i t i ons s e ve r a l l a va s f r om t hi s r i dge s e gm e nt ha ve e nr i c he d i nc om pa t i bl e e l e m e n t a bunda nc e s ( E M O R B ) s ugge s t i ng a n a ddi t i o na l e nr i c he d m a nt l e c om pone nt i s

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113 113 pe r i odi c a l l y a dde d t o t he m a gm a t i c s ys t e m be ne a t h t he E P R r i dge [ P e r f i t e t al 1994; N i u e t al 1999] H e t e r oge n e i t y I n d e x f or M i d O c e an R i d ge L ava F l ow s T he H e t e r oge ne i t y I nde x ( H I ) s e e ks t o qua nt i f y c he m i c a l va r i a t i on out s i de of a na l yt i c a l unc e r t a i nt y w i t hi n w e l l m a ppe d a nd s a m pl e d l a va f l ow s I ni t i a l l y de ve l ope d a nd na m e d t he hom oge ne i t y i nde x by R hode s [ 1983] f or c om pa r i s on of H a w a i i a n l a va s R ubi n e t al [ 2001] l a t e r r e na m e d i t t he he t e r oge n e i t y i nde x a nd c om pi l e d a da t a ba s e of 10 f l ow s f r om t he nor t he r n a nd s out he r n E P R [ P e r f i t and C hadw i c k 1998; R ubi n e t al 1998; Si nt on e t al 2002 ] J ua n de F uc a R i dge [ L oc k w ood and L i pm an 1987; E m bl e y and C hadw i c k 1994; Sm i t h e t al 1994; Sm i t h, 1999] G or da R i dge [ Si nt on 1997; Sm i t h 1999] a nd M i d A t l a nt i c R i dge [ B r y an e t al 199 4; C hadw i c k e t al 1995] ( T a bl e 7 1 ) Si nt on e t al [ 2002] a dde d a n a ddi t i ona l f our f l ow s f r om t he s out he r n E P R ( T a bl e 7 1) T he i nde x i s de f i ne d a s ( Si / P i ) / n w he r e Si / P i r e pr e s e nt s t he 2 s i gm a s t a nda r d de vi a t i on a bout t he m e a n of a n e l e m e nt i di vi de d by t he a na l yt i c a l pr e c i s i on f or e l e m e nt i a nd n r e f e r s t o t he num be r of e l e m e nt s us e d ( 10: S i T i A l F e M n, M g, C a N a K P ) I f t he H I of a f l ow i s 1, t he n t he obs e r ve d c he m i c a l he t e r oge ne i t y i s w i t hi n a na l yt i c a l bounda r i e s A s a m e a ns of c om pa r i s on be t w e e n f l ow s a t t he E P R a nd ot he r M O R f l ow s t he he t e r oge ne i t y i nde x f or t he s i x f l ow uni t s a t 9 5 0 N a nd t he t w o o f f a xi s m ounds a t 9 30 N w a s c a l c ul a t e d ( T a bl e 7 2) O ve r a l l i nd e xe s f or t he f l ow uni t s f r om di ve 3963 a r e < 1. 05, t he t hr e e f l ow uni t s f r om di ve 3974 a r e 1. 07, 2. 04, a nd 1. 56 r e s pe c t i ve l y, a nd t he t w o pi l l ow m ounds a r e 2. 88 a nd 0. 59. R u bi n e t al [ 2001] a t t r i but e d t he m or e he t e r oge ne ous na t ur e of t he 1996 nor t h G or da f l ow ( H I = 2. 06) S e r oc ki V ol c a no f l ow

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114 T a bl e 7 1 C ha r a c t e r i s i t i c s of M i d O c e a n R i dge L a va s

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115 T a bl e 7 2 H e t e r oge ne ous I nde x C a l c ul a t i ons f o r T he F l ow U ni t s ( 9 50' N ) a nd O f f A xi s M ounds ( 9 30' N )

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116

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117 117 ( H I = 3. 27) a nd t he nor t h C l e f t P i l l ow M ounds t o be t he r e s ul t of m a gm a m i xi ng a n d f r a c t i ona l c r ys t a l l i z a t i on. W hi l e t he m i nut e c he m i c a l he t e r oge ne i t y of t he f l ow uni t s a nd pi l l ow m ounds s t udi e d he r e h a ve pr e vi ous l y be e n de t e r m i ne d t o m a i nl y r e f l e c t m i xi ng be t w e e n m a gm a s of s l i ght l y di f f e r e nt c om pos i t i ons ot he r f a c t or s a f f e c t i ng H I m us t be c ons i de r e d. F or i ns t a n c e t he num be r of s a m pl e s ( n ) r e qui r e d f or a s t a t i s t i c a l l y s i gni f i c a nt r e s ul t i s m or e t ha n t he s a m pl i ng de n s i t y of t he f l ow uni t s a nd pi l l ow m ounds A l s o, us e of s t a nda r d de vi a t i ons t o de s c r i be t he e r r or a s s oc i a t e d w i t h t he m e a n a r e c ont i nge nt upo n a nor m a l di s t r i but i on of r e s ul t s a bout t ha t m e a n. A ge ne r a l l y nor m a l di s t r i but i on i s obs e r ve d f or m os t of t he m a j or e l e m e nt s a nd i s di s c us s e d i n A ppe ndi x B I n or de r t o i nc r e a s e t he num be r of s a m pl e s c ons i de r e d ( n ) ne w H I c a l c ul a t e d us i ng da t a f r om a l l t he f l ow f r ont s f r om di ve s 3963 ( ne w n = 10) a nd 3 974 ( ne w n = 12) w e r e 4. 41 a nd 2 36 r e s pe c t i ve l y, w hi l e t he ne w H I f or t he t w o pi l l ow m ounds ( ne w n = 4) w a s 2. 35 ( T a bl e 7 2) T he s e va l ue s a r e s t i l l c om pa r a bl e i n m a gni t u de i f not s l i ght l y hi ghe r t ha n t he m os t he t e r oge ne ous l a va f l ow s f r om t he a r e a M os t f l ow vol um e s a r e e s t i m a t e d f r om m a ppe d s pa t i a l e xt e nt s a nd de pt h s F or di ve s 3963 a nd 3974, pa r t s of e a c h f l ow uni t a r e i nva r i a bl y c ove r e d by a dva nc i ng f l ow s c om i ng f r om t he a xi s A l s o, t he r e i s no w a y t o de t e r m i ne t he t ot a l l a t e r a l e xt e nt s of e a c h f l ow s i nc e t he y c a n, f or i ns t a nc e e r upt a l ong t h e e nt i r e t y of a 7. 5 km f i s s ur e l i ke bot h t he 1991 1992 B B Q a nd A l do K i hi f l ow s di d. T he r e f or e t he vol um e s of e a c h f l ow uni t a r e e s t i m a t e d f r om a ge ne r i c w e dge s ha pe a ppr oxi m a t i ng t he a ve r a ge s i z e d l obe 1000 m w i de 500 m l ong, a nd 10 m hi gh f or a vol um e of 5x10 6 m 3 T he e r upt e d vol um e of pi l l ow l a va c om pr i s i ng e a c h of f a xi s m ound w a s e s t i m a t e d t o be a bout 2x10 6 m 3 f or t he

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118 118 ba s e d on m i c r oba t hym e t r y m e a s ur e m e nt s a nd a n e s t i m a t e d c one s ha pe w i t h r a di u s of 250 m a nd he i ght o f 30 m I n t he gl oba l da t a ba s e a g e ne r a l c or r e l a t i on e xi s t s be t w e e n t he H I of a f l ow a n d i t s e r upt e d vol um e ( F i gur e 7 1) [ R ubi n e t al 1998 ] T he e xc e pt i ons i nc l ude m a ny o f t he s out he r n E P R f l ow s s uc h a s A ni m a l F a r m w hi c h ha s one of t he l a r ge s t e r upt e d vol um e s ye t i s e xt r e m e l y hom oge ne ous P r e s um a bl y, s t e a dy s t a t e m a gm a r e s e r voi r s be ne a t h f a s t s pr e a di ng r i dge s l i ke t he E P R a r e w e l l m i xe d a nd pr om ot e c he m i c a l hom oge ne i t y w i t hi n t he l a va f l ow s a t t hos e r i dge s de s pi t e t he vol um e e r upt e d [ Si nt on and D e t r i c k 1992] T he H I of t he f l ow s i s a l s o i nve r s e l y c or r e l a t e d w i t h s pr e a di ng r a t e [ R ubi n e t al 2001] ( F i gur e 7 1) H ow e ve r bot h t he C l e f t P i l l ow M o unds a nd t he A l do K i hi F l ow ha ve H I hi ghe r t ha n w oul d b e pr e di c t e d a t t he i r r e s pe c t i ve s pr e a di ng r a t e s I n a ddi t i on, e a c h di ve w he r e a l l l a va s a m pl e s w e r e c ons i de r e d i n t he c a l c ul a t i ons a l s o hi ghe r t ha n e xpe c t e d H I M od e l s f or C r u s t al C on s t r u c t i o n O n t he s e a f l oor t he t hi c kne s s of s e di m e nt c ove r i nc r e a s e s w i t h t i m e a s c r us t s pr e a ds a w a y f r om t he a xi s of t he r i dge c r e s t T he r e f or e obs e r va t i ons r e ga r di ng t he a m ount of ove r l yi ng s e di m e nt i n a ddi t i on t o di s t a nc e f r om t he A S C T i n r e l a t i on t o s pr e a di ng r a t e pr ovi de r e l a t i ve a ge s of t he und e r l yi ng l a va s I n ge ne r a l l a va s c l os e s t t o t he A S C T a nd t ha t c om m onl y h a ve t he l e a s t s e di m e nt c ove r a r e a s s um e d t o be younge r t ha n l a va s f a r t he r a w a y f r om t he s pr e a di ng a xi s A f e w pr e vi ous s t udi e s us i ng t he s ubm e r s i bl e A L V I N doc um e nt e d a nom a l ous l y young l ooki ng l a va f l ow s a nd pi l l ow m ounds up t o 4 km f r om t he A S C T ne a r 9 30 N a nd 9 50 N E P R [ P e r f i t e t al 1994; M ac donal d e t al 1996; P e r f i t and C hadw i c k 1998] R e l a t i ve l y t hi n s e di m e nt c ove r f r e s h gl a s s l i ght M n c oa t i ng, a nd young U s e r i e s di s e qui l i br i um a ge s of ba s a l t s a m pl e s i ndi c a t e t ha t s om e of f a xi s l a va s w e r e younge r t ha n pr e di c t e d ba s e d on t he r e r e l a t i ve

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119 119 F i gur e 7 1 T he he t e r oge ne i t y i nde x c a l c ul a t e d f or i ndi vi dua l f l ow s v. t he i r f l ow vo l um e a nd s pr e a di ng r a t e ( m odi f i e d f r om R ubi n e t al [ 20 01] ) .

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120 120 di s t a n c e f r om t he a xi s [ G ol ds t e i n e t al 1994; P e r f i t e t al 1994; M ac donal d e t a l 1996; P e r f i t and C hadw i c k 1998; Si m s e t al 2003] M or e ove r s om e of f a xi s l a va s f r om t he E nde a vour s e gm e nt of t he J ua n de F uc a R i dge a l s o a ppe a r t o be younge r t ha n one w ou l d pr e di c t ba s e d on s pr e a di ng r a t e s ugge s t i ng t hi s p r oc e s s of of f a xi s vol c a ni s m oc c ur s on r i dge s of i nt e r m e di a t e s pr e a di ng r a t e a s w e l l [ G ol d s t e i n e t al 1992] A not he r obs e r va t i on s uppor t i ng t he hypot he s i s of of f a xi s vol c a ni s m i nvol ve s t he a ppa r e nt doubl i ng i n t hi c kne s s of s e i s m i c l a ye r 2 A w i t hi n s e ve r a l ki l om e t e r s of t he E P R r i dge a xi s a s s ugge s t e d by s e i s m i c r e f r a c t i on a nd s e i s m i c r e f l e c t i on pr of i l e s [ H ar di ng e t al 1993; V e r a and D i e bol d 1993; C hr i s t e s on e t al 1994, 1996; H oof t e t al 1996; C ar bot t e e t al 1997; Sc hout e n e t al 1999 2001] L a ye r 2A c o r r e s ponds t o t he 0. 5 km t hi c k uppe r vol c a ni c por t i on of oc e a ni c c r us t w hi c h i s l a r ge l y t hought t o f or m a t t he r i dge c r e s t H ow e ve r i f t he s e i s m i c da t a i s t o be be l i e ve d, t he n ha l f o f t he e xt r us i ve s e que nc e i s e m pl a c e d out s i de of t he A S C T a r e a E i t he r vol c a ni c e r upt i ons m us t oc c ur of f a xi s or l a r ge vol um e f l ow s e r upt e d i ni t i a l l y on a xi s m us t f l ow a s s he e t s i n c ha nne l s or i n l a va t ube s l ong di s t a nc e s ont o t he c r e s t a l pl a t e a u [ C h r i s t e ns on e t a l 1994; F or nar i e t al 1998; Sc hout e n e t al 1999 ] I n a ddi t i on, t he ~ 3. 5 t o 4 km w i de of t he C e nt r a l A nom a l y M a gne t i c H i gh ( C A M H ) w hi c h r e f l e c t s t he z one of r e c e nt l y e m pl a c e d l a va s [ M ar s hal l and C ox 1972; K l i t gor d e t al 1975] i m pl i e s a r e l a t i ve l y w i de n e ovol c a ni c z one [ F or nar i e t al 1998; Sc hout e n e t al 1999] T h e w i dt h of t he C A M H c or r e s pond s t o bot h t he l oc a t i ons of young of f a xi s l a va s a nd z o ne of t hi c ke ni ng o f s e i s m i c l a ye r 2A P i l l ow m ounds a nd r i dge s on t he c r e s t a l pl a t e a u, l a r ge r e di f i c e s s uc h a s s e a m ount s a nd f l ow s a s s oc i a t e d w i t h ne a r by a bys s a l hi l l s m a y a l s o c ont r i but e s ubs t a nt i a l vol um e s of

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121 121 l a va s f or c r us t a l c ons t r uc t i on [ e g. G ol ds t e i n e t al 1994; P e r f i t e t al 1994; Si m s e t al 2003] T w o c r us t a l ge ne s i s m ode l s ha v e be e n p r opos e d t o e xpl a i n bot h t he a nom a l ous l y young a ge s of s om e of f a xi s l a va s a nd a n obs e r v e d of f a xi s t hi c ke ni ng of s e i s m i c l a ye r 2A H oof t e t al [ 1996] s ugge s t t he e xt r us i ve c om pone nt of c r us t a l a c c r e t i on r e s ul t s f r o m num e r ous s m a l l a nd f r e que nt e r upt i ons c onf i ne d l a r ge l y t o t he A S C T w hi l e l e s s f r e que nt l a r ge r e r upt i ons e s c a pe a nd f l ow ont o t he c r e s t a l pl a t ue a u. H ow e ve r f e w f l ow s l a r ge e nough t o c ont r i but e s ubs t a nt i a l l y t o t he t hi c ke ni ng of t he e xt r us i ve c om pone nt of l a ye r 2A ha ve be e n obs e r ve d ne a r m i d oc e a n r i dge s [ e g D av i s 1982; M ac donal d e t al 1989 ; P e r f i t and C hadw i c k 1998] a nd pe t r ol ogi c s a m pl i ng s ugge s t s t he s e l a va f l ow s or f i e l ds a r e c om pos e d of m ul t i pl e t e m por a l l y r e l a t e d e r upt i ons [ H al l and Si nt on 1996] A n a l t e r na t i ve m ode l s ugge s t e d by P e r f i t e t al [ 1994] pr opos e s t ha t s m a l l vol um e e r upt i ons out s i de t he A S C T r e s ul t i n di s c ont i nuo us pi l l ow r i dge s ( < 10 m hi gh) s i t ua t e d a l ongs i de f i s s ur e s a nd f a ul t s t ha t pa r a l l e l t he r i dge ( F i gur e 7 2) T hi s m ode l a l s o s ugge s t s a pos s i bl e s our c e f o r t he s e of f a xi s e r upt i ons a s t he e dge of a s ub a xi a l m a gm a c ha m be r or di ke s t ha t e xt e nd l a t e r a l l y a w a y f r om t he a xi a l m e l t l e ns [ G ol ds t e i n e t al 1994; P e r f i t e t al 1994; P e r f i t and C hadw i c k 1998] P e t r ogr a phi c a nd ge oc he m i c a l a na l ys i s of t he f l ow uni t s c ha nne l s a nd pi l l ow m ounds f r om 9 30 N a nd 9 50 N E P R c a l l f or t h e s yne r gy of t he s e t w o m ode l s W hi l e r e c e nt vol c a ni s m f or t hi s s e gm e nt of t he r i dge i s l i m i t e d t o t he 1991 a xi a l f l ow t ha t e m pl a c e d a bout 4 6x10 6 m 3 of f a i r l y pr i m i t i ve l a va w i t hi n a nd ne xt t o t he A S C T [ H ay m on e t al 1991, 1993 ; V on D am m e t al 199 5; G r e gg e t al 1996] e a c h l obe d f l ow uni t l i ke t hos e i nve s t i ga t e d dur i ng di ve s 3963 a nd 3974, r e p r e s e nt s a s i m i l a r vol um e o f

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122 122 F i gur e 7 2 T h r e e di m e ns i ona l di a gr a m o f oc e a ni c c r us t f o r m a t i on a l ong t he nor t he r n E a s t P a c i f i c R i s e ( m odi f i e d f r om P e r f i t e t a l [ 199 4] a nd p r e s e nt e d i n P e r f i t and C hadw i c k [ 1998] ) S he e t a nd l oba t e f l ow s o r i gi na t i ng i n t he a xi a l s um m i t t r ough ( A S T ) a r e t he dom i na nt f or m of on a xi s e r u pt i ons w he r e a s pi l l ow l a v a s l i nke d t o f a ul t s a nd f i s s ur e s dom i na t e of f a xi s e r up t i ons O f f a xi s ba s a l t s ha ve di ve r s e c he m i c a l c om pos i t i ons ( N T a nd E M O R B ) c om pa r e d t o a xi a l ba s a l t s ( N M O R B ) l a va e r upt e d f r om t he a xi s a nd ont o t he c r e s t a l pl a t e a u. E a c h l obe i s c om po s e d dom i na nt l y of a phy r i c t o s pa r s e l y phyr i c N M O R B t he l a t e r a l e xt e nt s of w hi c h c oul d e xt e nd s e ve r a l km s a l ong t he r i dge a xi s S om e of t he s e f l ow s r e a c he d f a r t he r di s t a nc e s of f a xi s vi a t he c ha nne l s pl a c i ng f l ow s of va r i a bl e a ge a nd c he m i c a l c om pos i t i on ne xt t o e a c h ot he r H ow e ve r vol c a ni s m i s not l i m i t e d t o t he a xi s but a l s o oc c ur s ne a r t h e c onf l ue nc e of vol c a ni c a nd t e c t oni c f or c e s s e ve r a l km of f a xi s F e d by t he c ool e r m or e e vol ve d e dge s of t he m a gm a l e ns pi l l ow l a va s c ons t r uc t m ounds t ha t f i l l i n t he f i s s ur e f r om w hi c h t he y e r upt e d

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123 C H A P T E R 8 C O N C L U S I O N S G e oc he m i c a l a nd pe t r ogr a phi c r e s ul t s f or a s e r i e s of i ndi vi dua l f l ow uni t s c ha nne l s a nd of f a xi s pi l l ow m ounds ha vi ng good ge ol ogi c c ont r ol a dd t o t he pr e vi ous obs e r va t i ona l a nd i s ot opi c e vi de nc e i ndi c a t i ng l a ye r 2A of t he oc e a ni c c r us t a t t he E a s t P a c i f i c R i s e ( E P R ) i s f or m e d by t he a c c um ul a t i on of l a va s w i t hi n a m u c h w i de r ne ovol c a ni c z one t ha n pr e vi ous l y t hought A c om bi na t i on of e r up t i ons f r om on a nd of f a xi s s i t e s h e l p e xpl a i n bot h how l a ye r 2A t hi c ke n s w i t hi n a f e w km of t he a xi s a nd t he a nom a l ous l y young i s ot opi c a nd vi s ua l c ha r a c t e r i s t i c s of s om e l a va s f a r ont o t he c r e s t a l pl a t e a u. T he m a j or ge oc he m i c a l a nd pe t r og r a phi c r e s ul t s a r e s um m a r i z e d be l ow : T he bodi e s of e a c h f l ow u ni t a r e c om pos e d of l oba t e s a nd s he e t f l ow s w hi l e pi l l ow l a va s c om pr i s e t he f l ow f r ont s O n t he ot he r ha nd t he of f a xi s m ounds a r e c om pos e d of pi l l ow l a va s w hi l e t he c ha nne l s a r e c om pos e d of s he e t f l ow s T he f l ow uni t s a r e a phyr i c t o s pa r s e l y pl a gi oc l a s e phyr i c ba s a l t s w hi l e t he of f a xi s m ounds a r e m ode r a t e l y phyr i c pi l l ow ba s a l t s c ont a i ni ng ol i vi ne pl a gi oc l a s e a nd c l i nopyr oxe ne m i c r ophe noc r ys t s c om m onl y f ound i n c l ot s T he l a va s c om pr i s i ng e a c h f l ow uni t t o e i t he r s i de of t he a xi s a t 9 50 N t he of f a xi s pi l l ow m ounds a t 9 30 N a nd t he t w o c ha n ne l s a t 9 28 N a r e a l l i nc om pa t i bl e e l e m e nt de pl e t e d ( N M O R B ) t hol e i i t i c ba s a l t s M a j or a nd t r a c e e l e m e nt c onc e nt r a t i ons a nd m ode l i ng s how l a va s f r om e a c h f l ow uni t a nd pi l l ow m ound a r e c he m i c a l l y s i m i l a r a nd a ny e xt r a a na l yt i c a l c he m i c a l va r i a t i on c a n b e e xpl a i ne d by l i m i t e d ( < 4% ) f r a c t i ona t i on of ol i vi ne + pl a gi oc l a s e

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124 124 c l i nopyr oxe ne a s w e l l a s m i xi ng be t w e e n r e l a t i ve l y m or e e nr i c he d a nd m or e pr i m i t i ve pa r e nt a l m a gm a s i n t he a xi a l m a gm a c ha m be r M i xi ng i s a l s o e vi de nt i n t he f l ow uni t s a nd of f a xi s m ou nds a s obs e r ve d vi a t he pr e s e nc e of xe noc r ys t s xe noc r ys t a l c l ot s a nd di s e qui l i br i um f e a t ur e s T he s e e f f e c t s a r e pr i m a r i l y i m pr i nt e d on m a gm a c om pos i t i ons be f or e t he y a r e e r upt e d ont o t he s e a f l oor a nd r e s ul t i n h e t e r oge n e i t y be t w e e n f l ow s not w i t hi n f l ow uni t s T he hom oge ne i t y of t he f l ow uni t s i s c om pa r a bl e t o ot he r w e l l c ons t r a i ne d M O R f l ow s a nd t hus a dds t o t he l i m i t e d da t a ba s e

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125 A P P E N D I X A E 2 I C P M S A N A L Y T I C A L P R O T O C O L T he f ol l ow i ng d i s s ol ut i on a nd a na l yt i c pr oc e dur e f or t r a c e e l e m e nt a na l ys i s w a s r e f i ne d by G e or ge K a m e nov a t t he U ni ve r s i t y of F l or i d a ( U F L ) a nd r e f e r e nc e s t he c l e a n l a b f a c i l i t y a nd E l e m e nt I I I C P M S a t U F L D I S S O L U T O N P H A S E 1 1. C hoos e 2 3 s t a nda r ds of s i m i l a r c he m i c a l c om pos i t i on t o your s a m pl e s ( e g f or M O R B a na l ys i s w e c ho s e t w o c e r t i f i e d r oc k s t a n da r ds [ A G V 1 a nd B C R 2] a n d t w o i n hous e M O R B s t a nda r ds [ E N D V a nd 2392 9] ) 2. C l e a n a nd l a be l a t a l l ( 6 m l ) he x c a p S a v i l l e x t e f l on vi a l f o r e a c h s a m pl e a nd s t a nda r d, i nc l udi ng a bl a nk. 3. I n t he ba l a nc e r oom a dd 2 dr ops o f 4x w a t e r t o a l a be l e d vi a l T he n w e i gh a r ound 30 40 m g of t he s a m pl e / s t a nda r d r oc k po w de r or gl a s s c hi ps i nt o t he vi a l R e c or d t he s a m pl e w e i ght pr e c i s e l y! R e pe a t f or r e m a i ni ng s a m pl e s s t a nda r d s a nd bl a nk. 4. B r i ng vi a l s i nt o t he c l e a n l a b. I n t he hood, a dd 1 m l t r a c e m e t a l or opt i m a gr a de c onc e nt r a t e d *H F a nd 2 m l t r a c e m e t a l o r opt i m a gr a de c onc e nt r a t e d *H N O 3 t o e a c h vi a l C a p e a c h v i a l a nd t i gh t e n c a p us i ng t he gr e e n pl a s t i c w r e nc he s 5. H e a t vi a l s i n ove n i n out e r r oom a t 100 C f or 24 48 hour s O nc e e a c h s a m pl e i s i n s ol ut i on, r e m ove f r om ove n a nd t ur n i t o f f R e t ur n vi a l s t o c l e a n l a b a nd a l l ow vi a l s t o c ool

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126 126 6. O nc e c ool dr y dow n ( e va por a t e ) e a c h s a m pl e on t he hot pl a t e ( s e t t o no hi ghe r t ha n 3. 0) by f i r s t c a r e f ul l y s w i r l i ng t he s a m pl e t o c ol l e c t dr ops f r om t he s i de s a nd l i d. N e xt ope n t he vi a l a nd a dd 4x w a t e r t o l i d S w i r l a nd dum p t he s ol ut i on i nt o t he vi a l F i ni s h by pl a c i ng vi a l ont o hot p l a t e f or 12 24 hour s T he s a m pl e i s done dr yi ng w he n no l i qui d r e m a i ns a nd a l i ght ye l l ow / w hi t e pow de r i s pr e s e nt i n t he bot t om o f t he vi a l do not s c or c h! D I S S O L U T I O N P H A S E 2 1. A dd 4m l 5% H N O 3 s pi ke d w i t h R e a nd R h ( r e c or d t he a c i d w e i ght ) t o e a c h vi a l C a p t he vi a l s a nd t i ght e n c a ps w i t h g r e e n pl a s t i c w r e nc he s F o r t he bl a nk us e non s pi ke d 5% H N O 3 2. H e a t vi a l s on t he hot pl a t e a t 3. 0 ove r ni ght D I L U T I O N F O R A N A L Y S I S 1. P i pe t 200m l of s ol ut i on f r om e a c h vi a l a nd t r a n s f e r i t t o a n a ut os a m pl e r t ube ( r e c or d t he w e i ght of s a m pl e s ol ut i on t r a ns f e r r e d) 2. A dd 4m l of 5% H N O 3 s pi ke d w i t h R e a nd R h t o e a c h t ube ( r e c or d t he ne w w e i ght of s a m pl e s ol ut i on) c a p, a nd s ha ke F or t he bl a nk, a ga i n us e 4 m l of non s pi ke d 5% H N O 3 T hi s s houl d r e s ul t i n f i na l di l ut i on a r o und 2000x A N A L Y S I S B Y I C P M S U S I N G T H E E L E M E N T I I 1. A r r a nge s a m pl e s a nd s t a nda r ds i n t he a ut os a m pl e r A c c or di ng t o t hi s a r r a nge m e nt c om pi l e t he s e que nc e f i l e i n t he s e qu e nc e e di t or pr og r a m 2. E nt e r M e t hod f i l e ( w e us e d G K t r a c e 2) T une f i l e I nt e r na l s t a nda r d f i l e w e i ght s a nd di l ut i ons a c c or di ng. 3. E nt e r 1 m i nut e up t a ke t i m e a nd 2 m i nut e s w a s h t i m e pe r s a m pl e

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127 127 4. A l l s e l e c t e d e l e m e nt s w e r e a na l y z e d i n M e di um R e s ol ut i on m ode ; 4 r uns a nd 4 pa s s e s pe r a n a l ys e s ( t ot a l 16 m e a s ur e m e nt s pe r i s ot ope ) T he s pe c i f i c i s ot ope s us e d w e r e : S c 45, V 51, C r 52, C o59, N i 60, C u63 Z n66, G a 69, R b85, S r 88, Y 89 Z r 90, N b93, R h103 B a 137, L a 139, C e 140, P r 141, N d143 S m 149 E u153, G d157, T b159, D y163, H o165, E r 166 T m 169, Y b172, L u175, H f 178, T a 181, R e 185, P b208, T h232, U 238 A l l i s ot ope s l i ght e r t ha n B a s houl d be nor m a l i z e d w i t h R h, w hi l e a l l i s ot ope s he a vi e r t ha n B a s houl d be nor m a l i z e d w i t h R e 5. I n a ddi t i on, a d r i f t c or r e c t or s houl d be r un e ve r y 5 6 s a m pl e s ( e g. w e c hos e E N D V t o be our d r i f t c or r e c t or ) 6. D a t a w a s nor m a l i z e d O N a nd O F F l i ne f i r s t i n t he R e s ul t s E di t or by c he c ki ng t he c a l i br a t i on c ur ve s t he n i n E xc e l us i ng dr i f t c or r e c t i ons I f a t ot a l of 4 dr i f t c or r e c t or s w e r e us e d ( D C 1, D C 2, D C 3, D C 4) a v e r a ge t he f i r s t t w o, m i ddl e t w o, a nd l a s t t w o dr i f t c or r e c t or s f or e a c h e l e m e nt ( D C 1a nd2, D C 2a nd3, D C 3a nd4) N ot e t he t ot a l num be r of s a m pl e s m e a s ur e d be t w e e n e a c h s e t of dr i f t c or r e c t or s ( N 1a nd2, N 2a nd3, N 3a nd4) T he c or r e c t i on a ppl i e d t o e a c h e l e m e nt m e a s ur e d f o r t he f i r s t s e t of s a m pl e s be t w e e n D C 1 a nd D C 2 ( C or r e c t e d va l ue = O l d va l ue *[ 1 [ D C 1a nd2*[ pos i t i on of s a m pl e be t w e e n dr i f t c or r e c t or s / N 1a nd2] ] ) w i l l be di f f e r e nt t ha n t he m i ddl e s e t of s a m pl e s be t w e e n D C 2 a nd D C 3 ( C or r e c t e d va l ue = O l d va l ue *[ 1 D C 1a nd2] *[ 1 [ D C 2a nd3*[ po s i t i on of s a m pl e be t w e e n dr i f t c or r e c t or s / N 2a nd3] ] ] ) a nd t he l a s t s e t of s a m pl e s be t w e e n D C 3 a nd D C 4 ( C or r e c t e d va l ue = O l d va l ue *[ 1 D C 1a nd2] *[ 1 D C 2a nd3] *[ 1 [ D C 3a nd4] *[ pos i t i on of s a m pl e be t w e e n dr i f t c or r e c t or s / N 3a nd4] ] ] )

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128 128 7. A c c ur a c y a nd pr e c i s i on f or a n a na l yt i c a l r un w a s e va l ua t e d by m a ki ng a t a bl e c om pos e d o f t he ne w E N D V da t a a nd t he 3 f i r s t dr i f t c or r e c t or s A c c ur a c y w a s m e a s ur e d f r om how c l os e l y t he s e va l ue s c om pa r e d t o t r ue E N D V va l ue s w hi l e pr e c i s i on w a s c a l c ul a t e d f r om t he s t a nda r d de vi a t i on of t he 3 f i r s t dr i f t c or r e c t or s T hi s i nf or m a t i on i s a va i l a bl e i n A ppe ndi x B

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129 A P P E N D I X B E R R O R A N A L Y S I S A c c u r ac y R e pe a t a na l ys e s of t he i n hous e M O R B s t a nda r ds E N D V a nd 2392 9, i n a ddi t i on t o t he c e r t i f i e d r oc k s t a nda r d B H V O 1, w e r e c onduc t e d t o e va l ua t e t he a c c ur a c y of t he t r a c e e l e m e nt da t a pr e s e nt e d i n t hi s s t udy. T he t r a c e e l e m e nt c onc e nt r a t i ons f or B H V O 1 r e l a t i ve t o publ i s he d va l ue s [ G ov i ndar aj u 1994] pr oduc e d a n e xc e l l e nt c or r e l a t i on w i t h a n R 2 va l ue e qua l t o 0. 999 ( F i gur e B 1 ) T he c om pa r a t i ve r e s ul t s of E N D V m e a s ur e m e nt s w e r e a l s o r e m a r ka bl y s i m i l a r W he n t he m a ny E N D V d r i f t m e a s ur e m e nt s w e r e nor m a l i z e d t o a c c e pt e d E N D V c onc e nt r a t i ons t he r a nge i n ob s e r ve d v a l ue s w a s i ns i gni f i c a nt r e ga r dl e s s of w he t he r t he a ve r a ge of a l l dr i f t c o r r e c t or s or onl y t he f i r s t dr i f t c or r e c t or ( l e a s t a f f e c t e d by i ns t r um e nt dr i f t a nd t h e r e f or e m os t a c c ur a t e m e a s ur e m e nt pe r r un) f r om e a c h a na l yt i c a l r un w a s c ons i de r e d ( F i gur e B 2a ) A l l B a m e a s ur e m e nt s w e r e s l i ght l y m or e e nr i c he d t ha n E N D V w hi l e a f e w T a m e a s ur e m e nt s w e r e s l i ght l y m or e de pl e t e d. C oi nc i de nt l y, t he s e e l e m e nt s w e r e e x c l ude d, a l ong w i t h P b c onc e nt r a t i ons f r om t he da t a s e t of t hi s s t udy. T he gr e a t e r va r i a t i on s e e n i n t he s e e l e m e nt s w a s de t e r m i ne d t o be not a r e s ul t of na t ur a l va r i a t i on i n t he s a m pl e s but i ns t e a d due t o t he m i nut e c onc e nt r a t i ons of t he s e e l e m e nt s i n ge ne r a l or r e l a t i ve t o E N D V c onc e nt r a t i ons w hi c h w he n di l ut e d dur i ng t he f i na l s t e p of s a m pl e pr e pa r a t i on r e s ul t e d i n e xt r e m e l y l ow a bunda nc e s t ha t w e r e e i t he r e a s i l y c ont a m i na t e d or s w a m pe d by t he E N D V s i gna l F or e xa m pl e P b c onc e nt r a t i ons a r e s o l ow l e s s t ha n 1ppm t ha t w he n di l ut e d t o 2000x ( a s s um i ng m a x. 1ppm ) on l y 0 0005 ppm P b i n s ol ut i on c om i ng f r om t he s a m pl e

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130 130 F i gur e B 1. M e a s ur e d a nd a c c e pt e d t r a c e e l e m e nt c onc e nt r a t i ons of s t a nda r d B H V O 1. C om pa r i s on of t hi s da t a t o E N D V c onc e nt r a t i ons m e a s ur e d a t ot h e r L a bs a l s o r e s ul t e d i n r e m a r ka bl y s i m i l a r va l ue s f or m os t e l e m e nt s ( F i gur e B 2b) T he va r i a t i on s e e n i n t w o i n hous e 2392 9 a n a l ys e s r e l a t i ve t o t he r e s ul t s f r o m ot he r L a bs i s gr e a t e r t ha n t hos e f or E N D V a s s how n i n F i gur e B 3 W hi l e t he m os t i n c om pa t i bl e e l e m e nt s s how t he gr e a t e s t va r i a t i on, t he r e s ul t s of t he m or e c om pa t i bl e e l e m e nt s a nd r a r e e a r t h e l e m e nt s ( R E E ) a r e r e m a r ka bl e s i m i l a r O ve r a l l t he va r i a t i on s e e n i n E N D V 2392 9, a nd B H V O 1 r e ve a l c om pa r a t i ve l y a c c ur a t e r e s ul t s a nd t he r e l a t i ons hi ps be t w e e n t he m ul t i a na l ys e s on F i gur e s B 1 t o B 3 c a n be us e d t o e va l ua t e t he r e l a t i ons hi ps be t w e e n t he s a m pl e s of t hi s s t udy.

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131 131 A B F i gur e B 2. S pi de r di a gr a m s f or i n hous e s t a nda r d E N D V M e a s ur e d c on c e nt r a t i ons a r e nor m a l i z e d t o A ) a c c e pt e d E N D V va l ue s a nd B ) p r i m i t i ve m a nt l e va l ue s [ Sun and M ac D onough 1989]

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132 132 F i gur e B 3. S pi de r di a gr a m a nd R E E pl ot s f or s t a nda r d 2392 9 M e a s ur e d t r a c e e l e m e nt c onc e nt r a t i ons of 2392 9 a r e no r m a l i z e d t o pub l i s he d va l ue s [ Sun and M ac D onough 1989]

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133 133 P r e c i s i on P r e c i s i on i s of t e n qua nt i f i e d i n s c i e nt i f i c s t udy b y s t a nda r d de vi a t i on, a m e a s ur e of va r i a t i on a bout t he a r i t hm e t i c m e a n. I f a n or m a l di s t r i but i on ( e g. be l l c ur ve ) i s obs e r ve d f or t he m ul t i pl e a na l y s e s of a s i ngl e s t a n da r d, t he n 68% of a l l a na l ys e s w i l l f a l l w i t hi n one s t a nda r d de vi a t i on ( one s i gm a ) of t he m e a n va l ue 95 % o f a l l a na l ys e s w i l l f a l l w i t hi n t w o s t a nda r d de vi a t i ons ( t w o s i gm a ) of t he m e a n va l ue a nd 99% of a l l a na l ys e s w i l l f a l l w i t hi n t hr e e s t a nda r d de vi a t i on s of t he m e a n va l ue F or t hi s s t u dy, pr e c i s i on ha s be e n quot e d a t t he 95% c on f i de nc e l e ve l P r e c i s i on of t he m a j or e l e m e nt a n a l ys e s w a s c a l c ul a t e d f r om va r i a t i on be t w e e n 111 s e pa r a t e a na l ys e s of 2392 9. F i gu r e B 4 s how s nor m a l di s t r i but i ons f or m os t m a j or e l e m e nt s how e ve r M gO a nd K 2 O ha ve a f e w o ut l i e r s T he a na l ys e s a s s o c i a t e d w i t h t he s e out l i e r s w e r e not r e m ove d be c a us e t he s e s l i ght l y a nom a l ous r e s ul t s w e r e not a c c om pa ni e d by va r i a t i on i n ot he r e l e m e nt s a s w o ul d be s e e n i f s om e t hi ng ot he r t ha n t h e gl a s s s a m pl e w a s be i ng a na l yz e d ( e g. phe noc r ys t s gl a s s of s l i de e poxy, e t c ) T he m ul t i pl e a na l ys e s of t he s t a nda r ds E N D V a nd 2392 9, a l ong w i t h t he s a m pl e s 3963 10, 3970 10 a nd 3974 11 c a n be us e d t o m e a s ur e t he t r ue r e pr oduc i bi l i t y or pr e c i s i on of our r e s ul t s ( F i gur e B 5) N e w s a m p l e s pl i t s w e r e pr e pa r e d pr i or t o t he t hr e e s e pa r a t e a n a l yt i c a l r uns S a m pl e 3974 11 w a s i ni t i a l l y r e a na l yz e d due t o i t s a nom a l ous e nr i c hm e nt i n t he m os t i nc om pa t i bl e e l e m e nt s a nd l i ght R E E ( F i gur e B 5) T he s ubs e que nt t w o a n a l ys e s r e ve a l e d t he e nr i c hm e n t w a s f a l s e a nd pr oba bl y due t o a l t e r a t i on pr oduc t s on t he s ur f a c e of t he gl a s s c hi p t h a t w e nt unnot i c e d dur i ng a na l yt i c a l pr e pa r a t i on. T he r e s ul t s f r om t he a na l ys e s of t he ot he r s a m pl e s r e ve a l r e m a r ka bl y c ohe r e nt t r e nds de s pi t e t he m o r e t ha n f our m on t h b r e a k be t w e e n a na l yt i c a l r uns

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134 134 F i gur e B 4. D i s t r i but i ons o f m a j o r e l e m e nt a na l ys e s f or s t a nda r d 2392 9.

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135 135 F i gur e B 4. C ont i nue d

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136 136 F i gur e B 5. S pi de r di a gr a m a nd R E E pl ot s f or r e pe a t a na l ys e s of s e ve r a l s a m pl e s M e a s ur e d t r a c e e l e m e nt c onc e nt r a t i ons a r e nor m a l i z e d t o publ i s he d va l ue s [ Sun and M ac D onough 1989 ]

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137 L I S T O F R E F E R E N C E S A l e xa nde r R T a nd K C M a c dona l d, S e a be a m S e a M A R C I I a nd A L V I N ba s e d s t udi e s of f a ul t i ng on t he E a s t P a c i f i c R i s e 9 20 N 9 50 N M ar G e ophy s R e s 18 557 587 1996 A r i s ki n, A A G S B a r m i na M Y F r e nke l S i m ul a t i ng l ow pr e s s ur e t hol e i i t e m a gm a c r ys t a l l i z a t i on a t a f i xe d oxyge n f uga c i t y G e ok hi m i a, 11 1614 1627, 1986. B a t i z a R a nd Y N i u, P e t r ol ogy a nd m a gm a c ha m be r pr oc e s s e s a t t he E a s t P a c i f i c R i s e 9 30 N J G e ophy s R e s 97 6779 6797 1992 B e a t t i e P U r a ni um t hor i um di s e qui l i br i a a nd pa r t i t i oni ng on m e l t i ng o f ga r ne t pe r i dot i t e N at ur e 363 63 65, 1993. B i nde m a n, I N A M D a vi s a nd M J D r a ke I o n m i c r opr obe s t udy of pl a gi oc l a s e ba s a l t pa r t i t i on e xpe r i m e nt s a t na t ur a l c onc e nt r a t i o n l e ve l s of t r a c e e l e m e nt s G e oc hi m C os m oc hi m A c 62 ( 7) 1175 1193 199 8. B or i s ov, A A a nd A I S ha pki n A ne w e m pi r i c a l e qua t i on r a t i ng F e 3+ / F e 2+ i n m a gm a s t o t he i r c om pos i t i on, oxyge n f uga c i t y, a nd t e m pe r a t ur e G e oc he m I nt 2 7 111 116, 1990 B ouga ul t H a nd R H e ki ni a n, R i f t va l l e y i n t he A t l a nt i c O c e a n ne a r 36 de gr e e s 50 N : pe t r ol ogy a nd ge oc he m i s t r y o f ba s a l t r oc ks E ar t h P l ane t Sc i L e t t 24 ( 2) 249 261, 1974 B r ya n, W B S E H um p hr i s G T hom ps on, a nd J F C a s e y, C om pa r a t i ve vol c a nol ogy of s m a l l a xi a l e r upt i ve c e nt e r s i n t he M A R K a r e a J G e ophy s R e s 99 2973 2984, 1994 C a nn, J R A m ode l f o r oc e a ni c c r us t a l s t r uc t u r e de ve l ope d, G e ophy s J R A s t r on. Soc 39 169 187 1974 C a r bot t e S M J C M ut t e r a nd L X u, C ont r i but i on of vol c a ni s m a nd t e c t oni s m t o a xi a l a nd f l a nk m o r phol ogy o f t he s out he r n E a s t P a c i f i c R i s e 17 10 17 40 S f r om a s t udy of l a ye r 2A ge om e t r y J G e ophy s R e s 99 10165 10184, 1997 C a r bot t e S M A S ol om on a nd G P onc e C or r e a E va l ua t i on of m or p hol ogi c a l i ndi c a t or s of m a gm a s uppl y a nd s e gm e nt a t i on f r o m a s e i s m i c r e f l e c t i on s t udy of t he E P R 15 30 17 N J G e ophy s R e s 105 2737 2759, 2000

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138 138 C ha dw i c k, J r W W a nd R W E m bl e y, L a va f l o w s f r om a m i d 1980s s ubm a r i ne e r upt i on on t he C l e f t s e gm e nt J ua n de F uc a R i dge J G e ophy s R e s 99 4761 4776, 1994 C ha dw i c k, W W J r R W E m bl e y, a nd C G F ox, S e a B e a m de pt h c ha nge s a s s oc i a t e d w i t h r e c e nt l a va f l ow s C oA xi a l s e gm e nt J ua n de F uc a R i dge : E vi de nc e f o r m ul t i pl e e r upt i ons be t w e e n 198 1 1993, G e phy s R e s L e t t 22 167 170 199 5. C hr i s t e ns on, G L G M P ur dy a nd G J F r ye r S e i s m i c c ons t r a i nt s on s ha l l ow c r us t a l e m pl a c e m e nt pr oc e s s e s a t t he f a s t s pr e a di ng E a s t P a c i f i c R i s e J G e ophy s R e s 99 17957 17973 1994 C hr i s t e ns on G L G M K e nt G M P ur dy a nd R S D e t r i c k, E x t r us i ve t hi c kne s s va r i a bi l i t y a t t he E a s t P a c i f i c R i s e 9 10 N c ons t r a i nt s f r om s e i s m i c t e c hni que s J G e ophy s R e s 101 2859 2873, 1996. C oc hr a n, J R D J F or na r i B J C oa kl e y, R H e r r a nd M A T i ve y. C ont i nuous ne a r bot t om gr a vi t y m e a s ur e m e nt s m a de w i t h a B G M 3 gr a vi m e t e r i n D S V A l v i n on t he E a s t P a c i f i c R i s e c r e s t ne a r 9'N a nd 9'N J G e ophy s R e s 104 10841 10861 1999. C r a w f or d, W C a nd S C W e bb, V a r i a t i ons i n t he di s t r i but i on o f m a gm a i n t he l ow e r c r us t a nd a t t he M oho be ne a t h t he E a s t P a c i f i c R i s e a t 9 10 N E ar t h P l ane t Sc i L e t t 203 117 130 2002 C r a w f or d, W C S C W e bb, a nd J A H i l de br a n d, C ons t r a i nt s on m e l t i n t he l ow e r c r us t a nd M oho a t he E a s t P a c i f i c R i s e 9 48 N us i ng s e a f l oor c om pl i a nc e m e a s ur e m e nt s J G e ophy s R e s 104 2923 2939, 1999. C r ow de r L K a nd K C M a c dona l d, N e w c ons t r a i nt s on t he w i dt h of t he z one o f a c t i ve f a ul t i ng on t he E a s t P a c i f i c R i s e 8 30 N 10 00 N f r om S e a B e a m B a t hym e t r y a nd S e a M A R C I I S i de s c a n S ona r M ar G e ophy s R e s 21 513 527, 2000. D a nyus he vs ky, L V T he e f f e c t of s m a l l a m ount s of H 2 O on c r ys t a l l i z a t i on o f m i d oc e a n r i dge a nd ba c ka r c ba s i n m a gm a s J V ol c an. G e ot h. R e s 110 265 280, 2001. D a vi s E E vi de nc e f or e xt e ns i ve ba s a l t f l ow s on t h e s e a f l oor G e ol Soc A m B ul l 93 1023 1029, 1982 D e t r i c k, R S P B uhl E V e r a J M ut t e r J O r c ut t J M a ds e n, a nd T B r oc he r M ul t i c ha nne l s e i s m i c i m a gi ng of a c r us t a l m a gm a c ha m be r a l ong t he E a s t P a c i f i c R i s e N at ur e 326 35 41, 1987. D unn, R A a nd D R T oom e y, S e i s m ol ogi c a l e vi de nc e f or t h r e e di m e ns i ona l m e l t m i gr a t i on be ne a t h t he E a s t P a c i f i c R i s e N at ur e 3 88 1997

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149 B I O G R A P H I C A L S K E T C H J i l l i a n S a r a y H i nds w a s bor n one s now y ni ght i n T r a ve r s e C i t y, M i c hi ga n. A s s he gr e w up, s he e nj oye d m a ny a c t i vi t i e s s pe c i f i c t o t he c ha ngi ng s e a s ons i nc l udi ng s w i m m i ng e ve r y s um m e r hi ki ng dur i ng t he s pr i ng a nd f a l l a nd s ki i ng a l l w i nt e r I n 1999, J i l l i a n gr a dua t e d w i t h honor s f r om T r a ve r s e C i t y C e nt r a l H i gh S c hool a t 1: 00 A M dur i ng t he S e ni or A l l N i ght P a r t y. S he w a s u na bl e t o a t t e nd t he r e gul a r g r a dua t i on c e r e m ony be c a us e s he w a s c om pe t i ng a t t he T r a c k a nd F i e l d S t a t e F i na l s w e r e s he be c a m e t he pol e va ul t i ng s t a t e c ha m pi on w i t h a r e c or d j um p o f 11 f e e t A f t e r hi gh s c hool J i l l i a n m ove d t o M ount P l e a s a n t M i c hi ga n, t o r un f or a nd a t t e nd c l a s s e s a t C e nt r a l M i c hi ga n U ni ve r s i t y. A f t e r f our ye a r s s he gr a dua t e d m a gna c um l a ude w i t h a B a c he l or of S c i e nc e i n g e ol ogy. N ot s a t i s f i e d w i t h he r l e v e l of e duc a t i on, s he m ove d t o G a i ne s vi l l e F l or i da t o a t t e nd gr a dua t e c l a s s e s a t t he U ni ve r s i t y of F l or i da I n 2005, s he c om pl e t e d a M a s t e r of S c i e nc e i n ge ol og y.


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Title: Construction of the Oceanic Crustal Layer 2A: A Detailed Petrographic and Geochemical Study of Lavas from 9 Degrees 30 Minutes North and 9 Degrees 50 Minutes North East Pacific Rise
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Material Information

Title: Construction of the Oceanic Crustal Layer 2A: A Detailed Petrographic and Geochemical Study of Lavas from 9 Degrees 30 Minutes North and 9 Degrees 50 Minutes North East Pacific Rise
Physical Description: Mixed Material
Copyright Date: 2008

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CONSTRUCTION OF THE OCEANIC CRUSTAL LAYER 2A: A DETAILED
PETROGRAPHIC AND GEOCHEMICAL STUDY OF LAVAS FROM 9030' N AND
9050' N EAST PACIFIC RISE
















By

JILLIAN SARAY HINDS


A THESIS PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF SCIENCE

UNIVERSITY OF FLORIDA


2005















ACKNOWLEDGMENTS

I am in great debt to a number of friends and colleagues who provided technical,

analytical, financial, and mental support during the conceptualization, development, and

realization of my masters research. My greatest thanks go to my advisor, Mike Perfit, for

bringing me to the University of Florida to continue studying a subject I love, as well as

providing a safe, constructive environment to work in over the last two years. I would

also like to thank Dave Foster, Ray Russo, Ellen Martin, and Phil Neuhoff, whose doors

were always open when I needed their help.

Many thanks are due to the scientists and crew involved in collecting and

preparing lava samples during the AT11-7 cruise, especially Hans Schouten, the chief

scientist, along with fellow scientists Dan Fornari and Adam Soule. Continued

involvement by the latter two gentlemen was most welcome and resulted in two

published abstracts. Without the help of Ian Ridley, George Kamenov, Matt Smith, and

Susanna Blair, it is questionable whether the processing and analysis of the many

recovered lavas would have been completed in a timely matter or at an acceptable

scientific level. I thank them for all their help! Thanks go again to Adam Soule in

addition to John Jaeger, and Ray Thomas for their patience and technical help related to

my learning and the operation of ARC/GIS.

This research would not have been completed without the financial support of a

NSF grant and the mental support provided by my family and friends. I love them!
















TABLE OF CONTENTS

page

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

L IST O F T A B L E S ...................................................... v

L IST O F FIG U RE S ........................................................................................ .......vii

ABSTRACT ................. ........ .. ........... ............... .............

CHAPTER

1 INTRODUCTION .............................................................. .. .... ..............

Introduction to the Problem ......................................... .. .... ......................
Sample Localities: AT11-7 Cruise and Dive Information ........................................5

2 B A CK G R O U N D ................................................................................................ 15

Construction of Oceanic Crust at the Fast-Spreading East Pacific Rise ................ 15
Regional Geology of The 9-10 N segment of the East Pacific Rise ................... 18

3 ANALYTICAL METHODS......................... ........ ..............24

4 PETROGRAPHY ................ .......... .................. 27

Petrography ................. ........... ..........................................27
Flow Units With Fronts, Dives 3963 and 3974.................................... ..... 27
C channels, D ive 3968 .................................................. 46
Off-Axis M ounds, Dive 3970........................................ .............. 46
P h ase C h em istry ............. .... ..... .... ......... .. ...................................................... 4 9
S u m m a ry ............. ......... .. .. ......... .. .. ................................................. 5 8

5 MAJOR AND TRACE ELEMENT CHEMISTRY ............. ............. ............. 59

M ajor Elem ent D ata ......................................5...................9........
Flow Units With Fronts, Dives 3963 and 3974 .............. ..............................69
C channels, D ive 3968 ........................................ ..........................70
Off-Axis M pounds, D ive 3970...................................................................... 71
T race E lem ent D ata ........................................................ .............................. 71
Flow Units With Fronts, Dives 3963 and 3974................................................ 85









C channels, D ive 3968 ........................ ..................... .. ........................... 86
Off-Axis Mounds, Dive 3970............ ........................... ..............86
Summ ary .............. .. ......... ..... ........................ 86

6 PETROGENESIS ................. ......... .................. 88

M major Elem ent M models ............................................................................. 88
Flow Units, Dives 3963 and 3974 ....................................... ..............89
O ff-A xis M ounds, D ive 3970.................................... .................................... 97
Trace Elem ent M models ............................................................................ 99
Flow Units, Dives 3963 and 3974 .......................................... ............. 100
Off-Axis M ounds, Dive 3970.................................................... 102
Magma Mixing and Source Characteristics..................................................... 103
Sum m ary .................................... .................................. .......... 105

7 DISCUSSION ................................... ............................ ............ 109

Combining and Evaluating Petrographic and Geochemical Results....................... 109
Heterogeneity Index for Mid-Ocean Ridge Lava Flows ............................... 113
M odels for Crustal Construction ................................................................. ..... 118

8 CONCLUSIONS ................................... ............................ ........... 123

APPENDIX

A E2 ICP-MS ANALYTICAL PROTOCOL ..................................................... 125

B ERR OR AN A LY SIS ......................................................................... .............. 129

A accuracy .................................... ................................ .......... 129
Precision .................................... .................................. .......... 133

LIST OF REFEREN CES.......................................................................... .............. 137

BIO GRAPH ICAL SK ETCH .......................................................................... 149
















LIST OF TABLES


Table page

4-1. Thin Section Petrography of Lavas Collected during Alvin Dive 3963 Near
950'N East Pacific Rise ............................................................... 28

4-2. Thin Section Petrography of Lavas Collected During ALVIN Dive 3974 Near
950'N East Pacific Rise ............................................................... 32

4-3. Thin Section Petrography of Lavas Sampled During ALVIN Dive 3968 Near
930'N E ast Pacific R ise .............................................. ............................ 36

4-4. Thin Section Petrography of Lavas Sampled During ALVIN Dive 3970 Near
930'N E ast Pacific R ise .............................................. ............................ 37

4-5. Petrologic Characteristics of Flow Units From ALVIN Dives 3963 and 3974.........47

4-6. Olivine Chemical Compostions by Electron Microprobe for 3970-9 and 3974-1 It..50

4-7. Plagioclase Chemical Compositions by Electron Microprobe for 3970-9 ..............52

4-8. Clinopyroxene Chemical Compositions by Electron Microprobe for 3970-9..........53

5-1: Major Element Chemistry of Lavas Collected From 90-100N East Pacific Rise
D during A T 11-7 Cruise................. ........... ................................... ....... ......... 60

5-2. Select Trace elements For Lavas Collected During Dives 3963, 3968, 3970, and
3 9 7 4 ......... ..... ............. ..................................... ..............................7 2

5-3. Rare Earth Element Concentrations for Dives 3963, 3968, 3970, and 3974.............74

6-1. Petrogenetic Parameters and Conditions Used in Flow Unit (Dive 3963)................90

6-2. Petrogenetic Parameters and Conditions Used in Flow Unit (Dive 3974) ..............91

6-3. Petrogenetic Parameters and Conditions Used in Off-Axis Mounds (Dive 3970)
Calculations .............. .......... ................................ ............. 92

6-4. Partition Coefficients Used in Modeling of Trace Element ............................. 101

7-1. Characterisitics of M id-Ocean Ridge Lavas ......................................................... 114









7-2. Heterogeneous Index Calculations for The Flow Units (9050' N) and Off-Axis
M pounds (9030' N ) ................................................ ............................... 115
















LIST OF FIGURES


Figure pae

1-1. Location and bathymetry of 90 to 100 N segment of the East Pacific Rise between
the Clipperton and Siqueiros fracture zones ........................................ .................2

1-2. Microbathymetry map showing the location of Alvin dives during AT 11-7 along
the East Pacific Rise ................................................................. .......... .... 6

1-3. Location of lavas sampled during dives 3963 and 3974 ..........................................7

1-4. Typical lava morphologies observed at the East Pacific Rise .............. ....................9

1-5. Location of lavas sampled during dive 3968 .............. .......................................... 10

1-6. Location of lavas sampled during dive 3970......................................................... 13

1-7. Bathym etric profile for dive 3963 ................................ ........................ ........ 14

2-1. Interpretative cross-section of the subaxial crustal structure beneath the East
Pacific R ise ...................................................................... ......... 17

2-2. Cross-axial profiles for two sections of the East Pacific Rise .............. .................... 19

2-3. Typical lava morphologies and stratigraphic positions within the axial summit
collapse trough at the East Pacific Rise....................................... .............. 20

4-1. Photomicrographs of common textures found in basalts collected near 9030' and
9051'N E PR ................................................... ................... ...... ........ 4 1

4-2: Photomicrographs of olivine, clinopyroxene and plagioclase crystal
m orphologies, all in crossed polars .................................................................... 43

4-3. Photomicrographs of xenoglomerocrysts found in basalts collected during dive
3963 near 9051'N EPR ............................................................ .............. 45

4-4. Photomicrographs of basaltic samples retrieved during Alvin dive 3970 to a series
of off-axis mounds 1.23-1.58 km west of the AST near 9030'N EPR..................48

4-5. O livine com positions ................................................ .............................. 54









4-6. Zoning in olivine, plagioclase, and clinopyroxene........................................ 56

5-1. Major element oxide variation diagrams for lavas sampled during dive 3963,
3968, 3970, and 3974. ........................................................................... 66

5-2. MgO content of lavas from dives 3963, 3968, 3970, and 3974 as a function of
their latitude. ...................................................................... ........ 68

5-3. Compositional distributions for sheet, lobate, and pillow lavas.............................69

5-4. Spider diagram and REE plot for lavas from dive 3963............... ..............76

5-5. Spider diagram and REE plot for lavas from dive 3974............... ..............77

5-6. Spider diagram and REE plot for lavas from dive 3968....................... ..............78

5-7. Spider diagram and REE plot for lavas from dive 3970....................... ..............79

5-8. Trace element variation diagrams for lavas from dives 3963, 3968, 3970, and
3974. ...................... ..................... .......... 80

6-1. Liquid lines of descent calculated for flow units sampled during dives 3963 and
3974 under both anhydrous and hydrous conditions............ .......................93

6-2. Liquid lines of descent calculated for off-axis pillow mounds sampled during
dive 3970 under both anhydrous and hydrous conditions............. .............. 98

6-3. Liquid lines of descent for select trace elements calculated for the flow units
sampled during dives 3963 and 3974 under anhydrous (A) and hydrous (H)
co n d itio n s ............................................. ........ .............. 10 2

6-4. Liquid lines of descent for select trace elements calculated for the off-axis pillow
mounds sampled during dive 3970............................... .............. 103

6-5. Zr v. Y for the flow units (dive 3963 and 3974), channels (dive 3968), and off-
axis mounds (dive 3970).. ................................ 104

6-6. Zr/Y v. Y for the flow units (dive 3963 and 3974), channels (dive 3968), and off-
axis mounds (dive 3970).. ................................ 107

6-7. Ce/Yb v. Ce for the flow units (dive 3963 and 3974), channels (dive 3968), and
off-axis m ounds (dive 3970) ........................................................... ...... ........ 108

7-1. The heterogeneity Index calculated for individual flows v. their flow volume and
spreading rate. ................................... ........................... ... ........ 119

7-2. Three-dimensional diagram of oceanic crust formation along the northern East
Pacific Rise ...................................... ........................... .... ......... 122









B-1. Measured and accepted trace element concentrations of standard BHVO-1..........130

B-2. Spider diagrams for in-house standard ENDV ..................................................... 131

B-3. Spider diagram and REE plots for standard 2392-9 ............. ....................... 132

B-4. Distributions of major element analyses for standard 2392-9................. ............ 134

B-5. Spider diagram and REE plots for repeat analyses of several samples. .................136















Abstract of Thesis Presented to the Graduate School
of the University of Florida in Partial Fulfillment of the
Requirements for the Degree of Master of Science

CONSTRUCTION OF THE OCEANIC CRUSTAL LAYER 2A: A DETAILED
PETROGRAPHIC AND GEOCHEMICAL STUDY OF LAVAS FROM 9030' N AND
950' N EAST PACIFIC RISE

By

Jillian Saray Hinds

December 2005

Chair: Michael Perfit
Major Department: Geological Sciences

Layer 2A of oceanic crust is composed of a continuum of overlapping lava flows

erupted from both on-axis and off-axis sites at the East Pacific Rise (EPR). Between

925' N and 9055' N EPR, microbathymetry and side scan sonar maps in addition to in

situ observations reveal most flows form large lobes 1-2 km wide extending up to 1 km

from the ridge axis. The body of each lobed flow unit mostly consists of sheet and lobate

flows that transition into pillows at steep flow fronts. Channel systems are found

interspersed amongst these flow units in 50-1000 m long segments and serve to move

lava up to 3 km off-axis onto the crestal plateau. Adding to this complexity are pillow

mounds erupted from fissures >1 km from the spreading ridge. The accumulation of

lavas close to the axis explains both the apparent doubling in thickness of layer 2A within

1-2 km of the axis and the observations and isotopic evidence that suggest active









volcanism occurs over a much wider zone about the EPR axis (up to 4 km) than

previously thought.

Lavas from a series of flow-fronts on either side of the axial summit collapse

trough (ASCT) at 950' N EPR and two channels at -929' N EPR are mainly aphyric to

sparsely plagioclase phyric (<2% by volume) basalts with normal incompatible element

depletions (N-MORB). In contrast the more evolved (lower MgO) lavas from off-axis

mounds at -930' N EPR are relatively more incompatible element depleted and contain

as much as 10% microphenocrysts of olivine, plagioclase, and clinopyroxene. The data

suggest these lavas may have come from the cooler edges of the axial magma chamber

and were derived from a distinctly different source than the flows emanating from the

axis. Pillow basalts comprising the flow fronts of each flow unit are consistently more

evolved (0.2-0.77 lower wt.% MgO) than their associated lobate and sheet flows in the

body of the flow and have slightly higher concentrations of incompatible elements. The

quantities of microphenocrysts in pillow lavas at the flow front are -1.0-2.4 vol. %

greater than those in the body of the flow. Fractional crystallization models at low

pressure predict 4-14 vol. % crystallization of parental magmas is required to produce the

observed changes in chemistry within individual flow units, at odds with the measured

crystallinity variation. Furthermore, some mixing between primitive and evolved melts is

required to explain all of the variations in major element and trace element characteristics

of the flow units. Mixing is supported by the presence of xenocrysts and other

disequilibrium relationships between phenocrysts and their host melt. The results of our

study suggest that the majority of chemical imprinting of lavas precedes eruption onto the

seafloor and that little variation occurs once magmas have been erupted.














CHAPTER 1
INTRODUCTION

Introduction to the Problem

Oceanic crust makes up nearly 70% of the rocks on Earth's surface yet the nature

and significance of the magmatic and volcanic processes that create them are, in detail,

uncertain. Most oceanic crust is thought to form within the narrow neovolcanic zone (1-4

km wide) centered along the axis of a mid-ocean ridge (MOR). Spanning nearly 65,000

km along the ocean bottoms, the remoteness and largely inaccessible nature of the MOR

system has limited its study to only a few representative ridges (e.g., East Pacific Rise

Gorda Ridge, Juan de Fuca Ridge, and Mid-Atlantic Ridge). The 9-10 N segment of

the East Pacific Rise (EPR) is one section of the MOR where the integration of numerous

disciplines, such as seafloor morphology [Fornari et al., 1998; Kurras et al., 2000], lava

geochemistry [Batiza and Niu, 1992; Perfit et al., 1994], hydrothermal activity [Haymon

et al., 1991; Von Damm et al., 1992, 1995], detailed mapping [Macdonald et al., 1984;

Alexander and Macdonald, 1996; Crowder and Macdonald, 2000] and seismic

investigations [Detrick et al., 1987; Christenson et al., 1994, 1996; Hooft et al., 1996;

Crawford and Webb, 2002], has resulted in one of the most extensive multidisciplinary

databases of any MOR.

At the 9-10 N segment of the EPR (Figure 1-1), lavas that are erupted from

within or closely proximal (<100 m) to the axial summit collapse trough (ASCT) amass

to form the extrusive portion of oceanic crust (Layer 2A). According to seismic studies

this layer doubles in thickness with 1-2 km of the ridge axis [Christenson et al., 1994,
















* I.. ,
'Mutti~eam bathymetry
Co .chrfn et a1., 1999

L1.



















V I


II




$1














Sea m 2- bachymetry
ac aMjlal, 1992
AMLM


-2524
S 10 kilometers


110W 105'W 100'W


Depth (mi From Soule et al. (2005)


Figure 1-1. Location and bathymetry of 90 to 100 N segment of the East Pacific Rise
between the Clipperton and Siqueiros fracture zones. The axial summit
collapse trough is marked by a dotted line and the location of the DSL-120A
side-scan sonar survey is outlined in black. The Multibeam bathymetry
contour interval is -84 m (Revised from Soule et al. [2005]).

1996; Hooft et al., 1996]. In areas up to 4 km from the spreading axis, anomalously


young lava flows and pillow mounds have been identified through both fresher


appearance as well as younger U-series model eruption ages than would be predicted


based on their distance from the axis [Goldstein et al., 1994; Perfit et al., 1994; Perfit and


Chadwick, 1998; Smith et al., 2001; Sims et al., 2003]. Thus, recent studies suggest


active volcanism occurs over a much wider zone at the EPR than previously believed and


[ I I I


104"101W


104'5'W


i

I
h









off-axis volcanic activity contributes notable volumes towards crustal construction

[Goldstein et al., 1994; Perfit et al., 1994; Sims et al., 2003].

The relative volumes of lavas that erupt off-axis compared to those that originate in

the ASCT and then flow long distances off-axis is still unclear. Only a few individual

flows have been identified, mapped, and sampled at MORs [e.g., Embley et al., 1991;

Haymon et al., 1993; Chadwick and Embley, 1994; Rubin et al., 1994, 2001; Gregg et al.,

1995, 1996; Sinton et al., 2002], and all are the consequence of recent volcanism

(probably within a few years of eruption) within the neovolcanic zone. As flows age,

younger flows on the axial crest are harder to distinguish from neighboring older flows

due to increased sediment cover, sea floor weathering, and complex stratigraphic

relationships.

Although many sections of the MOR have been sampled and thousands of lavas

have been chemically analyzed very few individual MOR flows have been unequivocally

identified (see Rubin et al. [2001]). Consequently, scientists do not have much

information regarding the extent of chemical heterogeneity in flows and instead use

regional chemical trends, in conjunction with the relative position of a sample to the

spreading axis, to characterize the spatial and temporal aspects of processes operating

beneath the ridge axis. Without placing lavas into the context of single eruptions, it

cannot be known if the observed chemical variations over small spatial scales are related

by one or more processes, including mixing of different magmas during eruption,

fractional crystallization during surficial cooling, flow contamination during eruption, or

incomplete mixing of magmas prior to eruption.









Careful inspection of side-scan sonar and microbathymetry maps of the 9O25' to

955' N region of the EPR reveals the neovolcanic zone is dominantly comprised of

overlapping lobe-shaped reflectors, 0.25-1 km in length and 1-2 km in width, that

emanate from the ASCT and extend up to a few km down the ridge flanks in an

overlapping and anastomosing pattern. Each lobe has been inferred to represent lava

flow surfaces ending in a ringed flow front [Fornari et al., 1998; Kurras et al., 2000;

Schouten et al., 2001]. Interspersed within this volcanic terrain are sinuous to linear

channel systems (50-1000 m segments) that are oriented roughly perpendicular to the

ASCT and are found up to 3 km from the axis [Schouten et al., 2001; Soule et al., 2005].

In addition, elongate ridge-parallel pillow mounds -10-30 m high, -100 m wide, -1 km

long, and located >1 km off-axis add to the complex volcanologic panorama of the sea

floor. These features suggest three main types of volcanism active within the

neovolcanic zone of the EPR: (1) overlapping eruptions of lobates and sheet flows

originating from the axis and extending up to 1 km onto the crestal plateau, (2) slope-

driven lava channels and tubes which move lava flows greater distances, and (3) off-axis

eruptions from fissures at the edge of the neovolcanic zone

In order to better understand the distribution of lavas that comprise the neovolcanic

zone of the EPR and to better constrain models of upper oceanic crustal construction at

fast-spreading mid-ocean ridges like the northern EPR, the degree of geochemical

heterogeneity, petrographic diversity, and morphological variation within carefully

mapped and sampled lava flow units from the ridge crest were determined. In addition to

steep pillow flow fronts and sheet and lobate flow tops that comprise the flow units









adjacent to the ASCT, samples from distributary lava channels and off-axis pillow

mounds have also been investigated.

Sample Localities: AT11-7 Cruise and Dive Information

In January and February of 2004 during Voyage 11, Leg 7 of the R/V Atlantis, the

submersible Alvin collected 107 samples of lava along 9025'-55'N of the EPR (Figure 1-

2). Geologic settings and spatial relations of the sample sites were documented using

photographs and video images while the Doppler navigation system in Alvin provided

sample positions on the seafloor. The selection of sample sites was guided by ABE

microbathymetry and DSL-120A side scan sonar images for the area with the primary

goal to map and sample features that appear to be discrete flow units emanating within or

near the ASCT and extending off-axis. Of particular interest were flow units situated

along the crestal plateau that appear as brightly reflected scalloped features on the side

scan images (Figure 1-3). Each flow unit is presumed to represent either a single eruptive

unit or several lava flows that are closely related in time and space. The body of flow

unit is composed of mainly sheet and lobate flows while pillow lavas comprise the fronts

of the farthest-reaching flows (Figure 1-4). More than 25 individual lobes were directly

observed and/or sampled to evaluate these conjectures. At many sites, lava samples were

collected from the body and the flow front of a single lobe as well as from adjacent lobes.

Also of interest were inferred lava channel systems identified on microbathymetry

maps as prevalent non-reflective, linear features situated largely perpendicular to the

ASCT (Figure 1-5). These channels are thought to represent distributary channels that

serve to move lavas off-axis. Seven sites in five separate channel systems were examined

in detail and sampled (discussed by Soule et al. [2005]). Like the flow units, variable


















9 -& .
i,


I1




__, I
-I 2 I- -v



i ,,


















-|11L4 24 -i4 2N0 -14 16 -I,4 12 -114 8

Figure 1-2. Microbathymetry map showing the location of Alvin dives during AT11-7
along the East Pacific Rise. The contour interval is -25 m. The black
transects along or across the axial summit collapse trough include dives where
samples of lavas were recovered. The northernmost cluster of dives
3963,3965, 3973, 3974, and 3976 recovered a total of 50 samples from
9050.7'-49' N. The middle cluster of 3966, 3967, 3971, 3976 dives recovered
24 samples from 9044'-43.3' N. The southernmost cluster of dives 3968, 3970,
3975 recovered 33 samples from 9030.8'-28.7' N.
3975 recovered 33 samples from 9o3O.8'-28.7' N.













A



104'18'30"W 104"18'0"W 104"17'30"W 104'17'0"W 104'16'30"W

z z
0 0 0 p














10418'3W 104"180"W W 104'170W 10416'30W
oi0


























Figure 1-3. Location of lavas sampled during dives 3963 and 3974. A) Microbathymetry








samples 4, 5, and 8, and (3) samples 3 and 6. Dive 3974 sampled three flow
units: (1) samples 10 and 11, (2) samples 5, 6, and 9, and (3) samples 1-5. C)
Photomosaic of flow front from dive 3963.
.. .i "
., "":L -; ;. C)







0
7,3 ,., ..........








0 260 520 780
10418'30W 104 18'0W 1044130W 104'17"0W 104 1630'W




Figure 1-3. Location of lavas sampled during dives 3963 and 3974. A) Microbathymetry
and B) side-scan sonar maps of flow fronts showing locations of lava samples
collected by Alvin during dives 3963 (number symbols East of ASCT) and
3974 (number symbols West of ASCT) and by earlier rock-cores (circles) and
Alvin dives (squares). A symbols color refers to the wt.% MgO for the
sample. The ASCT is located in the center of each map. Dive 3963 sampled
a flow front (samples 10 and 11) and three flow units: (1) samples 7 and 9, (2)
samples 4, 5, and 8, and (3) samples 3 and 6. Dive 3974 sampled three flow
units: (1) samples 10 and 11, (2) samples 5, 6, and 9, and (3) samples 1-5. C)
Photomosaic of flow front from dive 3963.


















104'17 30"W


104'18'30"W 104'18'0'W


Figure 1-3. Continued


104'18'30*W


104'18'0"W


104'1T7O'W


104'16'30'W























A















B "















C

Figure 1-4. Typical lava morphologies observed at the East Pacific Rise. A) Ropy sheet
lavas. B) Lobate lavas. C) Decorated pillow lavas.
















A




104'14'30"W







Z z
SDive 3968
MGO
C: 1,t .,- t I, IIII1


I ,I 1.. I ,


...... ... .. ... ...
















F igure 1-5. Location of lavas s ample during dive 3968. A) Microbathymetry and B)
from the northern most channel while samples 1-5 were taken from the


104southern channel. C) Photomosaic of a channel from dive 3968.
Figure 1-5. Location of lavas sampled during dive 3968. A) Microbathymetry and B)
side-scan sonar maps channels showing locations of lava samples collected by
Alvin during dive 3968 and by earlier rock-cores (circles) and Alvin dives
(squares). A symbols color refers to the wt.% MgO for the sample. The
ASCT is located on the right-hand side of each map. Samples 6-10 come
from the northern most channel while samples 1-5 were taken from the
southern channel. C) Photomosaic of a channel from dive 3968.















104'14'30*W


104'14'30VW


Figure 1-5. Continued









lava morphology prompted the systematic sampling across and along the sides of the

channel. Discrete pillow mounds located on the crestal plateau were also investigated

due to their possible off-axis origins (Figure 1-6).

In this investigation, only four dives and the basalts sampled during those dives

are discussed in detail: 3963 and 3974 (flow units), 3968 (two channels), and 3970 (two

off-axis mounds). Historically, most of the submersible dives and rock sampling in the

9-100 N EPR region have been concentrated in and adjacent to the ASCT [Perfit and

Chadwick, 1998; Kurras et al., 2000]. However, dives 3963 and 3974 began over 2 km

off-axis near 950' N and proceeded towards the axis, sampling a continuum of flow

units in order to evaluate intra- and inter-flow unit chemical and petrographic variability

of spatially related flows (Figure 1-3). Both dives sampled parts of six flow units that

were distinguished by both changes in morphology and topographic relief at the front of

each flow unit (Figure 1-7). The division of flow units is less clear for dive 3974 due to a

slightly chaotic track. Two channels were extensively sampled near 929' N during dive

3968. Lava samples from the center, transition zone, and margin of the southern channel

were recovered from only one locality -450 m from the axis whereas the center (and

margin in one case) of the more northern channel was sampled at 300, 450, and 750 m

from the axis (Figure 1-5). Lobate flows composing the surrounding terrain were also

sampled. The final dive of interest (3970) sampled two pillow mounds located 1.5 km

from the spreading axis between 929'30" N and 931' N (Figure 1-6). Several pillow

lavas from each constructional mound were sampled in addition to several fissure-filling

lavas and surrounding older lobate and hackly lava flows.







13



S104"14'30"W 104'14'0"W 104'13'30"W 104 13'0'W





P- ,0 j \, /

) i "J, '. \, | .
,, o
I I










_1

I (\


104'1430"W 104'14'0"W 104'1330W 104*13'0"W



S104"1430W 104'140"W 104'1330OW 104 130"W


















104"14'30"W 104'14'0"W 104 1330"W 104 13'0"W


Figure 1-6. Location of lavas sampled during dive 3970. A) Microbathymetry and B)
side-scan sonar maps of off-axis mounds showing locations of lava samples
collected by Alvin during dive 3970 (number symbols) and by earlier rock-
cores (circles) and Alvin dives (squares). A symbols color refers to the wt.%
MgO for the sample. On the left side of each map, the ASCT is denoted by
the near-vertical trend in rock core and Alvin sampling. The northernmost
pillow mound consists of samples 4, 5, and 6 while the southernmost pillow
mound consists of samples 10 and 11.
R : f1




















mound consists of samples 10 and 11.





















-2510 ,,,,,

-2520 ASCT "+

-2530
E Flow Front 1

-2540 Flow Unit 2
S^.I Flow Unit 1

-2550


-2560 Flow Unit 3

-2570 -
0 100 200 300 400 500 600 700 800 900
distance (m)






Figure 1-7. Bathymetric profile for dive 3963. Each flow front sampled during dive 3963
and dive 3974 are marked by not only a morphological change in the lavas but
also a change in topography. The axial summit collapse trough (ASCT) is to
the west.














CHAPTER 2
BACKGROUND

Construction of Oceanic Crust at the Fast-Spreading East Pacific Rise

Magmatic processes focused beneath MORs are responsible for creating a multi-

layered crustal structure within the first few km of the spreading axis. The three main

seismically-defined layers comprising oceanic crust include sediments (Layer 1) covering

extruded basalts (Layer 2A) at the top, a sheeted dike complex in the middle (Layer 2B)

and gabbros with some cumulate ultramafic rocks at the bottom (Layer 3). Much of our

understanding regarding the composition and formation of this oceanic crustal lithology

has been obtained from geophysical results related to geologic observations. Deep drill

holes (e.g., ODP Hole 504B) into oceanic crust, ophiolites believed to represent slices of

oceanic crust preserved on land [e.g., Cann, 1974; Nicolas, 1989], and "tectonic

windows" along rift and transform zones such as the Hess Deep Rift and Blanco

Transform scarp [Karson et al., 2002a, 2002b; Stewart et al., 2005] provide unique

perspectives into the cross-sectional structure and composition of the uppermost oceanic

crust. These direct observations combined with seismic studies [Detrick et al., 1987;

Toomey et al., 1990, 1994; Harding et al., 1993; Vera and Diebold, 1993; Christenson et

al., 1994, 1996; Hooft et al., 1996; Crawford and Webb, 2002] and detailed sampling and

mapping of lavas from the EPR [Macdonald andFox, 1988; Haymon et al., 1993; Perfit

et al., 1994; Fornari and Embley, 1995] have been used to better understand MOR

magmatic and volcanic processes.









Lavas found at the EPR are primarily low-potassium tholeiitic basalts, otherwise

referred to as mid-ocean ridge basalts (MORB). They are fed from some type of magma

"chamber" at depth to the surface by dikes. Magmas fed by dikes that do not erupt onto

the seafloor as glassy to microcrystalline sheet, lobate, and pillow lavas, solidify to form

the sheeted basaltic dikes that comprise seismic Layer 2B. Dikes stem from the magma

chamber and transport magmas both vertically to the ASCT as well as long distances

laterally along the spreading axis [Dziak et al., 1995; Fox et al., 1995; Embley et al.,

2000]. A large, fully or mostly liquid magma chamber that extends from the bottom of

Layer 2 down to the Moho is no longer believed to exist at MOR [e.g., Cann, 1974].

Instead, seismic reflection and refraction data indicate a 1-2 km-wide, thin melt lens

(<100 m thick) exists at depths -1.5 km above a larger crystal mush zone 6-7 km wide

[Detrick et al., 1987; Toomey et al., 1990, 1994; Sinton and Detrick, 1992; Harding et

al., 1993; Vera and Diebold, 1993; Hooft et al., 1996; Carbotte et al., 1997].

Solidification of the composite magma chamber on either side of the axis forms the

gabbroic rocks of Layer 3 (5 km thick), with cumulate ultramafic rocks at its base. A

second, deeper melt lens located at the Moho below the mush zone has also been

identified [Garmany, 1989; Dunn and Toomey, 1997; Crawford et al., 1999].

Crawford and Webb [2002] developed interpretive cross sections of the subaxial

crust for three areas of the EPR (948' N, 933' N, and 9008'N) based on previous

studies regarding the placement and size of the axial melt lens [Kent et al., 1993a,

1993b], the distribution of melt in the low-velocity crystal mush zone [Dunn et al., 2000;

Toomey et al., 1990], and the presence of melt near the Moho [Dunn et al., 2001] (Figure

2-1). At the northern end of the 9-10 N segment of the EPR, the melt lens is narrow






















1 0 1 . . -. L -
-5 0 5 10





.8




.. -5 0 5 10






.8 -melt sill


10
-10 -5 0 5
Distance across, axis (kmn)



From Crawford and Webb [2002]

Figure 2-1. Interpretative cross-section of the subaxial crustal structure beneath the East
Pacific Rise. Note the changing locations and sizes of the axial magma lens
above its associated mush zone (partial melt area) from 948'-08' N. The
presence of melt sills at the Moho is intermittent along this section of the
ridge.









and centered at the spreading axis but becomes wider and offset to the west of the ridge

axis closer to the overlapping spreading center (OSC) at 9003' N. Past the OSC, the

ridge axis is once again aligned with a narrow melt lens.

Regional Geology of The 9-10 N segment of the East Pacific Rise

The fast spreading (110 mm/yr) EPR between 9-10o N is bounded by the

Clipperton Transform Fault at 10010' N and a large overlapping spreading center (OSC)

at 9003' N (Figure 1-1). Axial linearity is disrupted at 9037' N by a smaller OSC, which

Sih, et al. [2001] interpret to be both a hydrothermal and volcanological divide for areas

north and south of it. In addition to detailed mapping and numerous in situ observations,

this segment of the EPR remains one of the most systematically sampled segments of all

MORs [Fornari et al., 1990, 1992, 1994, 1998; Batiza and Niu, 1992; Haymon et al.,

1993; Perfit et al., 1994; Fornari and Embley, 1995; Gregg et al., 1995, 1996; Perfit and

Chadwick, 1998; S.nith et al., 2001]. More than 1200 rock samples have been collected

from both the ridge axis and ridge flanks by means of rock core, dredge and submersible.

However, most of these samples come from within or close to the ASCT where the towed

vehicle ARGO and the submersible ALVIN were used to investigate the volcanic,

tectonic, and hydrothermal features active within the ridge axis [Haymon et al., 1991,

1993; Wright et al., 1995a, 1995b; Kurras et al., 2000].

The ASCT between 9022' N and 9051' N is a sinuous and nearly continuous feature

of the ridge axis with width that varies from 40 m to 100 m wide. Between 9037' N

and 9O50' N, a smooth, broad, and rounded bathymetric profile indicates a robust

magmatic history for this portion of the ridge (Figure 2-2) [Scheirer and Fox, 1993;

Perfit and Chadwick, 1998; Fornari et al., 1998; Siilt et al., 2001]. Here, the ASCT is











A

Axis

-2520
-2540
-2560 -
-2580
S-2600
W -2620
-2640
-2660

0 500 1000 1500 2000 2500 3000 3500 4000 4500
Distance (m)


B

Axis
-2600
-2620
-2640
0. -2660
-2680
-2700
-2700 0 1000 2000 3000 4000 5000
Distance (m)


Figure 2-2. Cross-axial profiles for two sections of the East Pacific Rise. A) shows the
smooth, broad profile of the EPR at 9050'N is a typical of magmatically
robust sections while B) shows the cross-axial profile at 9028' N EPR has a
greater influence by tectonism. Profiles were constructed using Cochran et
al., 1999 microbathymetry. Each profile is shown west to east.

narrowest (-40-80 m wide) with steeply sloping (>600) to near vertical sidewalls less


than 15 m high. Recent volcanic activity (April to May, 1991) within the ASCT between

9046' N and 951' N was identified and documented by Haymon et al. [1993] and dated


by Rubin et al. [1994]. The small-volume eruptions appeared to emanate from an 8.5 km

long, 1-4 m wide fissure in the axial floor [Fornari et al., 1994; Lutz et al., 1994]. Since

these recent volcanic events, new diffuse and high-temperature hydrothermal venting has

commenced [Von damm et al., 1992, 1995; Haymon et al., 1993]. The stratigraphic

relationships and morphologies of lavas on the floor and sides of the ASCT reflect

modification of, and deposition by, the last volcanic event (Figure 2-3). Both lava pillars



















v'dIfCAI WAiL uf LM.ykI ScB-rT tHI.Hi W


| '

PRVItOUS tVEI. O*IA;L .1 .i i?
4V-


YOUNG SJIr AM)I i(WY FM IWS


-YOXM LAUVA PILLOJUT
2' *"t a


From Perfit and Chadwick [1998]


Figure 2-3. Typical lava morphologies and stratigraphic positions within the axial summit
collapse trough at the East Pacific Rise. A) An artists rendition of the ASCT
at the EPR is based on Alvin observations of B) collapsed features, C) lava
pillars, and D) and E) complex stratigraphic relations which often reflect the
most recent episode of volcanism









D E













Figure 2-3. Continued

5-8 m tall and lava that breached the rim of the ASCT were used to estimate the thickness

of the 1991 flow before all but 2-3 m of lava drained back into the fissure. The

remaining lava was dominantly composed of complex sheet flows (ropy, platy, and

jumbled varieties) exhibiting extensive collapse features [Haymon et al., 1993; Von

Damm et al., 1995; Gregg et al., 1996].

In contrast, a less robust magmatic portion of the ridge between 9O30' N and 9o35'

N is marked by a deeper axial trough, more evolved and older looking lavas, and

tectonically dominated axial morphology (Figure 2-2) [Fornari et al., 1998; Perfit and

Chadwick, 1998; S.mith et al., 2001]. The ASCT in this area is up to 100 m wide, 10-25

m deep, and is located asymmetrically within an axial graben 250-350 m wide [Fornari

et al., 1998]. Fracturing of the youngest flows attests to the tectonic dominance in an

otherwise magmatically robust region.

Three types of volcanic emplacement were recognized by Kurras et al. [2000]

between 949'N and 952'N of the EPR axial ridge: 1) mainly sheet and lobate flows

extending <500 m from the ASCT; 2) channelized lava flows transported off-axis from

the site of eruption at or near the ASCT; and 3) lavas erupted off-axis (0.5 to 1.5 km)









resulting in the construction of pillow mounds and ridges. In general, the sheet and

lobate flows close to the ASCT between 9o30' N and 951' N (50-100 m from axis) show

extensive collapse features intermingled with lava tubes and channels resulting from the

effusive nature of these lava morphologies [G/ if/i/hh and Fink, 1992; Gregg and Fink,

1995, Engels et al., 2003]. The lavas are also more mafic and have fewer phenocrysts

than flows further away from the ASCT [Kurras et al., 2000]. The off-axis pillow

mounds near 931' N EPR form pillow ridges 10-15 high and 50 m wide paralleling the

axis [Perfit et al., 1994; Sims et al., 2003]. When compared to on-axis lavas, the off-axis

flows near 9031' N and 9050'N are compositionally more diverse with substantial inter-

flow geochemical variations over small spatial scales of less than 600 m [Perfit et al.,

1994]. Other areas of the EPR, specifically 1 120'N and 12-13N, possess off-axis lavas

showing similar compositional complexity and diversity [Hekinian et al., 1989; Reynolds

et al., 1992].

Off-axis volcanism is thought to be associated with the development of normal

faulting and the initiation of fissuring parallel to the spreading axis [Luyendyk, 1970;

Lonsdale, 1977; Macdonald, 1982; Perfit et al., 1994; Alexander and Macdonald, 1996;

Macdonald et al., 1996]. The regional and local stresses responsible for brittle crustal

failure and the resulting rolling topography of the abyssal hills 2-6 km from the axis are

related to a variable magmatic supply [Edwards et al., 1992], movement of the crust

away from this supply and the associated cooling of the lithosphere [Lonsdale, 1977;

Searle, 1984; Goff 1991; Goff et al., 1993], nearby discontinuities in the ridge axis

[Pollard and Aydin, 1984; Sempere and Macdonald, 1986], or bending and snapping of

the lithosphere [Sohn and Sims, 2005]. According to Alexander and Macdonald [1996],









80% of fault scarps that face the ridge axis (inward-facing) stop lengthening and reach

final heights of 60-70 m within 30 km of the axis. Alternatively, more than 95% of all

fault scarps facing away from the ridge axis (outward-facing) reach a final height of -60

m within 5-7 km of the axis. Submersible observations of the outward-facing fault scarps

reveal slopes of -300 and evidence of recent volcanism in the form of draped lava flows

and fissure-filling pillows [Edwards et al., 1992; Macdonald et al., 1996; Kurras et al.,

2000]. Feeder dikes most likely represent the sources of these syntectonic features as

interpreted from narrow Bouguer gravity highs measured above pillow ridges formed

over fissures [Cochran et al., 1999]. In addition, several lavas collected from sites of off-

axis mound construction are more chemically evolved, alkali enriched transitional

MORB (T-MORB) which suggests the source of the dikes to be different than that of the

dikes at the axis [Perfit et al., 1994; Perfit and Chadwick, 1998].














CHAPTER 3
ANALYTICAL METHODS

Natural glasses from the outer quenched surfaces of lavas were separated during

description and photographing for each of the samples during the cruise. The glass chips

and remaining hand specimens were brought back to the University of Florida to process

for major and trace element analysis. Approximately 100 mg of the cleanest glass from

each sample was handpicked using a binocular microscope. Vesicles, cristolites,

carbonate and Mn-coatings were avoided as much as possible. To remove any remaining

surficial coatings, the sub-samples were ensonified three times in 2X H20 for five

minutes each, an additional five minutes in a mixture containing 100 ml H202, 80 ml 2X

H20, and 20 ml 12N HCL, and finally three five minute sessions in 4X H20. After

completing the ensonification process, samples were dried under a heat lamp.

Once dry, several of the freshest glass chips from each of the 111 samples were

made into polished thin-sections and subsequently were analyzed for major elements by

Ian Ridley at the USGS Microbeam Laboratory using a JEOL 8900 Electron Microprobe.

Analysis of seven to ten separate points (including spots on separate chips of the same

sample) were averaged for each sample and then corrected for instrument drift by

applying a correction factor based on the normalization of the measured values to

established values for the in-house standards JdF-D2 and 2392-9 (see Sinitl et al.

[2001]). SiO2 was also corrected for error due to auto-focusing of the electron beam

according to the methods of Reynolds and Langmuir [1997]. Corrections were typically

less than 1.22 relative % of the measured value. The 2-sigma errors (precision) calculated









from variation in the analyses of 2392-9 during these analytical runs are as follows: Si02

(0.29 wt.% or 0.57 relative %), TiO2 (0.07 wt.% or 5.4%), A1203 (0.17 wt.% or 1.1%),

FeO (0.17 wt.% or 1.8%), MnO (0.05 wt.% or 26.8%), MgO (0.11 wt.% or 1.3%), CaO

(0.14 wt.% or 1.2%), Na20 (0.09 wt.% or 3.6%), K20 (0.02 wt.% or 18.8%), and P205

(0.04 wt.% or 29.8%).

Additional sample splits (-40 mg) of the freshest glass from 48 of the samples

(from four dives; 3963, 3968, 3970, 3974) were selected for trace element analysis by

ICP-MS. An in depth description of the dissolution and analytic procedure is provided in

Appendix A. Several certified rock standards (AGV-1 and BCR-2) and two in-house

MORB standards (ENDV and 2392-9) were used to calibrate the machine while repeated

chemical analyses of the enriched MORB "ENDV" was used to evaluate and correct for

instrument drift. Precision was found to be better than 1% (relative to the

concentration) for La, Nd; 2% for Zr, Dy, Yb; + 3% for Co, Ga, Sm, Eu, Gd; + 4% V,

Ni, Cu, Y, Nb, Ce, Tb, Ho, Hf, Th; + 5% Sc, Rb, Sr, Ba, Pr, Tm, Lu, Pb; + 6% for Er, Ta;

7% Cr, U; and 9% for Zn. Accuracy was evaluated using the values we measured for

BHVO-1. A discussion of accuracy and precision (error) for this study is available in

Appendix B.

Thirty-five lavas were selected for petrographic analysis using thin-sections that

included the outer glassy margins as well as the more crystalline portions of each sample.

They were examined with a petrographic microscope to describe the mineralogy and

cotectic relationships, as well as estimate the modal abundances, textural proportions, and

vesicularity (by point counting). Representative photomicrographs from several of the

thin-sections were taken. The compositions of olivine, plagioclase, and clinopyroxene in









two thin-sections, 3970-9 and 3974-11, were analyzed by Michael Perfit using the JEOL

8900 Electron Microprobe at the USGS in Denver. Precision was estimated from

multiple major element analyses of glass in samples 3970-9o, 3970-9i, and 3974-11. For

the An# ((Ca/(Ca+Na))*100) and Mg# ((Mg/(Mg+Fe))*100), precision was 0.76 and

1.04 relative % respectively.

Photographs obtained by the submersible ALVIN and the Rabbit Cam towed

camera system, dive transcripts, ABE microbathymetry, and DSL-120A side scan sonar

images were used in conjunction with the major and trace element analyses to catalog and

map: 1) the physical extent of lava flows, including flow fronts and channels, and off-axis

mounds; 2) the distribution of lava morphologies and related contact relationships; 3)

chemical variations within and between flows, including possible genetic relationships;

and 4) relative crustal ages based on sediment cover and infill. Compiled into ARC/GIS,

this information was used to construct geologic maps and bathymetric profiles for the

four dives of interest: 3963 and 3974 (flow fronts), 3968 (channels), and 3970 (off-axis

mounds).














CHAPTER 4
PETROGRAPHY

Petrography

The 31 samples selected for petrographic study were collected during ALVIN

dives to the three main sites of interest along the 90-100N segment of the East Pacific

Rise (EPR): a series of flow-fronts on either side of ASCT at 9050'30"N, two lava

channels at -9029'N EPR, and a series of off-axis pillow mounds at 9030'N. The

samples range from mafic to moderately evolved (7.23-8.91 wt.% MgO) and all have

depleted incompatible element concentrations (N-MORB). A total of 35 thin-sections

were cut from the outer glassy rinds down through the more crystalline interiors of the 31

lavas (Figure 4-1). Composed of sheet flows, lobate flows and pillow basalts, the lavas

were collected up to 2.17 km from the ASCT and are mainly aphyric to sparsely

plagioclase phyric basalts (Figure 4-2). A comprehensive description for each

petrographically examined sample is provided in Tables 4-1 to 4-4.

Flow Units With Fronts, Dives 3963 and 3974

The flows sampled from both sides of the ASCT at 9050'30"N EPR are mainly

aphyric to sparsely plagioclase phyric plagioclase basalts (Table 4-1 and 4-2). Only two

pillow basalts collected from flow fronts contain more than 1.5% microphenocrysts

(3963-3, 3.9% and 3963-10, 1.72%). Subhedral to euhedral Carlsbad twinned plagioclase

laths (0.1-1 mm) along with more acicular (0.1-0.4 mm) forms represent the most

abundant microphenocryst phase (Figure 4-2). In addition, the majority of plagioclase














Table 4-1. Thin Section Petrography of Lavas Collected during Alvin Dive 3963 Near 9050'N East Pacific Rise
Thin Section Location Sample Locale Lava Type MgO % Plagioclase Characteristics
3963-1 950.653N 2.2 Km East ASCT sheet 7.89 an-euhedral laths (0.1-0.55 mm), rare Carlsbad twins, glomerocrysts
3963-1 9050.653' N 2.2 Km East ASCT sheet 7.89
(0.2-0.6 mm), microlites (<0.1 mm)
39632 95065N 2.2 KmEastASCT sheet sub-euhedral laths (0.2-0.4 mm), Carlsbad twins, zoning,
3963-2* 9050.651' N 2.2 Km East ASCT sheet x
xenoglomerocyrst (1.2 mm)
sub-euhedral laths (0.1-0.6 mm), Carlsbad twins, skeletal,
3963-3* 9050.610' N Flow Front #1 pillow 7.71 glomerocrysts (0.4-0.6 mm), acicular/microlites (<0.2 mm),
intergrown with mmp
sub-euhedral laths (0.1-0.4 mm), Carlsbad twins, skeletal,
3963-4 9050.502' N Flow Front #2 pillow 7.73 glommerocrysts (0.3-0.8 mm), acicular/microlites (<0.3 mm),
intergrown with mmp
sub-euhedral laths (0.1-0.9 mm), skeletal, Carlsbad twins,
3963-6* 9050.501' N Under Flow Front # 2 sheet 7.83 glomerocrysts (0.5-1.1 mm), acicular/microlites (<0.3 mm),
intergrown with mmp
3963-7* 9050.450' N Flow Front #3 pillow 8.73 embayed xenoglomerocrysts (2.5 mm), skeletal acicular/microlites
(<0.2 mm), intergrown with mmp

subhedral laths (<0.6 mm), Carlsbad twins, glomerocrysts (0.8 mm),
3963-8* 9050.452' N Under Flow Front #3 lobate 8.00
skeletal acicular/microlites (<0.4 mm), intergrown with mmp

3963-10* 9050.294N Flow Front #4 pillow 8.91 embayed xenoglomerocryst (1 mm), acicular/microlites (<0.15 mm),
3963-10* 9050.294' N Flow Front #4 pillow 8.91
Carlsbad twins, intergrown with 01
3963-11 9050.305' N Flow Front #4 pillow 8.89 N/A

Notes: Representative photomicrographs, which illustrate common mineralogies and textures, are available for the thinsections possessing an astrix after their
sample number (e.g., 3963-2*). Sample localities are based on in situ observations. Percentages of microphenocrysts, glass, groundmass, and vesicles were
first estimated visually. The point counts (1000 per slide) for several representative thinsections are provided in parenthesis preceding the estimate. Note that
when significant glass is present at the ends of slides, the point counting apparatus, which won't extend to measure the ends, underestimates the amount of
glass present and thus the other point count values for that slide are compromised. Thickness of the glassy outer rind of many lavas (in cm) was measured for
both the hand specimen (Glass H.S.) and the thin section (Glass T.S.).

Sample locality: ASCT = axial summit collapse trough, F.F.= flow front, P.M. = pillow mound; Crystal Shapes: an = anhedral, sub = subhedral; Mineralogy:
Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G-
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2% microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 = dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases
include iddingsite and palagonite.














Table 4-1. Continued


Thin Section Clinopyroxene Characteristics Olivine Characteristics Cotectic
3963-1 N/A N/A

3963-2* N/A N/A


3963-3* sub-euhedral (<0.1 mm) intergrown with Plag sub-euhedral (<0.1 mm), embayed, intergrown with Plag


3963-4 anhedral mass (<0.1 mm) intergrown with Plag euhedral (<0.1 mm), intergrown with Plag 01 + Plag


3963-6* an-subhedral mass (<<0. Iam) intergrown with Plag euhedral (<0.1 mm), skeletal, xenocryst (0.6 mm), intergrown with 01 + Plag
3963-6* an-subhedral mass (<<0.1 mm) intergrown with Plag 01 + Plag
Plag

3963-7* anhedral (<0.1 mm) intergrown with Plag euhedral (<0.2 mm), integrown with Plag 01 + Plag


3963-8* an-subhedral mass (<<0.1 mm) intergrown with Plag an-subhedral mass (<<0.1 mm) intergrown with Plag


3963-10* N/A euhedral (<0.1 mm), embayed, hopper, integrown with Plag 01 Plag
3963-11 N/A sub-euhedral (<0.1 mm), hopper, glomerocrysts (<0.1 mm) 01

Notes: Representative photomicrographs, which illustrate common mineralogies and textures, are available for the thinsections possessing an astrix after their
sample number (e.g., 3963-2*). Sample localities are based on in situ observations. Percentages of microphenocrysts, glass, groundmass, and vesicles were
first estimated visually. The point counts (1000 per slide) for several representative thinsections are provided in parenthesis preceding the estimate. Note that
when significant glass is present at the ends of slides, the point counting apparatus, which won't extend to measure the ends, underestimates the amount of glass
present and thus the other point count values for that slide are compromised. Thickness of the glassy outer rind of many lavas (in cm) was measured for both
the hand specimen (Glass H.S.) and the thin section (Glass T.S.).

Sample locality: ASCT = axial summit collapse trough, F.F.= flow front, P.M. = pillow mound; Crystal Shapes: an = anhedral, sub = subhedral; Mineralogy:
Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G-
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2% microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 = dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases include
iddingsite and palagonite.














Table 4-1. Continued


Thin Section % mPheno %Glass %G-mass %Vesicles Vesicle size Diseq Alteration Glass T.S. Glass H.S.
3963-1 <1(0.9) 0 (0) 98 (98.7) <1 (0.4) <0.2 N R/O 0 0.1

3963-2* <1 30 69 -0 0.2-0.25 Y -0.4 1


3963-3* 3 (3.9) 5 (0.9) 90 (93.2) 2 (2) 0.1-0.3 Y R/O 0 0.5


3963-4 <1 8 90 2 0.2 Y -0.2 1


3963-6* 1.5 10 85 4 <0.1-0.5 Y R/O 0.3 0.4


3963-7* <1 30 68 1 0.1-0.3 Y 0.5 1.5


3963-8* <1 20 78 2 0.1-0.3 Y 0.5 0.5


3963-10* 1(1.72) 0 (0) 97 (97) 2 (1.1) 0.1-0.25 Y 0 0.4
3963-11 <1 99 0 <1 0.4 N 1.5 All

Notes: Representative photomicrographs, which illustrate common mineralogies and textures, are available for the thinsections possessing an astrix after their
sample number (e.g., 3963-2*). Sample localities are based on in situ observations. Percentages of microphenocrysts, glass, groundmass, and vesicles were
first estimated visually. The point counts (1000 per slide) for several representative thinsections are provided in parenthesis preceding the estimate. Note that
when significant glass is present at the ends of slides, the point counting apparatus, which won't extend to measure the ends, underestimates the amount of
glass present and thus the other point count values for that slide are compromised. Thickness of the glassy outer rind of many lavas (in cm) was measured for
both the hand specimen (Glass H.S.) and the thin section (Glass T.S.).

Sample locality: ASCT = axial summit collapse trough, F.F.= flow front, P.M. = pillow mound; Crystal Shapes: an = anhedral, sub = subhedral; Mineralogy:
Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G-
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2% microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 = dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases
include iddingsite and palagonite.














Table 4-1. Continued


Thin Section Crystallinity Phyric Vitrophyric Microlitic Spherulitic Variolitic Trachytic Fascicular Glomeroporphoritic
3963-1 quench no 0 0 GRAD, 1 0 0 3 0

3963-2* hyaline-quench no 2 0 GRAD 2 0 GRAD 0


3963-3* quench sparse 0 2 GRAD, 1 1 SUB 3 0


3963-4 quench no 0 2 GRAD, 2 1 2 GRAD, 2 0


3963-6* quench sparse 0-1 2 GRAD, 2 GRAD, 2 SUB, 2 GRAD, 2 0


3963-7* hyaline-quench no 0 2 0 2 2 1 0


3963-8* quench no 0-1 3 0 1 0 0 0


3963-10* quench no 0 0 GRAD, 3 0 0 GRAD, 3 0
3963-11 hyaline no 3 0 0 1 0 0 0

Notes: Representative photomicrographs, which illustrate common mineralogies and textures, are available for the thinsections possessing an astrix after their
sample number (e.g., 3963-2*). Sample localities are based on in situ observations. Percentages of microphenocrysts, glass, groundmass, and vesicles were first
estimated visually. The point counts (1000 per slide) for several representative thinsections are provided in parenthesis preceding the estimate. Note that when
significant glass is present at the ends of slides, the point counting apparatus, which won't extend to measure the ends, underestimates the amount of glass
present and thus the other point count values for that slide are compromised. Thickness of the glassy outer rind of many lavas (in cm) was measured for both the
hand specimen (Glass H.S.) and the thin section (Glass T.S.).

Sample locality: ASCT = axial summit collapse trough, F.F.= flow front, P.M. = pillow mound; Crystal Shapes: an = anhedral, sub = subhedral; Mineralogy:
Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G-
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2% microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 = dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases include
iddingsite and palagonite.














Table 4-2. Thin Section Petrography of Lavas Collected During ALVIN Dive 3974 Near 9050'N East Pacific Rise

Thin Section Location Sample Locale Lava Type MgO % Plagioclase Characteristics
3974-1* 9050.136'N 1.9 Km West ASCT sheet 7.64 microlites (<0.1 mm), intergrown with Cpx
4-2 90.1 N Flow Front # pilw 7 skeletal acicular/microlites (<0.3 mm), Carlsbad twins, intergrown
3974-2 950.164' N Flow Front #1 pillow 7.59p
with mmp
3974-3 9050.202' N Between F.F. #1 and 2 hackly 8.05 subhedral laths (0.1-0.2 mm), acicular/microlites (<0.1 mm)
39744* subhedral laths (<0.7 mm), Carlsbad twins, glomerocrysts (1-1.4mm),
3974-4* 9050.254' N Between F.F. #1 and 2 lobate 8.03
skeletal microlites (<0.2 mm), intergrown with mmp
3974-5 9050.299' N Flow Front #2 pillow 7.65 skeletal acicular/microlites (<0.2 mm), intergrown with mmp
3974-6 9050.349' N Flow Front #2 pillow 7.68 Carlsbad twins, skeletal acicular/microlites (<0.1 mm), intergrown
3974-6 950.349' N Flow Front #2 pillow 7.68
with mmp
3974-8t* 9050 N B en F.F. #I and 2 pillow knob 7.91 Carlsbad twins, skeletal acicular/microlites (<0.3 mm), intergrown
3974-8t* 9050.252'N Between F.F. #1 and 2 pillow knob 7.91
with mmp
3974-8b 9050. N B en F.F. #I and 2 pillow knob 7.91 Carlsbad twins, skeletal acicular/microlites (<0.3 mm), intergrown
3974-8b 9050.252'N Between F.F. #1 and 2 pillow knob 7.91
with mmp
3974-9 9050.284' N Between F.F. #2 and 3 lobate 8.45 skeletal microlites (<0.1 mm), intergrown with mmp
sub-euhedral laths (0.2-1 mm), Carlsbad twins, skeletal,
3974-10 9050.348' N Flow Front #3 pillow 8.33 glomerocrysts (1.1 mm), acicular/microlites (<0.2 mm), intergrown
with mmp
3974-11t 9050.386' N 700 m East of ASCT lobate 8.70 skeletal microlites (<0.1 mm)
3974-1 lb 9050.386' N 700 m East of ASCT lobate 8.70 skeletal acicular/microlites (<0.4 mm), intergrown with mmp

Notes: Representative photomicrographs, which illustrate common mineralogies and textures, are available for the thinsections possessing an astrix after their
sample number (e.g., 3963-2*). Sample localities are based on in situ observations. Percentages of microphenocrysts, glass, groundmass, and vesicles were first
estimated visually. The point counts (1000 per slide) for several representative thinsections are provided in parenthesis preceding the estimate. Note that when
significant glass is present at the ends of slides, the point counting apparatus, which won't extend to measure the ends, underestimates the amount of glass
present and thus the other point count values for that slide are compromised. Thickness of the glassy outer rind of many lavas (in cm) was measured for both the
hand specimen (Glass H.S.) and the thin section (Glass T.S.).

Sample locality: ASCT = axial summit collapse trough, F.F.= flow front, P.M. = pillow mound; Crystal Shapes: an = anhedral, sub = subhedral; Mineralogy:
Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G-
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2% microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 = dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases include
iddingsite and palagonite.














Table 4-2. Continued


Clinopyroxene Characteristics
anhedral (<<0.1 mm) intergrown with Plag

anhedral (<0.1 mm) intergrown with Plag

N/A

an-subhedral (<0.1 mm) intergrown with Plag
an-subhedral (<0.1 mm) intergrown with Plag
an-subhedral (<0.1 mm) intergrown with Plag

an-subhedral (<0.1 mm) intergrown with Plag

anhedral mass (<0.1 mm) intergrown with Plag
anhedral mass (<0.1 mm) intergrown with Plag

an-subhedral (<0.1 mm) intergrown with Plag

sub-euhedral (<<0.1 mm)
anhedral (<<0.1 mm) intergrown with Plag


Olivine Characteristics
euhedral (<0.1 mm), hopper, embayed

sub-euhedral (<0.15 mm), hopper, intergrown with Plag

subhedral (<<0.1 mm)

sub-euhedral (<0.1 mm), intergrown with Plag
sub-euhedral, skeletal, hopper, intergrown with Plag
sub-euhedral (<0.15 mm), xenocryst (0.55 mm), skeletal, hopper,
intergrown with Plag
sub-euhedral (<0.15 mm), skeletal, hopper, intergrown with Plag

subhedral (<0.1 mm), skeletal, intergrown with Plag
subhedral (<<0.1 mm) intergrown with Plag

sub-euhedral (<0.1 mm), skeletal, hopper, intergrown with Plag

sub-euhedral (<<0.1 mm)
subhedral (<<0.1 mm), skeletal, intergrown with Plag


Cotectic
01 Plag

01+ Plag


01 Plag
01+ Plag


01

01 Plag

01


Notes: Representative photomicrographs, which illustrate common mineralogies and textures, are available for the thinsections possessing an astrix after their
sample number (e.g., 3963-2*). Sample localities are based on in situ observations. Percentages of microphenocrysts, glass, groundmass, and vesicles were
first estimated visually. The point counts (1000 per slide) for several representative thinsections are provided in parenthesis preceding the estimate. Note that
when significant glass is present at the ends of slides, the point counting apparatus, which won't extend to measure the ends, underestimates the amount of
glass present and thus the other point count values for that slide are compromised. Thickness of the glassy outer rind of many lavas (in cm) was measured for
both the hand specimen (Glass H.S.) and the thin section (Glass T.S.).

Sample locality: ASCT = axial summit collapse trough, F.F.= flow front, P.M. = pillow mound; Crystal Shapes: an = anhedral, sub = subhedral; Mineralogy:
Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G-
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2% microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 = dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases
include iddingsite and palagonite.


Thin Section
3974-1*

3974-2

3974-3

3974-4*


3974-5
3974-6

3974-8t*

3974-8b
3974-9

3974-10

3974-11t
3974-11b














Table 4-2. Continued


Thin Section % mPheno %Glass %G-mass %Vesicles Vesicle size Diseq Alteration Glass T.S. Glass H.S.
3974-1* <<1 5 -84 10 <4 Y 0.3 0.6

3974-2 1.5 30 68 1 <0.3 Y 0.7 1

3974-3 <<1 0 99 <<1 <<0.1 N R/O 0 0.6

3974-4* -1(1.4) 0 (0) 98 (96.6) 2 (2) <<0.1 Y 0 0.3
3974-5 -1 5 92 2 <0.4 Y 0.2 0.8
3974-6 <1 15 84 1.5 <0.4 Y 0.3 1

3974-8t* <1 0 98 2 0.1 Y 0 0.02

3974-8b -) 0 98 1.5 0.2 Y 0
3974-9 (1.3) 10 (0.4) 88 (97.6) 2 (0.7) <0.7 Y 0.3 1.25

3974-10 1 15 83 1 <0.4 Y 0.5 0.5

3974-1 It -) 2 97 1 <0.8 Y 0.3 1
3974-11b -4 0 99 <1 <0.8 Y 0

Notes: Representative photomicrographs, which illustrate common mineralogies and textures, are available for the thinsections possessing an astrix after their
sample number (e.g., 3963-2*). Sample localities are based on in situ observations. Percentages of microphenocrysts, glass, groundmass, and vesicles were
first estimated visually. The point counts (1000 per slide) for several representative thinsections are provided in parenthesis preceding the estimate. Note thai
when significant glass is present at the ends of slides, the point counting apparatus, which won't extend to measure the ends, underestimates the amount of glass
present and thus the other point count values for that slide are compromised. Thickness of the glassy outer rind of many lavas (in cm) was measured for both
the hand specimen (Glass H.S.) and the thin section (Glass T.S.).

Sample locality: ASCT = axial summit collapse trough, F.F.= flow front, P.M. = pillow mound; Crystal Shapes: an = anhedral, sub = subhedral; Mineralogy:
Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G-
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2% microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 = dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases
include iddingsite and palagonite.














Table 4-2. Continued


Thin Section Crystallinity Phyric Vitrophyric Microlitic Spherulitic Variolitic Trachytic Fascicular Glomeroporphoritic
3974-1* quench no 0 GRAD GRAD, 1 1 COMPLEX 3 0

3974-2 hyaline-quench sparse 1 3 0 2 SUB 1 0

3974-3 quench no 0 2 GRAD, 3 0 1 GRAD, 3 0

3974-4* quench no 0 3 0 0 3 1 0
3974-5 quench no 0 3 0 1 0 1 0
3974-6 quench no 0 3 GRAD, 1 2 0 GRAD, 1 0

3974-8t* quench no 0 3 GRAD, 1 0 3 GRAD, 1 0

3974-8b quench no 0 3 0 0 3 0 0
3974-9 quench no 0 1 GRAD, 1 2 0 GRAD, 1 0

3974-10 quench sparse 0 2 GRAD, 1 2 SUB GRAD, 1 0

3974-1It quench no 0 1 0 3 0 2 0
3974-1 lb quench no 0 GRAD,2 GRAD, 2 1 0 GRAD, 2 0

Notes: Representative photomicrographs, which illustrate common mineralogies and textures, are available for the thinsections possessing an astrix after their
sample number (e.g., 3963-2*). Sample localities are based on in situ observations. Percentages of microphenocrysts, glass, groundmass, and vesicles were first
estimated visually. The point counts (1000 per slide) for several representative thinsections are provided in parenthesis preceding the estimate. Note that when
significant glass is present at the ends of slides, the point counting apparatus, which won't extend to measure the ends, underestimates the amount of glass
present and thus the other point count values for that slide are compromised. Thickness of the glassy outer rind of many lavas (in cm) was measured for both the
hand specimen (Glass H.S.) and the thin section (Glass T.S.).

Sample locality: ASCT = axial summit collapse trough, F.F.= flow front, P.M. = pillow mound; Crystal Shapes: an = anhedral, sub = subhedral; Mineralogy:
Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G-
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2% microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 = dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases include
iddingsite and palagonite.









Table 4-3. Thin Section Petrography of Lavas Sampled During ALVIN Dive 3968 Near
930'N East Pacific Rise


Thin Section
Location
Sample Locale
Lava Type
MgO %

Plagioclase
Characteristics

Vesicle size
Diseq
Alteration
Crystallinity
Phyric
% mPheno
%Glass
%G-mass
%Vesicles
Glass TS.
Glass H. S.
Vitrophyric
Microlitic
Spherulitic
Variolitic
Trachytic
Fascicular
Glomeroporphoritic


3968-2T
9028.770' N
Channel #1
lobate
7.57

sub-euhedral laths
(<0.5 mm), Carlsbad
twins, zoning
<0.1
Y

quench
no
<1
20
80
~0
0.4
0.5
0
0
1
3
0
1
0


3968-6T
9028.946' N
Channel #2
folded sheet
7.50
sub-euhedral laths (0.1-0.8 mm),
acicular (0.3-0.4 mm), Carlsbad
twins, zoning, glomerocrysts
(0.6 and 1.7 mm)
N/A
Y

hyaline
no
<1
99
0
0
1.2
0
3
0
0
1
0
0
0


Notes: Sample locations based on in situ observations. Thickness of the glassy outer
rind of many lavas (in cm) was measured for both the hand specimen (Glass H.S.) and
the thin section (Glass T.S.). Crystal Shapes: an = anhedral, sub = subhedral;
Mineralogy: Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic
microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2%
microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 =
dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases include
iddingsite and palagonite.










Table 4-4. Thin Section Petrography of Lavas Sampled During ALVIN Dive 3970 Near 9030'N East Pacific Rise
Thin Section Location Sample Locale Lava Type MgO % Plagioclase Characteristics
subhedral laths (<0.3 mm), acicular (0.4-0.7 mm), Carlsbad twins, glomerocrysts
3970-1" 9030.729' N 1.25 Km East ASCT lobate 7.37
+Cpx (0.3-0.75 mm), microlites (<0.1 mm)
3970-2 930729N 125 Km East ASCT pillow 748 subhedral laths (0.1-0.4mm), Carlsbad twins, microlites (<0.1 mm), intergrown with
3970-2 9030.729' N 1.25 Km East ASCT pillow 7.48
Cpx
3970-3 subhedral laths (0.1-0.5 mm), acicular (0.1-0.7 mm), Carlsbad twins, zoning,
3970-3 9030.763' N 1.25 Km East ASCT pillow tube 7.39
glomerocrysts (0.3 mm), microlites (<0.1 mm), intergrown with mmp
laths (0.2-0.7 mm), acicular (<1.4mm), Carlsbad twins, embayed, glomerocrysts (<3
3970-4* 9030.724' N Pillow Mound #1 pillow 7.38 intergrown with Cpx or
mm), intergrown with Cpx or 01
3970-5* 930.707'N Pillow Mound #1 pillow 7.23 laths (1 mm), acicular (<1.3 mm), Carlsbad twins, embayed, glomerocrysts (<5.8
3970-5* 9030.707' N Pillow Mound #1 pillow 7.23
mm), intergrown with Cpx or 01
laths (0.2-0.7 mm), acicular (<0.8 mm), Carlsbad twins, embayed, glomerocrysts
3970-6t 9030.603' N Pillow Mound #1 pillow 7.76
(<1.6 mm), microlites (<0.1 mm), intergrown with 01 and Cpx
laths (0.2-1.4 mm), acicular (<1.4 mm), subequant, Carlsbad twins, zoning,
3970-6b 930.603' N Pillow Mound #1 pillow 7.76 '
glomerocrysts (<3.2 mm), microlites (<0.1 mm), intergrown with 01 and Cpx
N F p 7 laths (0.2-0.7 mm), acicular (0.2-0.6 mm), Carlsbad twins, microlite network (<0.1
3970-7t 9030.337' N Fissure pillow 7.52
mm), intergrown with Cpx
subhedral laths (<0.3 mm), acicular (<0.3 mm), Carlsbad and Albite twins, microlite
3970-7b 930.337' N Fissure pillow 7.52
network (<0.1 mm), intergrown with Cpx
subhedral laths (0.2-0.7 mm), acicular/microlites (<1.1 mm), Carlsbad twins,
3970-9 9030.052' N Hornito pillow 7.64 intergrown with Cpx or 1
intergrown with Cpx or 01
39700* 9 N Pilw Mund # pil 77 sub-euhedral laths (0.2-1.4 mm), acicular (<1 mm), Carlsbad twins, embayed,
3970-10* 9030.020' N Pillow Mound #2 pillow 7.67 '
zoning, glomerocrysts (0.6-1.4 mm), intergrown with 01 and Cpx
laths (0.3-1.2 mm), acicular (<1.4 mm), Carlsbad twins, skeletal, zoning,
3970-11* 9029.753' N Pillow Mound #2 pillow 7.77
w wglomerocrysts (1.2-1.6 mm), intergrown with Cpx
Notes: Representative photomicrographs, which illustrate common mineralogies and textures, are available for the thinsections possessing an astrix after their
sample number (e.g., 3963-2*). Sample localities are based on in situ observations. Percentages of microphenocrysts, glass, groundmass, and vesicles were
first estimated visually. The point counts (1000 per slide) for several representative thinsections are provided in parenthesis preceding the estimate. Note that
when significant glass is present at the ends of slides, the point counting apparatus, which won't extend to measure the ends, underestimates the amount of glass
present and thus the other point count values for that slide are compromised. Thickness of the glassy outer rind of many lavas (in cm) was measured for both
the hand specimen (Glass H.S.) and the thin section (Glass T.S.).
Sample locality: ASCT = axial summit collapse trough, F.F.= flow front, P.M. = pillow mound; Crystal Shapes: an = anhedral, sub = subhedral; Mineralogy:
Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G-
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2% microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 = dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases
include iddingsite and palagonite.











Table 4-4. Continued


T]


Olivine Characteristics
sub-euhedral (<0.1-0.65 mm), intergrown with Plag,
embayed
euhedral (<<0.1 mm)


Cotectic


01 plag


T


anhedral mass (<0.1) intergrown with Plag

an-subhedral mass (<0.1 mm), intergrown with Plag


sub-euhedral (0.2-0.7 mm), embayed, hopper (<0.1)

euhedral (<0.4 mm), hopper, embayed, intergrown with Plag


01+ Plag + Cpx


01+ Plag


Notes: Representative photomicrographs, which illustrate common mineralogies and textures, are available for the thinsections possessing an astrix after their
sample number (e.g., 3963-2*). Sample localities are based on in situ observations. Percentages of microphenocrysts, glass, groundmass, and vesicles were first
estimated visually. The point counts (1000 per slide) for several representative thinsections are provided in parenthesis preceding the estimate. Note that when
significant glass is present at the ends of slides, the point counting apparatus, which won't extend to measure the ends, underestimates the amount of glass
present and thus the other point count values for that slide are compromised. Thickness of the glassy outer rind of many lavas (in cm) was measured for both the
hand specimen (Glass H.S.) and the thin section (Glass T.S.).
Sample locality: ASCT = axial summit collapse trough, F.F.= flow front, P.M. = pillow mound; Crystal Shapes: an = anhedral, sub = subhedral; Mineralogy:
Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G-
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2% microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 = dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases include
iddingsite and palagonite.


hin Section Clinopyroxene Characteristics
an-subhedral mass (<0.1 mm) intergrown with Plag, euhedral
(0.5 mm), glomerocryst with Plag (0.75 mm)
3970-2 an-subhedral mass (<0.1 mm) intergrown with Plag

3970-3 an-euhedral (<<0.1) intergrown with Plag

3970-4* an-subhedral (<0.4), embayed, intergrown with Plag and 01

3970-5* an-subhedral (<0.6 mm) intergrown with plag and 01

3970-6t an-subhedral mass (<0.3 mm) intergrown with Plag (and 01)

3970-6b an-subhedral mass (<0.1 mm), intergrown with Plag

3970-7t an-subhedral mass (<0.1 mm), intergrown with Plag

3970-7b an-subhedral mass (<0.1 mm), intergrown with Plag

3970-9 anhedral mass (<0.1 mm) intergrown with Plag


3970-10*

3970-11*


an-euhedral (<<0.1) intergrown with Plag
sub-euhedral (<0.3), embayed, hopper, intergrown with Plag Plag Cpx
and Cpx
subhedral (<0.4 mm), hopper, embayed, intergrown with
Plag and Cpx
hopper (<0.3 mm), intergrown with Plag (and Cpx)

subhedral (<0.15 mm) intergrown with Plag

N/A Plag + Cpx

N/A
sub-euhedral (0.7 mm), skeletal, hopper, intergrown with
Plag










Table 4-4. Continued
Thin Section % mPheno %Glass %G-mass %Vesicles Vesicle size Diseq Alteration Glass T.S. Glass H.S.
3970-1* <1 (1.1) 3 (1.2) 96 (97) <1 (0.7) 0.1 Y R/O 0.1 0.2

3970-2 <1 10 90 -1 <0.2 N R/O 0.3 0.2

3970-3 <1 1 98 <1 <0.1 Y R/O 0 0.5

3970-4* 20 (6.3) 10 (0) 69 (92.4) 1(1.3) 0.1-0.2 Y -0.3 0.4

3970-5* 10 (10.3) 0 (0) 87 (86.8) 3 (2.9) <1.4 Y R/O 0 0.5

3970-6t 5 0 95 <1 <0.2 Y R/O 0 0.4

3970-6b 1 0 98 <1 0.1-0.5 Y R/O 0

3970-7t 1.5 3 95 <1 <0.1 N R/O 0.1 0.1

3970-7b ~0 0 99 1 0.1 N R/O 0

3970-9 1.5 10 86 1.5 0.4-3 Y -0.3 0.5

3970-10* -10 8 81 1 <0.3 Y -0.3 0.8

3970-11* 5 8 86 1 0.1-1.4 Y -0.2 0.4

Notes: Representative photomicrographs, which illustrate common mineralogies and textures, are available for the thinsections possessing an astrix after their
sample number (e.g., 3963-2*). Sample localities are based on in situ observations. Percentages of microphenocrysts, glass, groundmass, and vesicles were first
estimated visually. The point counts (1000 per slide) for several representative thinsections are provided in parenthesis preceding the estimate. Note that when
significant glass is present at the ends of slides, the point counting apparatus, which won't extend to measure the ends, underestimates the amount of glass
present and thus the other point count values for that slide are compromised. Thickness of the glassy outer rind of many lavas (in cm) was measured for both the
hand specimen (Glass H.S.) and the thin section (Glass T.S.).
Sample locality: ASCT = axial summit collapse trough, F.F.= flow front, P.M. = pillow mound; Crystal Shapes: an = anhedral, sub = subhedral; Mineralogy:
Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G-
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2% microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 = dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases include
iddingsite and palagonite.










Table 4-4. Continued
Thin Section Crystallinity Phyric Vitrophyric Microlitic Spherulitic Variolitic Trachytic Fascicular Glomeroporphoritic
3970-1* quench no 0 0 1 1 0 3 0

3970-2 quench no 0 0 GRAD, 1 1 SUB, 1 3 0

3970-3 quench no 0 0 GRAD, 1 1 0 3 0

3970-4* quench phyric 0 0 2 2 0 0 2

3970-5* quench mod 0 0 3 1 0 0 3

3970-6t quench mod 0 2 0 0 SUB, 1 3 2

3970-6b quench sparse 0 3 0 0 0 1 0

3970-7t quench sparse 0 2 GRAD, 1 2 SUB, 1 GRAD, 2 1

3970-7b quench no 0 3 0 0 0 GRAD, 1 1

3970-9 quench sparse 0 1 0 3 SUB 0 1

3970-10* quench mod 0.5 0 GRAD, 2 2 0 GRAD, 2 2

3970-11* quench mod 0 0 2 2 0 GRAD, 1 -1

Notes: Representative photomicrographs, which illustrate common mineralogies and textures, are available for the thinsections possessing an astrix after their
sample number (e.g., 3963-2*). Sample localities are based on in situ observations. Percentages of microphenocrysts, glass, groundmass, and vesicles were
first estimated visually. The point counts (1000 per slide) for several representative thinsections are provided in parenthesis preceding the estimate. Note that
when significant glass is present at the ends of slides, the point counting apparatus, which won't extend to measure the ends, underestimates the amount of glass
present and thus the other point count values for that slide are compromised. Thickness of the glassy outer rind of many lavas (in cm) was measured for both
the hand specimen (Glass H.S.) and the thin section (Glass T.S.).
Sample locality: ASCT = axial summit collapse trough, F.F.= flow front, P.M. = pillow mound; Crystal Shapes: an = anhedral, sub = subhedral; Mineralogy:
Plag = plagioclase, Cpx = clinopyroxene, 01 = Olivine, mmp = mafic microphenocryst (either 01 or Cpx); Modal Abundances: mPheno = microphenocryst; G-
mass = quenched groundmass; Phyric: no = 0-1% microphenocrysts, sparse = 1-2% microphenocrysts, mod = 2-10% microphenocrysts, phyric = 10-100%
microphenocrysts; Textural Proportions: 0 = none, 1 = sparse, 2 = abundant, 3 = dominant; Diseq = disequilibrium; R/O = orangish-red accessory phases include
iddingsite and palagonite.















































Figure 4-1. Photomicrographs of common textures found in basalts collected near 930'
and 9051'N EPR. The common gradational sequence of textures starting at
the outer glassy rind and continuing into the more crystalline interior of the
slide is as follows: A) vitrophyric transitional into variolitic (3963-2), B)
bands of variolites can develop in sheet flows (3963-6), C) a gradation
between fasicular and D) less common spherulitic (3963-6), and finally E)
microlitic (3963-8), referred to as F) trachytic if individual microlites are
aligned (3974-4), G) which can be quite complex (3974-1). G) A
glomeroporphoritic texture (3970-5) is found throughout this sequence.
Textures pictured in F) and H) are shown in crossed polars while all others are
pictured in plane polarized light. The scale is the same for A), B), and E)-G).


























































TT


Figure 4-1. Continued





















C D











E F













Figure 4-2: Photomicrographs of olivine, clinopyroxene and plagioclase crystal
morphologies, all in crossed polars. A) Rare olivine (3963-6) and B)
clinopyroxene (3970-1) xenocrysts are out numbered by smaller crystals (<0.1
mm). C) Mostly indistinguishable, they are referred to as mafic
microphenocrysts unless an olivine exhibits a characteristic hopper form
(3974-8t). Plagioclase crystals constitute the majority of microphenocrysts
and appear as skeletal microlites forming parts of the groundmass, D) and E)
clusters of Carlsbad twinned laths known as glomerocrysts (3963-3 and 3963-
3), E) and acicular forms. F) Many plagioclase laths are zoned (3970-11).
The scale is the same for A) and C)-F).









are clustered together in crystal clots or glomerocrysts. Samples from east of the axis

commonly contain glomerocrysts (0.2-1.1 mm in diameter) except for three samples

(3963-2, 3963-7, and 3963-10) which each contain a xenocrystal clot as indicated by

larger sizes (1-2.5 mm across) and crystal faces with embayments and rounded edges; all

evidence for disequilibrium with the host melt (Figures 4-2 and 4-3). In contrast, only

two samples (3974-4 and 3974-10) recovered from the west side of the axis contained

glomerocrysts (1-1.4 mm). Further evidence of crystal-melt disequilibrium includes

prevalent skeletal plagioclase laths and microlites, as well as the presence of zoned

plagioclase from 3963-2 (Figure 4-2). In addition, two rare olivine xenocrysts were

found in samples 3963-6 (0.6 mm) and 3974-6 (0.55 mm) (Figure 4-2).

The majority of each section is quenched groundmass containing variable

amounts of (skeletal) plagioclase microlites intergrown with anhedral to subhedral

clinopyroxene and olivine; all commonly <0.1 mm. These equilibrium intergrowths are

arranged in variolitic, spherulitic, and fascicular masses (Figure 4-1) composed usually of

clinopyroxene nucleating around a plagioclase microlite. In over half the sections,

microlites are (sub-) aligned due to shear forces created as the lava cooled and flowed

over the seafloor. Tiny euhedral crystals of olivine and clinopyroxene also appear in the

groundmass but are indistinguishable from each other unless the characteristic hopper

shape of olivine is observed (Figure 4-2). Vesicles constitute 1-2% of most slides and are

0.2-0.4 mm in diameter.

Microbathymetry, side-scan sonar, and Alvin observations were used to identify

and sample three complete flow units each from dives 3963 and 3974. The lobate and

sheet flows comprising the main bodies of the lava units were also sampled in addition to









A B














C




























Figure 4-3. Photomicrographs of xenoglomerocrysts found in basalts collected during
dive 3963 near 9051'N EPR. Samples A) 3963-2, B) 3963-7, and C) 3963-10
show obvious disequilibrium features including embayments, as well as
rounded and corroded shapes. All pictures are shown in crossed polars. The
scale is the same for A) and B).









the pillowed flow fronts. Petrographic examination of the slides of lavas from different

parts of each flow unit reveal that the pillow basalts comprising the flow fronts are -1-

2.4 % (by volume) more crystalline than the rest of the flow (Table 4-5).

Channels, Dive 3968

The two thin sections, one from each channel investigated during dive 3968, are

both aphyric basalts (Table 4-3). The section from the first channel (3968-2T) is mainly

variolitic with rare (<1%) subhedral to euhedral plagioclase laths. The entirely hyaline

section from the northern channel (3968-6T) also contains rare plagioclase

microphenocrysts, including acicular (0.3-0.4 mm) and lath (0.1-0.8 mm) varieties,

clustered together into glomerocrysts 0.6 and 1.7 mm across. Each sample also contains

both plagioclases that exhibit Carlsbad twinning and zoning, as well as, subhedral to

euhedral olivine (<<1 mm).

Off-Axis Mounds, Dive 3970

Both pillow mounds are composed of moderately phyric pillow basalts containing

crystals of plagioclase, olivine, and clinopyroxene (Table 4-4). However the sheet and

lobate flows surrounding them are mainly aphyric basalts. Acicular and lath forms of

plagioclase (both 0.1-1.4 mm) exhibiting Carlsbad twins are common to all the pillow

samples. Microphenocrysts in rocks from the two pillow mounds constitute 5-10% of

each thin section and frequently form glomerocrysts composed of plagioclase intergrown

with both subhedral to euhedral olivine (<0.7 mm) and anhedral to subhedral

clinopyroxene (<0.6 mm) (Figure 4-3 and 4-4). These crystal clots are up to 5.8 mm

across and form a dominant glomeroporphyritic texture (Figure 4-1). Many olivine and

plagioclase crystals comprising the clots are embayed indicating disequilibrium with the














Table 4-5. Petrologic Characteristics of Flow Units From ALVIN Dives 3963 and 3974
Apparent change in Apparent change
Sample Sample Locale Lava Type wt.% MgO % mPheno arnt ca in Apparent c
% crvstallinitv in wt.% MgO
Dive 3963: Fin Front 1
3963-10 Flow Front closest to axis pillow 8.91 1.72
<1.72
3963-11 Flow Front closest to axis pillow 8.89 <1
Dive 3963: Fin,- Front 1
3963-7 Flow Front #1 pillow 8.73 <1
<1 0.27
3963-9 Between Flow Front #1 and 2 sheet 9.00
Dive 3963: Fi, i.' Front 2
3963-4 Flow Front #2 pillow 7.73 <1
3963-5 Flow Front #2 pillow 7.7 <1 0.30
3963-8 Under Flow Front #1 lobate 8.00 <1
Dive 3963: Flow Unit 3
3963-3 Flow Front #3 pillow 7.71 3.9
2.4 0.12
3963-6 Under Flow Front # 2 sheet 7.83 1.5
Dive 3974: Flow Unit 1
3974-10 Flow Front #1 pillow 8.33 1
3974-11t 700 m East of ASCT lobate 8.70 -0 <1 0.37
3974-1 lb 700 m East of ASCT lobate 8.70 -0
Dive 3974: Flow Unit 2
3974-5 Flow Front #2 pillow knob 7.65 ~1
3974-6 Flow Front #2 pillow 7.68 <1 <1 0.8
3974-9 Between Flow Front #2 and 3 lobate 8.45 1.3
Dive 3974: Flow Unit 3
3974-2 Flow Front #3 pillow 7.59 1.5
3974-3 Between Flow Front #2 and 3 hackly 8.05 <<1
3974-4 Between Flow Front #2 and 3 lobate 8.03 1.4
<1.5 0.52
3974-7 Between Flow Front #2 and 3 sheet 8.11
3974-8t Between Flow Front #2 and 3 pillow knob 7.91 <1
3974-8b Between Flow Front #2 and 3 pillow knob 7.91 -0


Notes: mPheno = Microphenocrysts.









A B














C
























Figure 4-4. Photomicrographs of basaltic samples retrieved during Alvin dive 3970 to a
series of off-axis mounds 1.23-1.58 km west of the AST near 9030'N EPR; all
in crossed polars. A) A rare olivine xenocryst intergrown with Carlsbad
twinned, acicular plagioclase was observed in a lobate lava west of the
northernmost pillow mound (3970-1). B) and C) Glomerocrysts composed of
embayed olivines (3970-10), acicular plagioclase, and clinopyroxene
microphenocrysts compose the glomeroporphoritic texture of pillow basalts
recovered from both pillow mounds (3970-4). The scale is the same for both
(A) and (B).









host melt (Figure 4-4). In addition, zoned plagioclases were observed in half the mound

samples (Figure 4-3).

Several glomerocrysts (0.3-0.75 mm across) were observed in rocks from the

aphyric lobate and pillow lavas located to the west of the northern most mound, and only

sample 3970-1 contained euhedral olivine (up to 0.65 mm) and clinopyroxene (0.5mm)

xenocrysts that are as large as the crystals in the pillow mounds (Figure 4-2). Another

rare olivine xenocryst (0.7 mm) was found in sample 3970-9; a pillow basalt from of a

hornito just north of the second, more southern pillow mound.

The quenched groundmass for these thin sections has a similar composition to that

of the rocks collected during dives to the flow units (3963 and 3974). Plagioclase

microlites act as nucleation sites for clinopyroxenes, which then forms a dominantly

gradational mosaic of variolitic, spherulitic, and/or fascicular masses (Figure 4-1)

interspersed with variable amounts of subhedral to euhedral olivine microphenocrysts (<1

mm). Sub-alignment of microlites was noted in several slides. Vesicles constitute -1%

of most slides and are commonly 0.1-0.4 mm in diameter.

Phase Chemistry

The compositions of olivine, plagioclase, and clinopyroxene were measured in

two representative thin sections: 3970-9 and 3974-11t (olivine only) (Tables 4-6 to 4-8).

The aphyric 3974-1 it contained only minute crystals of groundmass, non-zoned olivines

with Fo87 (one Fo86) (Figure 4-5 and 4-6; Table 4-6). On the other hand, the sparsely

phyric 3970-9 contained olivine (Fo85-86), plagioclase (An66.3-73.1), and clinopyroxene

(Mg# 80-87) crystals, often intergrown in large crystal clots (Figure 4-5; Tables 4-6 to 4-

8). The majority of olivines, whether apart of the groundmass or large phenocrysts, were

Fo86 (5 of 6 crystals measured and 19 of 25 crystals measured, respectively) while












Table 4-6. Olivine Chemical Compostions by Electron Microprobe for 3970-9 and 3974-1 1t


Sample SiO2 TiO2 A1203 FeO MnO MgO CaO


3974-11t 40.05 0.07
3974-1 t 39.25 0.03
3974-11t 39.23 0.06
3974-1 t 39.29 0.01
3974-1 t 39.00 0.05
3974-1 t 39.96 0.00
3970-9 39.46 0.05
3970-9 39.50 0.04
3970-9 40.22 0.01
3970-9 40.04 0.00
3970-9 40.08 0.02
3970-9 39.66 0.03
3970-9 40.05 0.00
3970-9 39.23 0.00
3970-9 38.73 0.00
3970-9 39.97 0.04
3970-9 40.32 0.06
3970-9 39.19 0.01
3970-9 39.25 0.07
3970-9 39.67 0.01
3970-9 40.42 0.01
3970-9 38.80 0.06
3970-9 39.82 0.04


0.05 12.71 0.19 46.19 0.41
0.11 12.54 0.20 46.32 0.33
0.18 12.65 0.17 45.81 0.35
0.08 12.38 0.19 45.48 0.34
0.06 13.22 0.25 44.68 0.36
0.04 12.29 0.18 46.00 0.35
0.09 13.90 0.26 44.60 0.45
0.05 13.88 0.23 44.68 0.44
0.05 13.81 0.21 45.71 0.35
0.02 13.87 0.19 45.70 0.32
0.04 14.26 0.19 45.84 0.31
0.12 13.24 0.26 44.90 0.43
0.02 13.11 0.23 44.89 0.35
0.05 13.19 0.25 44.70 0.32
0.03 13.05 0.23 44.04 0.34
0.02 13.41 0.19 45.46 0.37
0.03 13.45 0.25 45.64 0.39
0.04 12.89 0.24 44.01 0.44
0.04 13.02 0.19 43.84 0.41
0.05 13.33 0.22 44.14 0.36
0.08 14.02 0.24 44.17 0.42
0.07 13.37 0.20 43.65 0.41
0.06 13.14 0.23 44.77 0.36


Na20
0.01
0.00
0.02
0.02
0.01
0.03
0.00
0.01
0.00
0.00
0.01
0.00
0.00
0.00
0.02
0.00
0.02
0.02
0.00
0.01
0.02
0.01
0.00


P205 Cr203 Total Fo Comment


K20
0.00
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.02
0.01
0.02
0.00
0.00
0.01
0.00
0.00
0.00
0.01
0.01
0.00
0.00
0.01
0.01


0.10
0.09
0.00
0.10
0.07
0.00
0.10
0.08
0.00
0.09
0.09
0.02
0.05
0.09
0.10
0.10
0.07
0.07
0.14
0.10
0.03
0.07
0.00


0.04
0.08
0.10
0.08
0.10
0.10
0.02
0.07
0.02
0.03
0.06
0.01
0.02
0.06
0.05
0.06
0.04
0.01
0.05
0.08
0.02
0.01
0.03


Notes: Fo = ((Mg/(Mg + Fe))*100); 01 = Olivine; Plag = Plagioclase; Cpx


Clinopyroxene.


99.82 86.62 Groundmass
98.97 86.81 Groundmass
98.56 86.58 Groundmass
97.99 86.74 Groundmass
97.80 85.76 Groundmass
98.95 86.96 Groundmass
98.93 85.12 Olivine A core
98.99 85.15 Olivine A rim
100.39 85.50 Ol-Plag clot
100.26 85.44 01-Plag clot2
100.92 85.14 01-Plag clot3
98.67 85.80 Olivine
98.72 85.92 Olivine
97.88 85.79 Olivine B core
96.59 85.74 Olivine B rim
99.60 85.79 Olivine C core
100.27 85.81 Olivine C rim
96.91 85.88 Olivine in clot
97.01 85.71 Olivine in clot
97.97 85.50 Olivine in clot2
99.43 84.88 Olivine in clot2
96.67 85.33 Olivine in clot3
98.45 85.86 01 with Cpx

















Table 4-6. Continued


Sample SiO2 TiO2 A1203 FeO MnO MgO CaO Na20 K20 P205 Cr203 Total Fo Comment
3970-9 39.74 0.00 0.04 13.14 0.25 44.66 0.39 0.00 0.00 0.04 0.05 98.32 85.82 01 with Cpx
3970-9 39.72 0.02 0.04 12.92 0.24 44.80 0.40 0.00 0.00 0.03 0.09 98.26 86.07 01 with Cpx 2
3970-9 39.35 0.01 0.03 13.26 0.17 44.83 0.34 0.02 0.01 0.08 0.03 98.12 85.76 01-Plag clot 4
3970-9 39.75 0.03 0.02 13.12 0.23 44.69 0.35 0.01 0.01 0.09 0.06 98.38 85.85 01-Plag clot 4
3970-9 39.84 0.03 0.01 12.76 0.23 44.74 0.32 0.00 0.00 0.05 0.07 98.04 86.20 Ol-Plag clot 4
3970-9 39.74 0.00 0.03 13.02 0.19 45.09 0.34 0.01 0.00 0.02 0.07 98.50 86.05 01-Plag clot 4
3970-9 39.32 0.00 0.02 12.95 0.27 44.76 0.32 0.00 0.01 0.02 0.01 97.68 86.03 Ol-Plag clot 4
3970-9 39.86 0.02 0.03 13.01 0.22 44.86 0.33 0.00 0.00 0.03 0.01 98.37 86.00 Ol-Plag clot 4
3970-9 39.25 0.02 0.03 13.08 0.25 44.12 0.31 0.00 0.01 0.06 0.03 97.15 85.73 Ol-Plag clot 5
3970-9 39.50 0.04 0.03 13.15 0.24 44.90 0.32 0.00 0.00 0.05 0.04 98.27 85.89 Ol-Plag clot 5
3970-9 39.95 0.03 0.02 13.12 0.26 45.21 0.34 0.00 0.00 0.01 0.05 98.97 86.00 Ol-Plag clot 5
3970-9 39.32 0.00 0.03 12.92 0.22 44.42 0.34 0.02 0.00 0.04 0.04 97.35 85.96 O1 in glass
3970-9 39.64 0.01 0.04 13.27 0.26 44.83 0.38 0.02 0.01 0.01 0.06 98.52 85.75 Ol in glass
3970-9 40.16 0.00 0.02 13.65 0.21 45.33 0.36 0.01 0.01 0.00 0.04 99.79 85.54 Ol in glass


Notes: Fo = ((Mg/(Mg + Fe))*100); 01 = Olivine; Plag = Plagioclase; Cpx


Clinopyroxene.










Table 4-7. Plagioclase Chemical Compositions by Electron Microprobe for 3970-9


SiO2 TiO2 A1203 FeO
50.27 0.04 29.00 0.67
50.65 0.06 29.45 0.64
51.27 0.10 28.94 0.85
51.45 0.08 29.76 0.60
51.02 0.07 30.20 0.54
51.96 0.07 30.31 0.53
51.83 0.04 30.26 0.60
50.57 0.05 30.31 0.61
49.60 0.02 29.42 0.50
49.02 0.01 29.97 0.52
49.46 0.06 29.52 0.55
50.64 0.03 28.94 0.73
48.89 0.07 28.39 0.60
49.45 0.07 28.09 0.62
50.46 0.07 28.37 0.66
50.78 0.08 28.07 0.73
50.40 0.05 28.30 0.70
50.42 0.10 28.35 0.70
50.57 0.06 28.38 0.60
49.89 0.06 28.89 0.57
50.57 0.07 29.08 0.67
48.79 0.06 29.60 0.57
49.06 0.08 29.30 0.67
50.31 0.09 28.63 0.67
50.02 0.08 28.81 0.58
49.09 0.03 28.89 0.67
50.00 0.04 28.38 0.49
49.85 0.06 28.36 0.52
50.36 0.05 28.51 0.68
50.12 0.03 28.85 0.58
48.80 0.04 29.09 0.60


MnO
0.00
0.01
0.00
0.02
0.00
0.03
0.00
0.00
0.02
0.07
0.00
0.00
0.01
0.03
0.00
0.01
0.00
0.01
0.00
0.00
0.01
0.04
0.00
0.04
0.02
0.01
0.00
0.01
0.00
0.02
0.03


MgO CaO Na20
0.27 13.84 3.41
0.25 13.91 3.37
0.30 13.27 3.66
0.30 13.85 3.53
0.24 14.24 3.34
0.26 13.77 3.65
0.26 13.87 3.64
0.24 14.50 3.35
0.26 14.22 3.21
0.21 14.66 2.98
0.22 14.21 3.23
0.28 13.65 3.56
0.25 14.10 3.32
0.27 13.68 3.42
0.33 13.62 3.60
0.36 13.61 3.69
0.30 13.60 3.55
0.27 13.64 3.59
0.32 13.25 3.71
0.24 14.04 3.42
0.27 13.96 3.58
0.20 14.67 2.99
0.21 14.59 3.03
0.28 13.87 3.48
0.23 13.83 3.54
0.24 13.96 3.31
0.24 13.83 3.48
0.30 13.98 3.45
0.27 13.79 3.48
0.28 13.93 3.42
0.24 14.21 3.19


K20
0.02
0.02
0.02
0.02
0.02
0.01
0.02
0.03
0.02
0.02
0.03
0.03
0.03
0.02
0.03
0.03
0.02
0.03
0.03
0.02
0.03
0.03
0.02
0.02
0.02
0.03
0.01
0.02
0.02
0.03
0.03


P205
0.00
0.04
0.02
0.02
0.02
0.00
0.01
0.00
0.02
0.02
0.01
0.02
0.01
0.04
0.06
0.00
0.00
0.00
0.00
0.06
0.01
0.00
0.00
0.01
0.04
0.03
0.01
0.02
0.02
0.01
0.00


Cr203
0.02
0.00
0.01
0.01
0.00
0.03
0.00
0.00
0.02
0.04
0.00
0.00
0.00
0.00
0.00
0.02
0.00
0.00
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.01
0.00
0.02
0.00
0.01
0.00


Total An Comment
97.54 69.05 Plagioclase A rim
98.40 69.44 Plagioclase A middle
98.43 66.64 Plagioclase A core
99.63 68.31 Plagioclase B core
99.69 70.08 Plagioclase B rim
100.61 67.51 Plagioclase C core
100.53 67.73 Plagioclase C middle
99.67 70.42 Plagioclase C rim
97.31 70.93 Plagioclase D core
97.50 73.06 Plagioclase D middle
97.30 70.71 Plagioclase D rim
97.87 67.85 Plagioclase core
95.66 69.98 Plagioclase E core
95.67 68.80 Plagioclase E rim
97.18 67.55 Plagioclase in clot
97.38 66.96 Plagioclase in clot
96.92 67.84 Plagioclase in clot 2
97.09 67.61 Plagioclase in clot 3
96.91 66.27 Plagioclase in clot 4
97.18 69.31 Plagioclase in clot 4
98.24 68.16 Plag with Cpx
96.95 72.95 Ol-Plag clot
96.96 72.64 Ol-Plag clot
97.38 68.69 Ol-Plag clot
97.17 68.27 Ol-Plag clot
96.26 69.87 Plag in 01
96.46 68.70 Plag in glass
96.58 69.04 Plag in glass
97.18 68.56 Plag in glass
97.27 69.12 Plag in glass
96.23 70.99 Plaa in alass


Notes: An = ((Ca/(Ca + Na))* 100); Cpx = Clinopyroxene; Plag = Plagioclase; 01 = Olivine.


LI LI










Table 4-8. Clinopyroxene Chemical Compositions by Electron Microprobe for 3970-9


SiO2 TiO2 A1203 FeO MnO


48.43 1.18
49.81 0.51
49.40 0.86
49.30 0.92
49.21 0.63
49.07 0.55
49.31 0.57
49.55 0.95
49.58 1.14
49.05 0.98
49.85 1.15
49.66 0.97
49.70 0.94
49.76 0.76
50.17 0.71
50.18 0.61
50.76 0.55
51.06 0.67
47.28 1.20
49.74 0.83
50.25 0.84
51.72 0.53
52.67 0.35
52.08 0.43
51.99 0.64
51.69 0.73


5.69
3.37
4.02
4.40
3.56
3.65
3.48
4.18
4.21
4.68
4.82
4.60
4.56
4.28
4.06
3.91
3.49
3.42
5.79
4.12
3.92
2.67
1.85
2.17
3.55
3.62


6.01
4.63
5.30
5.57
4.87
4.86
4.80
5.43
5.27
5.47
7.05
5.52
5.32
5.13
5.11
4.64
4.56
4.95
5.69
4.95
6.00
5.14
5.68
5.63
5.50
5.39


0.18
0.13
0.16
0.13
0.14
0.12
0.14
0.15
0.11
0.13
0.17
0.17
0.17
0.18
0.15
0.14
0.12
0.15
0.13
0.12
0.19
0.19
0.19
0.18
0.16
0.18


MgO CaO Na20
15.58 18.49 0.46
16.60 20.27 0.29
16.76 19.41 0.29
17.29 18.06 0.28
16.88 19.32 0.27
16.86 19.06 0.24
16.49 19.60 0.27
16.43 19.58 0.26
15.93 20.38 0.30
16.11 19.09 0.27
16.07 17.55 0.41
16.64 18.72 0.27
16.47 19.33 0.30
16.70 18.99 0.29
16.75 19.48 0.27
16.63 19.82 0.27
16.60 20.21 0.29
17.00 19.81 0.27
15.17 19.10 0.35
16.17 20.58 0.28
17.81 17.42 0.25
18.05 19.01 0.25
19.84 16.52 0.18
19.20 17.47 0.20
17.59 19.39 0.23
17.31 19.42 0.26


P205 Cr203 Total Mg # Comment


K20
0.01
0.00
0.00
0.01
0.00
0.00
0.00
0.01
0.00
0.00
0.01
0.00
0.00
0.00
0.00
0.00
0.00
0.01
0.00
0.01
0.00
0.00
0.01
0.00
0.00
0.01


0.02
0.05
0.00
0.04
0.00
0.02
0.01
0.00
0.01
0.04
0.06
0.01
0.05
0.01
0.00
0.02
0.03
0.00
0.01
0.00
0.01
0.01
0.00
0.01
0.02
0.00


Notes: Mg# = ((Mg/(Mg + Fe))*100); Cpx = Clinopyroxene; Plag = Plagioclase.


0.36 96.40 82.19 Cpx with Plag
1.04 96.70 86.45 Cpx 2 with Plag
0.46 96.66 84.93 Cpx 3 with Plag
0.40 96.40 84.69 Cpx 4 with Plag
0.93 95.80 86.06 Cpx with Plag 2
1.12 95.54 86.08 Cpx 2 with Plag 2
0.78 95.45 85.96 Cpx 3 with Plag 2
0.51 97.04 84.35 Cpx in clot
0.58 97.50 84.35 Cpx 2 in clot
0.57 96.40 84.00 Cpx 3 in clot
0.21 97.33 80.25 Cpx 4 in clot
0.71 97.26 84.31 Cpx A core
0.78 97.62 84.65 Cpx A core to rim
1.00 97.10 85.30 Cpx A core to rim
0.97 97.68 85.38 Cpx A core to rim
1.17 97.40 86.46 Cpx A core to rim
1.13 97.74 86.64 Cpx A core to rim
0.69 98.02 85.97 Cpx A core to rim
0.40 95.11 82.60 Cpx A rim
1.17 97.96 85.33 Cpx in glass
0.81 97.50 84.09 Cpx in glass
0.80 98.38 86.21 Cpx in glass
0.57 97.86 86.17 Cpx in glass
0.52 97.89 85.86 Cpx in glass
0.83 99.89 85.07 Cpx in glass
0.94 99.56 85.12 Cpx in glass






54



(A) Olivine Compositions from 3974-11


Si Groundmass'Cores
. I Crystal( li Rn ....


NlMg (mol.%)


(B) Olivine Compositions of 3970-9


85 86
NMg (mol.%)

Figure 4-5. Olivine Compositions. Compositions of olivine in lava samples A) 3974-11
and B) 3970-9. Compositions of C) plagioclase and D) clinopyroxene in lava
sample 3970-9.


. . . . . . . . . . . .











(C) Plagioclase Compositions from 3970-9
10I I
I Groundmass/Cores
O Crystal Clots/Rims
8


6


4


2


0
66 67 68 69 70 71 72 73
An# (mol.%)


(D) Clinopyroxene Compositions for 3970-9
10





6


o 11 -,-1 1,1 ,11 I1
80 81 82 83 84 85 86 87
NMg (mol.%)


Figure 4-5. Continued.







56




(A) Olivine


(B) Plagioclase


72 F


70 -

69 -

68 -

67 -


Core


d/
O-/


Middle


SPlagioclase A
Plagioclase B
Plagioclase C
x Plagioclase D
+Plagioclase E


Rim


Figure 4-6. Zoning in olivine, plagioclase, and clinopyroxene. A) There is no obvious
zoning in olivine (error is 01.04 relative %). B) Various types of zoning are
evident in plagioclase (error is 0.76 relative %) while C) a clinopyroxene
exhibits reverse zoning (error is 1.04 relative).


85.9


85.8 -

85.7

85.6


85.5

85.4

85.3


85.2 F


85.1


-- -------








Olivine A
Olivine B
Olivine C


--I I I


Rim


Core














(C) Clinopyroxene
87
Clinopyroxene A
86.5

86

5 85.5

; 85

84.5

84
Core Rim



Figure 4-6. Continued

the remaining crystals were Fos5. A larger compositional range was observed for both

plagioclase and clinopyroxene, especially in the clot-forming crystals. The groundmass

plagioclase, including the cores of larger crystals, were An69-71, with most at An69 (7 of

11 measurements) while the clot-forming plagioclases, including the rims of larger

crystals, were An66-73 with most at An68 (9 of 20 measurements). Five plagioclase

crystals (lettered A to E) exhibited various types of zoning including one with weak

normal zoning (Plagioclase E: core An70.o, rim An68.8), one with oscillatory zoning

(Plagioclase D: core An70.9, center An73.1, rim An70.7) and three with reversed zoning

(Plagioclase A: core An66.6, rim An69.1; Plagioclase B: core An68.3, rim An7o.1; Plagioclase

C: core An67.5, rim An70.4) (Figure 4-6). Most clinopyroxenes, whether forming the

groundmass or apart of crystal clots, were Mg# 84-86, however the compositions of the









clot-forming crystals varied between Mg# 80 and 87. One clinopyroxene crystal

displayed reversed zoning with a core Mg# 84 and rim Mg# 86 (Figure 4-6).

Summary

Overall the petrographic observations indicate only minimal amounts of

crystallization (< 4%) in the flow channels and units but there is some indication that the

greatest amount of crystallization has occurred in the distal parts of the flow in the

pillowed flow fronts. In contrast the pillow mounds are comprised of lavas with

significantly greater crystallinity (up to 10%). Mineral textures and compositions suggest

some of the larger crystals and clots are xenocrystal. There compositions are not

extremely out of equilibrium with their hosts but great enough to indicate they

crystallized from a magma with slightly different composition before being entrained in

their present host lava.














CHAPTER 5
MAJOR AND TRACE ELEMENT CHEMISTRY

Major Element Data

The basaltic lavas analyzed in this study were recovered up to several km from

the ASCT in one of three main areas along the 9-10 N segment of the EPR: 9o50' N,

9o43' N, and 928'-31' N (Figure 1-2). Glass chips from 107 of the sampled sheet,

lobate, and pillow lavas were analyzed for major element concentrations (Table 5-1).

Analytical uncertainty is discussed in Appendix B and the 2-sigma errors are presented in

the Methods section. Overall, the MgO content (in wt.%) of theses lavas is relatively

limited and ranges between 7.1 and 9.0%. Major element variations of the basalts

analyzed in this study mimic the apparent fractional crystallization trends documented in

other basaltic suites from the 9-10 N segment [e.g., Si.ii et al., 2001] (Figure 5-1). As

MgO decreases, TiO2, FeO, Na20, P205, MnO, SiO2, and K20 contents increase whereas

A1203 and CaO decrease.

The most primitive basalts were recovered from near the axis at 950' N (3963-9,

10, 11) and have MgO contents between 8.9 and 9.0 wt.%, while the most evolved

samples were found far to either side of the axis at 928'-30' N. Two hackly sheet flows

(3975-2, 2i) and one lobate flow (3975-li) west of the ASCT have MgO contents of 7.0-

7.1 wt.%. To the east of the ASCT, one pillow flow (3970-5) and one hackly flow (3970-

12) contain 7.2 wt.% MgO. All samples are low-K tholeiitic basalts that, as a group,

exhibit the classic iron-enrichment trend of tholeiitic suites and have less than 0.27 wt.%

K20. However, a few samples (3965-7, 3966-3, 3966-3s, 3966-5a, 3966-5b, 3966-5b-2)











Table 5-1: Major Element Chemistry of Lavas Collected From 90-100N East Pacific Rise During ATI 1-7 Cruise

Sample Deg. Lat Deg. Long. Depth Morph. SiO2b TiO2 A1203 FeO MnO MgO CaO Na20 K20 P205 Total Mg# K/Ti


3963-1
3963-3
3963-4
3963-5
3963-6
3963-7
3963-8
3963-9
3963-10
3963-11
3965-la
3965-lb
3965-2
3965-3
3965-4
3965-5a
3965-5b
3965-6a
3965-6b
3965-7
3965-8


9 50.653' 104 16.360' 2562
9 50.610' 104 16.458' 2557
9 50.502' 104 16.615' 2542
9 50.502' 104 16.615' 2543
9 50.501' 104 16.589' 2544
9 50.450' 104 16.746' 2532
9 50.452' 104 16.731' 2534
9 50.288' 104 16.848' 2524
9 50.294' 104 16.991' 2517
9 50.305' 104 16.989' 2518
949.681' 104 16.539' 2546
9 49.686' 104 16.536' 2546
9 49.685' 104 16.536' 2546
949.667' 104 16.541' 2546
9 49.592' 104 16.460' 2549
9 49.460' 104 16.457' 2557
9 49.460' 104 16.457' 2557
949.157' 104 16.321' 2566
949.157' 104 16.320' 2567
949.129' 104 16.467' 2561
9 48.975' 104 16.369' 2568


Sh
Pw
Pw
Pw
Sh
Pw
Lob
Sh
Pw
Pw
Sh
Sh
Pw
Lob
Pw
Sh
Sh
Hack
Hack
Pw
Pw


50.4
50.3
50.4
50.5
50.4
49.8
50.2
49.6
49.5
49.7
49.7
49.8
49.8
50.4
49.8
50.2
50.2
50.6
50.7
50.6
50.0


0.20
0.20
0.20
0.20
0.20
0.17
0.20
0.17
0.17
0.18
0.17
0.18
0.17
0.21
0.18
0.19
0.19
0.19
0.19
0.19
0.18


7.9
7.7
7.7
7.7
7.8
8.7
8.0
9.0
8.9
8.9
8.8
8.7
8.7
7.7
8.6
8.2
8.2
7.8
7.8
7.5
8.3


12.2 2.75 0.10 0.12
12.0 2.78 0.14 0.16
12.1 2.78 0.15 0.15
12.1 2.78 0.15 0.14
12.1 2.71 0.13 0.14
12.4 2.54 0.08 0.08
11.9 2.76 0.13 0.15
12.4 2.51 0.08 0.10
12.3 2.51 0.10 0.10
12.2 2.51 0.10 0.11
12.3 2.63 0.10 0.11
12.3 2.61 0.10 0.10
12.3 2.58 0.10 0.12
11.9 2.73 0.11 0.15
12.3 2.60 0.10 0.11
12.3 2.69 0.09 0.13
12.3 2.65 0.09 0.13
12.1 2.72 0.14 0.15
12.1 2.75 0.10 0.13
11.8 2.82 0.27 0.20
12.2 2.57 0.12 0.15


Notes: Each sample number is composed of the ALVIN dive followed by the station where the sample was collected during cruise AT11-7 (e.g., 3966-1).
Additional notation describes multiple samples taken from a site (e.g., 3965-la, 3965-1b), several sources of glass from a single sample (e.g., 3966-3, 3966-3s)
including distinct interior and exterior glass (e.g., 3970-9o, 3970-9i). The "depth" of each station is in reference to meters below sea-level while the "distance"
refers to the location of station in meters from the axial summit trough. Natural glasses were analyzed for major element concentrations (presented as weight
% oxides) using an electron microprobe. Lat = Latitude; Long = Longitude. Dist = distance from axis; Morph = lava morphology, Sh = sheet flow, Pw =
pillow flow, Lob = lobate flow, Hack = hackly flow; SiO2b = corrected SiO2; Mg# = ((Mg/(Mg+Fe))*100); K/Ti = ((K20/TiO2)*100).


99.91 60.68 6.93
100.03 59.57 8.93
99.97 60.37 9.81
99.93 60.14 9.62
99.93 60.57 8.69
99.87 65.62 7.34
99.91 60.84 8.27
99.93 66.41 7.18
99.90 66.68 8.11
99.78 66.80 8.38
100.04 65.85 8.27
99.92 65.73 8.05
100.02 65.46 8.12
99.94 59.05 7.02
99.93 64.99 7.89
99.96 62.18 6.72
99.95 61.98 6.71
99.86 60.70 9.56
99.83 60.44 6.81
99.67 59.97 16.26
99.87 63.44 8.78











Table 5-1. Continued

Sample Deg. Lat Deg. Long. Depth Morph. SiO2b TiO2 A1203 FeO MnO MgO


3966-1
3966-2
3966-3
3966-3s
3966-4
3966-5a
3966-5b
3966-5b-2
3967-2
3967-3
3967-4a
3967-4b
3967-5a
3967-5b
3967-6
3967-6s
3968-1
3968-2
3968-3
3968-4
3968-4si


943.516' 104 15.641' 2553
9 43.507' 104 15.617' 2554
9 43.500' 104 15.633' 2553
9 43.500' 104 15.633' 2553
943.516' 104 15.641' 2553
9 43.469' 104 15.690' 2550
9 43.469' 104 15.690' 2550
9 43.469' 104 15.690' 2550
9 43.700' 104 16.428' 2539
9 43.704' 104 16.374' 2539
9 43.700' 104 16.379' 2539
9 43.700' 104 16.379' 2539
943.671' 104 16.002' 2542
943.671' 104 16.002' 2542
943.661' 104 16.011' 2545
943.661' 104 16.011' 2545
9 28.774' 104 14.599' 2564
9 28.770' 104 14.567' 2564
928.801' 104 14.582' 2566
9 28.794' 104 14.586' 2566
9 28.794' 104 14.586' 2566


Lob
Sh
Hack
Hack
Pw
Lob
Lob
Lob
Pw
Lob
Lob
Lob
Sh
Lob
Hack
Hack
Lob
Lob
Sh
Sh
Sh


50.4
50.3
50.6
50.6
50.4
50.4
50.3
50.3
50.3
50.2
50.2
50.1
50.3
50.3
50.4
50.4
51.0
50.8
50.7
50.8
50.9


9.9 0.19
9.9 0.20
10.0 0.19
10.0 0.19
9.9 0.20
9.9 0.19
10.0 0.20
10.0 0.20
10.0 0.19
10.1 0.18
10.2 0.20
10.2 0.19
11.0 0.21
11.0 0.21
10.9 0.22
10.2 0.18
10.7 0.20
10.7 0.20
10.7 0.22
10.7 0.20
10.7 0.21


8.0
8.0
7.4
7.5
7.8
7.7
7.8
7.8
7.9
8.0
7.9
8.0
7.5
7.6
7.6
7.9
7.5
7.6
7.5
7.5
7.5


CaO Na20 K20 P205
12.0 2.68 0.15 0.15
12.1 2.72 0.16 0.15
11.8 2.84 0.26 0.18
11.7 2.87 0.27 0.22
12.1 2.76 0.15 0.16
11.9 2.80 0.22 0.18
12.0 2.77 0.21 0.16
11.9 2.83 0.21 0.18
11.9 2.66 0.12 0.13
12.0 2.72 0.12 0.13
11.9 2.73 0.12 0.16
11.9 2.76 0.12 0.14
11.6 2.82 0.12 0.18
11.6 2.81 0.13 0.17
11.6 2.78 0.12 0.16
11.8 2.71 0.11 0.15
11.6 2.67 0.13 0.15
11.7 2.70 0.13 0.15
11.7 2.73 0.12 0.16
11.7 2.71 0.13 0.17
11.6 2.72 0.12 0.17


Notes: Each sample number is composed of the ALVIN dive followed by the station where the sample was collected during cruise AT11-7 (e.g., 3966-1).
Additional notation describes multiple samples taken from a site (e.g., 3965-la, 3965-1b), several sources of glass from a single sample (e.g., 3966-3, 3966-3s)
including distinct interior and exterior glass (e.g., 3970-90, 3970-9i). The "depth" of each station is in reference to meters below sea-level while the "distance"
refers to the location of station in meters from the axial summit trough. Natural glasses were analyzed for major element concentrations (presented as weight
% oxides) using an electron microprobe. Lat = Latitude; Long = Longitude. Dist = distance from axis; Morph = lava morphology, Sh = sheet flow, Pw =
pillow flow, Lob = lobate flow, Hack = hackly flow; SiO2b = corrected SiO2; Mg# = ((Mg/(Mg+Fe))*100); K/Ti = ((K20/TiO2)*100).


Total Mg# K/Ti
99.78 61.59 10.22
99.84 61.57 10.67
99.72 59.35 15.43
99.72 59.92 16.13
99.80 61.04 10.16
99.78 60.68 13.74
99.82 60.61 13.27
99.84 60.62 13.58
99.76 61.03 7.89
99.84 60.91 7.93
99.85 60.67 7.81
99.95 60.85 7.66
99.90 57.59 6.97
99.87 57.72 7.35
99.85 57.89 7.13
99.64 60.73 7.38
100.01 58.27 7.41
99.99 58.39 7.71
99.99 58.30 7.38
99.97 58.12 7.72
99.87 58.24 7.35











Table 5-1. Continued

Sample Deg. Lat Deg. Long. Depth Morph. SiO2b TiO2 A1203 FeO MnO MgO CaO Na20 K20 P205 Total Mg# K/Ti


3968-4so
3968-5
3968-6
3968-7
3968-8ai
3968-8ao
3968-8b
3968-8c
3968-9
3968-10
3970-1
3970-2
3970-3
3970-4
3970-5
3970-6
3970-7
3970-7-2
3970-8a
3970-8b
3970-9o


9 28.794' 104 14.586' 2566
9 28.792' 104 14.585' 2566
9 28.946' 104 14.637' 2567
9 28.948' 104 14.640' 2567
9 28.925' 104 14.816' 2574
9 28.925' 104 14.816' 2574
9 28.924' 104 14.816' 2574
9 28.924' 104 14.816' 2574
9 28.958' 104 14.778' 2570
929.117' 104 14.524' 2565
9 30.729' 104 13.957' 2612
9 30.729' 104 13.963' 2610
9 30.763' 104 13.977' 2607
9 30.724' 104 13.881' 2616
9 30.707' 104 13.838' 2597
9 30.603' 104 13.782' 2644
9 30.337' 104 13.842' 2651
9 30.337' 104 13.842' 2651
930.181' 104 13.687' 2667
930.181' 104 13.687' 2667
9 30.052' 104 13.627' 2662


Sh
Hack
Sh
Hack
Lob
Lob
Lob
Lob
Sh
Sh
lob
Pw
Pw
Pw
Pw
Pw
Pw
Pw
Hack
Hack
Pw


51.1
51.0
51.0
51.0
50.8
50.8
50.8
50.9
50.8
50.8
50.9
50.8
50.8
50.6
50.7
50.8
50.7
50.9
50.7
50.6
50.8


10.7 0.21
10.7 0.20
10.7 0.21
10.7 0.22
10.8 0.22
10.7 0.22
10.7 0.22
10.7 0.19
10.7 0.20
10.7 0.21
10.9 0.19
10.8 0.21
10.9 0.22
11.1 0.22
11.1 0.22
10.1 0.20
10.7 0.21
10.7 0.20
10.7 0.20
10.9 0.23
10.2 0.20


7.5
7.5
7.5
7.5
7.4
7.5
7.5
7.4
7.5
7.5
7.4
7.5
7.4
7.4
7.2
7.8
7.5
7.5
7.6
7.5
7.6


11.6 2.55 0.13
11.6 2.57 0.13
11.6 2.56 0.13
11.6 2.58 0.13
11.7 2.72 0.13
11.7 2.73 0.13
11.6 2.74 0.13
11.7 2.72 0.13
11.6 2.73 0.12
11.7 2.71 0.13
11.6 2.88 0.11
11.7 2.83 0.11
11.7 2.87 0.11
11.7 2.82 0.13
11.7 2.85 0.14
12.1 2.79 0.10
11.8 2.91 0.11
11.7 2.91 0.11
11.8 2.81 0.12
11.8 2.81 0.12
12.1 2.73 0.11


Notes: Each sample number is composed of the ALVIN dive followed by the station where the sample was collected during cruise AT11-7 (e.g., 3966-1).
Additional notation describes multiple samples taken from a site (e.g., 3965-la, 3965-1b), several sources of glass from a single sample (e.g., 3966-3, 3966-3s)
including distinct interior and exterior glass (e.g., 3970-9o, 3970-9i). The "depth" of each station is in reference to meters below sea-level while the "distance"
refers to the location of station in meters from the axial summit trough. Natural glasses were analyzed for major element concentrations (presented as weight
% oxides) using an electron microprobe. Lat = Latitude; Long = Longitude. Dist = distance from axis; Morph = lava morphology, Sh = sheet flow, Pw =
pillow flow, Lob = lobate flow, Hack = hackly flow; SiO2b = corrected SiO2; Mg# = ((Mg/(Mg+Fe))*100); K/Ti = ((K20/TiO2)*100).


0.15 100.10 58.22 7.52
0.15 100.16 58.09 7.38
0.15 100.16 58.10 7.33
0.15 100.17 58.12 7.36
0.18 100.00 57.67 7.85
0.16 99.96 58.15 7.82
0.17 99.92 58.14 7.95
0.16 99.94 57.77 7.61
0.15 100.02 58.20 7.40
0.16 99.94 58.35 7.70
0.13 99.98 57.28 6.92
0.14 100.06 57.80 7.05
0.14 99.99 57.39 6.87
0.17 100.03 56.85 7.40
0.16 99.97 56.24 7.76
0.11 100.00 60.41 7.12
0.15 100.04 58.12 6.87
0.14 99.90 58.19 6.91
0.15 100.06 58.21 7.38
0.16 100.04 57.76 6.95
0.14 99.98 59.75 7.09











Table 5-1. Continued

Sample Deg. Lat Deg. Long. Depth Morph. SiO2b TiO2 A1203 FeO MnO MgO CaO Na20 K20 P205 Total Mg# K/Ti


3970-9i
3970-10
3970-11
3970-12
3971-1
3971-2
3971-3
3971-4
3971-5
3971-6
3971-7
3971-8
3971-9
3971-9-2
3971-10
3973-1
3973-2
3973-2ai
3973-2ao
3973-2bi
3973-3


9 30.052' 104 13.627' 2662
9 30.020' 104 13.624' 2653
9 29.753' 104 13.586' 2645
929.610' 104 13.646' 2658
9 43.288' 104 15.725' 2546
9 43.332' 104 15.652' 2547
9 43.456' 104 15.404' 2560
943.451' 104 15.404' 2559
9 43.485' 104 15.324' 2568
943.517' 104 15.128' 2578
9 43.588' 104 15.064' 2582
943.789' 104 15.177' 2581
9 43.774' 104 14.930' 2593
9 43.774' 104 14.930' 2593
9 43.697' 104 14.830' 2543
950.291' 104 17.197' 2510
9 50.133' 104 17.353' 2507
9 50.133' 104 17.353' 2507
9 50.133' 104 17.353' 2507
9 50.133' 104 17.353' 2507
9 50.230' 104 17.369' 2509


Pw
Pw
Pw
Hack
Pw
Pw
Hack
Pw
Lob
Pw
Pw
Lob
Pw
Pw
Lob
Sh
Lob
Lob
Lob
Lob
Lob


50.7
50.7
50.7
50.6
50.8
50.3
50.8
50.2
50.9
50.9
50.9
50.8
50.9
50.9
50.8
50.1
50.0
50.2
50.0
50.2
50.2


0.20
0.19
0.21
0.20
0.21
0.19
0.19
0.19
0.19
0.20
0.20
0.19
0.18
0.20
0.19
0.16
0.17
0.18
0.18
0.18
0.19


7.7
7.7
7.8
7.5
7.5
8.0
8.0
8.4
7.6
7.6
7.6
7.6
7.7
7.8
7.8
8.7
8.7
8.7
8.7
8.1
8.6


12.1 2.74 0.11 0.13
12.1 2.72 0.11 0.13
12.0 2.74 0.11 0.13
11.7 2.87 0.12 0.14
11.9 2.79 0.11 0.13
12.0 2.72 0.13 0.14
12.2 2.74 0.09 0.10
12.1 2.53 0.10 0.11
12.0 2.64 0.14 0.15
12.1 2.62 0.14 0.14
12.0 2.63 0.16 0.15
12.0 2.71 0.14 0.14
12.3 2.61 0.09 0.10
12.1 2.69 0.09 0.11
12.0 2.62 0.14 0.13
12.1 2.54 0.10 0.10
12.1 2.53 0.10 0.12
12.1 2.50 0.10 0.10
12.2 2.53 0.10 0.11
12.5 2.50 0.10 0.11
12.1 2.49 0.10 0.12


Notes: Each sample number is composed of the ALVIN dive followed by the station where the sample was collected during cruise AT11-7 (e.g., 3966-1).
Additional notation describes multiple samples taken from a site (e.g., 3965-la, 3965-1b), several sources of glass from a single sample (e.g., 3966-3, 3966-3s)
including distinct interior and exterior glass (e.g., 3970-9o, 3970-9i). The "depth" of each station is in reference to meters below sea-level while the "distance"
refers to the location of station in meters from the axial summit trough. Natural glasses were analyzed for major element concentrations (presented as weight
% oxides) using an electron microprobe. Lat = Latitude; Long = Longitude. Dist = distance from axis; Morph = lava morphology, Sh = sheet flow, Pw =
pillow flow, Lob = lobate flow, Hack = hackly flow; SiO2b = corrected SiO2; Mg# = ((Mg/(Mg+Fe))*100); K/Ti = ((K20/TiO2)*100).


100.07 59.55 7.02
100.03 59.78 7.33
99.96 60.45 7.45
100.02 58.20 7.20
100.03 58.16 6.99
99.90 61.36 8.71
99.96 62.23 6.51
99.94 63.93 7.60
99.88 60.47 9.38
99.89 60.19 9.63
99.86 60.48 10.76
99.74 60.63 9.52
99.93 60.81 6.29
99.91 60.72 6.57
99.87 60.91 9.82
100.05 65.84 7.94
99.87 65.14 7.96
99.97 65.95 7.96
100.12 65.78 8.29
100.06 63.73 7.97
100.06 65.10 7.97











Table 5-1. Continued

Sample Deg. Lat Deg. Long. Depth Morph. SiO2b TiO2 A1203 FeO MnO MgO CaO Na20 K20 P205 Total Mg# K/Ti


3973-4
3974-1
3974-2
3974-3
3974-4
3974-5
3974-6
3974-7
3974-8
3974-8-2
3974-9
3974-10
3974-11
3975-1
3975-li
3975-2
3975-2i
3975-3
3975-4
3975-5i
3975-50


9 50.217' 104 17.359' 2509
9 50.136' 104 18.571' 2603
9 50.164' 104 18.518' 2594
9 50.202' 104 18.461' 2595
9 50.254' 104 18.506' 2598
9 50.299' 104 18.505' 2599
9 50.349' 104 18.418' 2579
9 50.251' 104 18.326' 2569
9 50.252' 104 18.323' 2568
9 50.252' 104 18.323' 2568
9 50.284' 104 18.229' 2546
9 50.348' 104 18.137' 2532
9 50.386' 104 17.943' 2517
9 28.795' 104 15.603' 2590
9 28.795' 104 15.603' 2590
9 28.789' 104 15.685' 2601
9 28.789' 104 15.685' 2601
9 28.785' 104 15.816' 2605
9 28.678' 104 15.520' 2596
9 28.738' 104 15.303' 2588
9 28.738' 104 15.303' 2588


Sh
Sh
Pw
Hack
Lob
Pw
Pw
Sh
Pw
Pw
Lob
Pw
Lob
Lob
Lob
Hack
Hack
Pw
Hack
Hack
Hack


50.1
50.7
50.6
50.5
50.5
50.7
50.6
50.4
50.5
50.5
50.1
50.1
50.0
50.8
50.8
50.7
50.8
50.7
50.7
50.7
50.6


0.17
0.21
0.20
0.18
0.18
0.20
0.19
0.20
0.18
0.19
0.20
0.20
0.19
0.18
0.21
0.22
0.21
0.21
0.21
0.20
0.21


8.7
7.6
7.6
8.1
8.0
7.6
7.7
8.1
7.9
7.9
8.4
8.3
8.7
7.3
7.2
7.2
7.1
7.5
7.5
7.4
7.6


12.2 2.47 0.10 0.10
11.6 2.76 0.12 0.13
11.7 2.73 0.11 0.14
11.9 2.57 0.12 0.14
12.0 2.59 0.12 0.14
11.7 2.75 0.16 0.15
11.7 2.67 0.16 0.14
11.9 2.59 0.13 0.13
11.9 2.61 0.14 0.14
11.9 2.62 0.14 0.15
11.8 2.67 0.14 0.13
12.1 2.66 0.12 0.14
12.0 2.58 0.11 0.12
11.7 2.83 0.13 0.16
11.6 2.87 0.13 0.15
11.4 2.87 0.12 0.17
11.3 2.90 0.12 0.17
11.9 2.82 0.12 0.14
11.9 2.80 0.12 0.12
11.6 2.84 0.13 0.16
11.7 2.83 0.12 0.16


Notes: Each sample number is composed of the ALVIN dive followed by the station where the sample was collected during cruise AT11-7 (e.g., 3966-1).
Additional notation describes multiple samples taken from a site (e.g., 3965-la, 3965-1b), several sources of glass from a single sample (e.g., 3966-3, 3966-3s)
including distinct interior and exterior glass (e.g., 3970-9o, 3970-9i). The "depth" of each station is in reference to meters below sea-level while the "distance"
refers to the location of station in meters from the axial summit trough. Natural glasses were analyzed for major element concentrations (presented as weight
% oxides) using an electron microprobe. Lat = Latitude; Long = Longitude. Dist = distance from axis; Morph = lava morphology, Sh = sheet flow, Pw =
pillow flow, Lob = lobate flow, Hack = hackly flow; SiO2b = corrected SiO2; Mg# = ((Mg/(Mg+Fe))*100); K/Ti = ((K20/TiO2)*100).


100.07 65.71
99.89 59.43
99.90 59.37
99.93 62.11
99.92 62.01
99.90 60.01
99.86 60.23
99.87 62.76
99.90 61.89
99.87 61.97
99.97 63.66
100.07 63.51
100.06 64.44
99.91 57.30
99.98 56.58
99.98 55.65
99.94 55.66
99.97 58.53
99.98 58.43
100.03 57.62
100.08 58.23


8.68
7.31
7.14
8.75
8.72
9.91
9.76
9.43
9.99
9.96
9.86
8.50
8.24
7.60
7.93
6.90
6.65
7.67
7.78
7.69
7.31























Table 5-1. Continued

Sample Deg. Lat Deg. Long. Depth Morph. SiO2b TiO2 A1203 FeO MnO MgO CaO Na20 K20 P205 Total Mg# K/Ti
3975-6 928.877' 104 15.155' 2583 Lob 50.8 1.68 14.4 10.6 0.21 7.5 11.7 2.83 0.13 0.15 99.90 58.52 7.68
3975-7 928.950' 104 15.023' 2572 Lob 50.8 1.67 14.3 10.7 0.22 7.5 11.7 2.80 0.12 0.16 99.95 58.30 7.42
3975-8 928.945' 104 14.800' 2574 Pillar 50.6 1.68 14.4 10.7 0.21 7.7 11.6 2.83 0.13 0.15 100.05 58.55 7.51
3975-9 928.942' 104 14.790' 2574 Talus 50.6 1.69 14.3 10.8 0.20 7.6 11.6 2.80 0.12 0.16 100.04 58.29 7.37
3976-1 9 50.178' 104 17.226' 2510 Sh 49.8 1.21 16.0 9.1 0.17 8.8 12.2 2.61 0.10 0.10 100.07 65.73 8.13

Notes: Each sample number is composed of the ALVIN dive followed by the station where the sample was collected during cruise AT11-7 (e.g., 3966-1).
Additional notation describes multiple samples taken from a site (e.g., 3965-la, 3965-1b), several sources of glass from a single sample (e.g., 3966-3, 3966-3s)
including distinct interior and exterior glass (e.g., 3970-9o, 3970-9i). The "depth" of each station is in reference to meters below sea-level while the "distance"
refers to the location of station in meters from the axial summit trough. Natural glasses were analyzed for major element concentrations (presented as weight
% oxides) using an electron microprobe. Lat = Latitude; Long = Longitude. Dist = distance from axis; Morph = lava morphology, Sh = sheet flow, Pw =
pillow flow, Lob = lobate flow, Hack = hackly flow; SiO2b = corrected SiO2; Mg# = ((Mg/(Mg+Fe))*100); K/Ti = ((K20/TiO2)*100).







































































0 0 r








0:




6 6.5 7 7.5 8 8.5 9

MgO (wt.%)


12.5 k


0.85 F


0.75 F


S 0.65 L
9.5 5.5


6 6.5 7 7.5 8 8.5

MgO (wt.%)


Figure 5-1. Major element oxide variation diagrams for lavas sampled during dives 3963,
3968, 3970, and 3974. Error is indicated by cross-hairs.


13 o


SAll EPR
* AllAT11-7

* ni,'C ,"-
* i ),.. "-


1.5





1

4


9 9.5


I


I I II I I I


I I


I


I I I I I I


I I


o


I I I I I I I

















0.35


0.3


-0.25


0.2
tn
0
q 0.15


0.1


0.05

0.6


0.5


S0.4


0.3
O

0.2


0.1

0


0 o


c' 0




+ "


SAll EPR
* AllAT11-7

* Dive 3968
* Dive 3970


% E

I


.
Sc-..- if

'Ja c'14i


- 0
C
0
0a ag '




-
*~~~~ .


0.25 F


0.2 -



0.15 -


-
0 0


-o










-._L ;


5.5 6 6.5 7 7.5 8 8.5 9 9.5 5.5 6 6.5 7 7.5 8 8.5 9 9.5
MgO (wt.%) MgO (wt.%)


Figure 5-1. Continued


have slightly high K20/TiO2 ratios (> 11) (Table 5-1), which classifies them as enriched


mid-ocean ridge basalts (E-MORB) relative to all other samples, which are normal mid-


ocean ridge basalts (N-MORB) [Reynolds et al., 1992; Perfit et al., 1994].


In general, the lavas are more magnesian towards the north in the study area


(Figures 5-2 and 5-3). There is little evidence to suggest that the compositions of the


lavas are related to their morphology. The compositional distributions of sheets, lobates,


and pillows are remarkably similar both overall and at each major sampling site along the


EPR ridge (Figure 5-3).


oa o
00 0










00
oo o oO o




S aa





0 0


0.1

52.5

52

51.5

51

50.5

50

49.5

49

48 5

















* Dive 3963
* Dive 3965, 3973,
and 3976


Dive 3968
Dive 3970


1 1 1


*I
J.


9.45 9.5 9.55 9.6 9.65 9.7 9.75 9.8 9.85
Latitude (dec.)


Figure 5-2. MgO content of lavas sampled during dives 3963,
function of their latitude.


3968, 3970, and 3974 as a


9.5



9


0
0


8.5



8


............o._
0*


7.5



7












S70-7 49wt%MgO
7 75-799wt % MgO
0 80-849wt % MgO
S8 5-9 0 wt % MgO
ra


Sheet Pillow
Lava Morphology


Lobate


9 44' N EPR


9 50' N EPR


: 60








0
2O


100


80


60


40


1 2
Lava Morphology


9 28-30' N EPR


\ K II o--_ -----I I----- _
1 2 3 1 2 3
Lava Morphology Lava Morphology


Figure 5-3. Compositional distributions for sheet, lobate, and pillow lavas. A) All lavas
between 9028'N and 9050'N. B) All lavas at 9050'N. C) All lavas at 9044'N.
D) All lavas between 9028 and 9030'N.

Flow Units With Fronts, Dives 3963 and 3974

Two complimentary dives on the east and west sides of the EPR axis near 9050'N

sampled lavas with significant major element differences between flows proximal to the

ASCT and those farther away. In each case, the most primitive samples are those from


AT11-7









the first few flow units emanating from the ASCT, while those now located up to several

kilometers away from the axis were significantly more evolved (Figure 5-1).

Dive 3963 sampled three flow units, including a single flow front close to the

eastern edge of the ASCT. Two analytically separate groups are present within this suite

of basalts (Figures 5-1 and 5-2). The four samples closest to the ASCT (3963-7,9,10,11)

have higher MgO (8.7-9.0%) than the two flow units (3963-3,4,5,6,8) located further off-

axis (7.7-8.0%). Samples comprising the first two flow units exhibit only minor

compositional differences (only MgO contents are outside of analytical uncertainty) and

these follow regional fractional crystallization trends while lavas from the third flow unit

are analytically identical.

On the other side of the axis, dive 3974 also sampled three flow units but the

chemistries of these basalts are much more homogenous than those from dive 3963

(Figure 5-1). The samples comprising the third flow unit (3974-2,3,4,7,8,8-2) have lower

MgO contents (7.6-8.1%) than those of the first flow unit (3974-10,11), which is closest

to the axis (8.3-8.7%). The less morphologically distinct middle flow unit (3974-5,6,9)

has MgO values between these extremes (7.7-8.4%). For all flow units, the oxide

differentiation sequences parallel regional trends with only minor compositional

differences (again, only MgO contents are outside of analytical uncertainty) between the

samples of each flow unit.

Channels, Dive 3968

Both sheet flows that comprise channels, located west of the axis at 9029'N, have

MgO contents of 7.5% while the flows that surround them display slightly more variation

(7.4-7.6%) (Table 5-1). The major element chemistries of basalts from this dive are

analytically identical and plot mainly with the average differentiation trend for lavas from









the 90-100N segment (Figure 5-1), but some spread is noticeable for SiO2, MnO, and

P205 due to the larger error associated with these elements. While all Na20 contents are

low, samples 3968-4so, 3968-5, 3968-6, and 3968-7 are exceptionally low and represent

the lower extreme limit for this oxide.

Off-Axis Mounds, Dive 3970

The southern off-axis pillow mound at 9030'N (3970-9,10,11) shows little

variation in MgO contents (7.6-7.8%) compared to the northern pillow mound near

931'N (3970-4,5,6) which exhibits most of the MgO variation outside of analytical

uncertainty seen in samples from this dive (7.2-7.8%) (Table 5-1). Lavas from the

surrounding flows (3970-1,2,3,7,8,12) have MgO contents of 7.4-7.6%. A good

concordance exists between most average regional major oxide trends and basalts from

this area except values for both MnO and P205 show more scatter amongst the samples

than all other oxides (Figure 5-1).

Trace Element Data

Only lavas collected during dives 3963, 3968, 3970, and 3974 were analyzed for

trace element content (Tables 5-2 and 5-3). These lavas are N-MORB, as reflected in

their low K/Ti ratios (6.87 to 9.99) and depleted incompatible elements normalized to

chondritic and primitive mantle values (Tables 5-2 and 5-3 and Figures 5-4 to 5-7) [Sun

andMcDonough, 1989]. As a whole, the incompatible element contents of these samples

(Sc, V, Zn, Ga, Y, Zr, Nb, Hf, Sr, Rb, U, Th, and the rare earth elements [REE]) increase

and the compatible elements decrease (Ni and Cr) with decreasing MgO content while

several trace element contents appear to change very little (Figure 5-8).










Table 5-2. Select Trace elements For Lavas Collected During Dives 3963, 3968, 3970, and 3974


Sample
3963-1
3963-3
3963-4i
3963-4o
3963-5
3963-6
3963-6i
3963-7
3963-8
3963-9
3963-10
3963-10-2
3963-11
3968-1
3968-2
3968-3
3968-4
3968-5
3968-6
3968-7
3968-8b
3968-8c
3968-9
3968-10
3970-1
3970-2


V Cr Co
314 339 38.7
293 308 36.0
282 308 34.8
295 321 36.4
294 321 36.3
299 309 36.7
299 311 36.6
241 321 37.4
290 321 36.7
248 332 38.5
243 371 38.5
255 370 38.5
240 372 37.8
315 227 35.8
316 226 35.8
311 222 35.3
317 224 35.9
334 233 38.6
328 230 38.0
320 224 36.7
333 231 38.8
328 227 37.8
325 228 37.5
333 235 38.4
318 177 37.0
319 178 37.0


Ga Rb Sr Y
17.4 1.0 114 35
16.2 1.5 117 32
15.7 1.7 116 30
16.2 1.8 119 32
16.3 1.8 120 32
16.4 1.2 120 32
16.4 1.2 120 32
14.8 1.0 107 24
16.4 1.4 120 32
15.3 1.0 111 25
15.1 1.0 117 25
16.1 0.9 123 27
15.2 1.0 119 25
16.9 1.1 111 37
16.9 1.1 111 37
16.8 1.1 110 37
17.2 1.1 113 38
18.0 1.3 119 40
17.7 1.3 118 39
17.2 1.2 113 38
17.9 1.3 119 40
17.7 1.3 117 39
17.6 1.2 117 39
17.9 1.3 120 40
17.1 0.8 107 36
17.3 0.9 108 36


Zr
102
104
99
104
103
103
103
79
105
82
85
85
85
110
111
109
111
119
116
114
118
116
115
117
104
103


Nb Hf Th U
2.4 2.7 0.13 0.07
3.2 2.7 0.19 0.08
3.5 2.6 0.21 0.08
3.5 2.7 0.21 0.08
3.6 2.7 0.22 0.09
2.6 2.6 0.16 0.07
2.6 2.7 0.16 0.07
1.5 2.1 0.10 0.04
2.9 2.7 0.17 0.07
1.5 2.1 0.10 0.04
1.8 2.2 0.11 0.05
2.1 2.2 0.12 0.06
1.8 2.2 0.11 0.05
3.0 3.0 0.16 0.06
3.0 3.0 0.16 0.06
2.9 3.0 0.16 0.06
3.0 3.0 0.16 0.06
3.1 3.1 0.19 0.09
3.1 3.0 0.18 0.08
3.0 3.0 0.18 0.08
3.1 3.1 0.18 0.08
3.1 3.0 0.18 0.08
3.1 3.1 0.18 0.08
3.1 3.1 0.18 0.09
2.5 2.9 0.13 0.05
2.5 2.8 0.13 0.05


Notes: Trace element concentrations are in ppm.


Ti Fe Mn K P
8795 78695 1528 844 523
9556 80535 1573 1182 686
9123 78111 1564 1239 651
9123 78111 1564 1239 651
9410 78109 1521 1253 592
9194 78491 1573 1107 618
9194 78491 1573 1107 592
6876 70401 1346 699 370
9450 79264 1567 1082 674
6800 70283 1353 677 450
7065 68580 1288 794 436
7065 68580 1288 794 436
7045 68002 1361 818 463
10223 82886 1523 1050 653
10101 83060 1526 1078 656
10129 83106 1708 1036 700
9962 83558 1565 1065 739
10474 83333 1565 1070 633
10250 83222 1638 1040 645
10293 83290 1679 1049 652
10076 83097 1678 1109 733
10133 82831 1499 1068 717
10121 83269 1555 1037 652
10068 82848 1635 1073 689
9731 84697 1489 932 576
9766 84105 1603 954 617










Table 5-2. Continued


Ti Fe Mn
9911 84442 1697
10407 86275 1686


Sample Sc
3970-3 43
3970-4 45
3970-5 43
3970-6 42
3970-7 40
3970-8a 41
3970-8b 42
3970-90 41
3970-9i 40
3970-10 40
3970-10-2 43
3970-10-3 42
3970-11 40
3970-12 41
3974-1 39
3974-2 39
3974-3 38
3974-4 38
3974-5 42
3974-6 38
3974-7 37
3974-8 38
3974-9 37
3974-10 39
3974-11-2 37
3974-11-3 37.3


V
328
358
339
330
304
319
323
295
289
289
314
303
290
317
294
287
282
282
319
291
276
276
274
281
267
269


Notes: Trace element concentrations are in ppm.


Cr
184
234
237
235
238
219
218
276
268
273
290
267
270
209
255
255
344
337
331
301
343
339
333
357
331
322


Co
38.0
40.5
37.3
37.3
36.6
37.1
36.4
37.5
35.8
35.1
39.2
39.1
36.3
36.3
35.9
36.5
36.7
35.7
41.2
36.6
37.5
36.4
36.4
37.7
36.7
38.7


Ni
59
69
64
67
65
67
66
77
72
68
77
76.9
76
63
82
96
96
94
97
99
119
113
113
109
115
119


Ga
17.8
18.8
18.4
17.5
16.8
17.2
17.5
16.9
16.5
15.8
17.8
17.6
16.8
17.2
16.3
16.0
16.0
15.5
18.2
16.6
16.2
16.0
15.3
16.5
16.2
16.5


Rb Sr
0.9 111
1.2 123
1.1 115
1.1 113
0.9 110
1.0 106
1.0 107
0.8 114
0.8 110
0.7 108
0.9 120
0.8 117
0.8 111
1.2 115
0.9 115
0.8 114
1.5 123
1.2 119
3.6 163
2.1 141
1.9 132
1.7 130
1.6 126
1.2 128
1.1 116
1.0 120


Hf Th U
2.9 0.13 0.05
3.3 0.18 0.08
3.1 0.15 0.06
3.0 0.18 0.08
2.8 0.14 0.07
2.9 0.13 0.07
3.0 0.14 0.07
2.6 0.11 0.06
2.5 0.08 0.06
2.4 0.10 0.04
2.7 0.12 0.07
2.5 0.12 0.05
2.5 0.11 0.06
2.9 0.16 0.08
2.8 0.13 0.05
2.6 0.12 0.05
2.6 0.18 0.07
2.5 0.17 0.06
2.7 0.30 0.13
2.8 0.26 0.10
2.5 0.23 0.09
2.6 0.22 0.08
2.6 0.22 0.08
2.5 0.16 0.06
2.4 0.14 0.07
2.3 0.1 0.1


10605
8688
9806
9745
10006
8925
9029
8837
8837
8837
8653
10140
9432
9152
8437
8423
9468
9539
8551
8425
8440
8152
7886
7886


86649 1725
78289 1587
83397 1648
83504 1582
84442 1778
79263 1512
80276 1525
79447 1492
79447 1492
79447 1492
78251 1595
82831 1567
80295 1596
79941 1566
75661 1424
75737 1387
78444 1528
78036 1469
74111 1570
74977 1360
74232 1538
73656 1540
73944 1478
73944 1478


K P
943 601
1066 737
1139 698
856 492
933 638
997 661
962 684
876 620
877 576
898 571
898 571
898 571
893 551
1011 620
955 570
905 589
1022 599
1018 593
1300 637
1290 630
1117 580
1166 590
1153 588
960 602
900 542
900 542










Table 5-3. Rare Earth Element Concentrations for Dives 3963, 3968, 3970, and 3974


Sample La
3963-1 3.43
3963-3 3.80
3963-4i 3.80
3963-4o 3.90
3963-5 3.93
3963-6 3.54
3963-6i 3.54
3963-7 2.39
3963-8 3.71
3963-9 2.48
3963-10 2.67
3963-10-2 2.97
3963-11 2.68
3968-1 3.93
3968-2 3.97
3968-3 3.88
3968-4 3.90
3968-5 4.30
3968-6 4.23
3968-7 4.17
3968-8b 4.32
3968-8c 4.23
3968-9 4.21
3968-10 4.26
3970-1 3.43
3970-2 3.43


Ce Pr
11.02 1.9
11.90 2.0
11.55 2.0
11.99 2.1
12.05 2.0
11.37 2.0
11.34 2.0
8.19 1.5
11.82 2.0
8.41 1.5
8.94 1.6
9.43 1.6
8.93 1.6
12.38 2.1
12.52 2.1
12.23 2.1
12.41 2.1
13.29 2.2
13.00 2.2
12.85 2.2
13.34 2.2
13.02 2.2
13.00 2.2
13.14 2.2
11.27 2.0
11.25 2.0


Nd Sm Eu
10.15 3.43 1.26
10.31 3.43 1.22
9.81 3.25 1.16
10.20 3.37 1.21
10.18 3.37 1.20
9.91 3.33 1.18
9.88 3.34 1.19
7.20 2.49 0.93
10.34 3.45 1.22
7.48 2.54 0.96
7.88 2.62 0.98
8.28 2.73 1.07
7.82 2.58 0.97
11.05 3.70 1.30
11.06 3.72 1.30
11.00 3.70 1.30
11.14 3.73 1.31
12.18 4.08 1.41
11.93 4.00 1.35
11.66 3.89 1.34
12.14 4.05 1.40
11.92 3.95 1.36
11.80 3.97 1.36
12.05 4.04 1.39
10.29 3.51 1.27
10.40 3.53 1.28


Tb Dy Ho
0.88 5.56 1.19
0.84 5.41 1.17
0.79 5.08 1.10
0.83 5.29 1.14
0.83 5.28 1.14
0.82 5.27 1.13
0.83 5.27 1.14
0.64 4.05 0.85
0.84 5.37 1.15
0.67 4.15 0.88
0.67 4.16 0.87
0.69 4.38 0.93
0.67 4.15 0.87
0.93 5.90 1.29
0.93 5.96 1.30
0.91 5.91 1.29
0.92 5.94 1.30
1.02 6.37 1.35
1.00 6.23 1.32
0.97 6.05 1.29
1.02 6.34 1.34
0.99 6.22 1.31
0.98 6.20 1.31
1.01 6.31 1.34
0.91 5.79 1.27
0.91 5.81 1.26


Tm Yb Lu
0.51 3.44 0.54
0.51 3.32 0.52
0.48 3.10 0.49
0.51 3.23 0.51
0.50 3.22 0.51
0.50 3.22 0.51
0.51 3.22 0.51
0.41 2.43 0.38
0.51 3.27 0.52
0.42 2.51 0.39
0.42 2.48 0.39
0.43 2.66 0.42
0.42 2.49 0.39
0.54 3.65 0.54
0.55 3.67 0.55
0.54 3.68 0.55
0.55 3.72 0.55
0.56 3.95 0.62
0.55 3.86 0.61
0.54 3.76 0.59
0.56 3.94 0.61
0.55 3.83 0.60
0.55 3.85 0.60
0.56 3.93 0.61
0.54 3.60 0.54
0.54 3.60 0.53


Notes: Trace element concentrations are in ppm.










Table 5-3. Continued


Notes: Trace element concentrations are in ppm.


Sample La
3970-3 3.54
3970-4 4.34
3970-5 3.91
3970-6 3.98
3970-7 3.49
3970-8a 3.66
3970-8b 3.74
3970-90 3.17
3970-9i 3.08
3970-10 2.93
3970-10-2 3.40
3970-10-3 3.27
3970-11 3.16
3970-12 3.95
3974-1 3.53
3974-2 3.25
3974-3 3.64
3974-4 3.57
3974-5 5.00
3974-6 4.48
3974-7 4.01
3974-8 4.10
3974-9 4.01
3974-10 3.52
3974-11-2 3.38
3974-11-3 3.40


Ce Pr
11.58 2.1
13.65 2.3
12.50 2.2
12.49 2.1
11.30 1.9
11.73 2.0
12.01 2.1
10.34 1.8
9.92 1.7
9.75 1.8
11.04 1.9
10.77 1.9
10.31 1.8
12.36 2.1
11.34 2.0
10.57 1.9
11.44 1.9
11.04 1.9
14.26 2.4
13.35 2.2
12.09 2.0
12.08 2.0
11.92 2.0
10.90 1.9
10.58 1.8
10.76 1.8


Nd Sm Eu
10.64 3.66 1.30
12.81 4.34 1.48
11.58 3.95 1.40
11.66 3.95 1.36
10.53 3.52 1.27
10.90 3.68 1.31
11.08 3.75 1.33
9.45 3.20 1.20
9.08 3.06 1.16
8.84 3.09 1.12
10.11 3.39 1.27
9.95 3.39 1.26
9.36 3.17 1.19
11.33 3.78 1.32
10.29 3.48 1.23
9.59 3.26 1.17
9.69 3.20 1.15
9.26 3.10 1.10
11.81 3.74 1.31
11.00 3.52 1.25
9.93 3.20 1.14
9.90 3.20 1.13
9.60 3.12 1.10
9.23 3.12 1.12
9.06 2.92 1.12
9.42 3.13 1.16


Tb Dy Ho
0.92 5.91 1.30
1.10 6.83 1.45
0.98 6.31 1.38
1.00 6.23 1.32
0.91 5.72 1.21
0.95 5.89 1.26
0.97 6.07 1.29
0.83 5.23 1.11
0.79 5.05 1.06
0.80 5.08 1.10
0.88 5.56 1.18
0.85 5.32 1.18
0.81 5.20 1.10
0.96 6.00 1.27
0.87 5.54 1.20
0.82 5.26 1.13
0.79 5.00 1.08
0.79 4.99 1.07
0.83 5.22 1.11
0.84 5.33 1.13
0.77 4.88 1.04
0.79 5.01 1.08
0.79 4.97 1.05
0.79 5.00 1.07
0.74 4.80 1.01
0.78 4.83 1.06


Tm Yb Lu
0.55 3.70 0.55
0.59 4.26 0.67
0.57 3.93 0.58
0.55 3.87 0.61
0.52 3.54 0.55
0.54 3.69 0.58
0.55 3.79 0.60
0.49 3.23 0.51
0.47 3.09 0.48
0.49 3.12 0.47
0.52 3.45 0.54
0.50 3.33 0.50
0.49 3.17 0.50
0.53 3.69 0.58
0.52 3.41 0.51
0.50 3.22 0.48
0.48 3.05 0.48
0.48 3.06 0.46
0.50 3.13 0.49
0.50 3.20 0.50
0.47 2.97 0.47
0.48 3.04 0.46
0.48 3.00 0.45
0.48 3.06 0.46
0.46 2.93 0.45
0.47 2.97 0.45










Spider Diagram
I I I I I I I I I I 1 I I I I I I I I


1





100


REE Plot


SU P. M Qi Mk
(ppm)

Figure 5-4. Spider diagram and REE plot for lavas from dive 3963. Trace elements were
normalized to primitive mantle and chondrite values from Sun and
McDonough [1989].


S3963-1
3963-3
3963-4i
-x 3963-40
S3963-5

S3963-7
A 3963-9
- 3963-10


1- 3963-11



(ppm)







77


Spider Diagram


I I I I I I I I I I I I I I I I I I












3974-1 3974-5
3974-2 3974-11-3
A -3974-3 3974-11-2
/ t U












-x- 3974-4 3974-8



(ppm)


REE Plot


i-#




I I I I I I I I I I I I I I
SU E ;::s (ppm

(ppm)


Figure 5-5. Spider diagram and REE plot for lavas from dive 3974. Trace elements were
normalized to primitive mantle and chondrite values from Sun and
McDonough [1989].


1






100










Spider Diagram


I I I I I I I


100








,10
CI






1a





1






100


REE Plot


I I I I I I I I I I I I I I
Ct U E ; O (ppm)


Figure 5-6. Spider diagram and REE plot for lavas from dive 3968. Trace elements were
normalized to primitive mantle and chondrite values from Sun and
McDonough [1989].


S3968-1
3968-2 3968-8b
0- 3968-3 3
x -3968-4 A 3968-9
+ 3968-5 V 3968-10


(ppm)


I I I I I I I I I I I I I I











100


REE Plot


^ x- X, -><-
x- x











(ppm)


Figure 5-7. Spider diagram and REE plot for lavas from dive 3970. Trace elements were
normalized to primitive mantle and chondrite values from Sun and
McDonough [1989].


79


Spider Diagram


3970-1 3970-10-2
3970-2 3970-8a
3970-3 39
x 3970-4 A 3970-9i
3970-5 3970-90




SX- -X




.m/





(ppm)


100



100




































36 k


350


300



U 250


200 h


100 h


xx
XX


x


7 7.5 8 8.5
MgO (wt.%)


340 F


320


300


280


260 k


40

39

, 38

U 37

36

35


9 9.5 7 7.5 8 8.5
MgO (wt.%)


Figure 5-8. Trace element variation diagrams for lavas from dives 3963, 3968, 3970, and

3974. Error is represented by cross-hairs.


So Dive 3963
0 Dive 3968
o o o Dive3970
x Dive 3974



xx


I x
|x x


0





0
x 0


x x
o x ) K


x
Ox x x x



S9
0




XX


o
0 0


0



0 x

o x o
o oX

JxX 0 ox x
0 oo x x
ih v


0



C0
00
00 0
fo o x
8x
0 0
x x


0
E o
xx x

-i
[] [


i i i


9 9.5


I I





















95

90

85

80

S75
N
70

65

60

4


^35



30


25 +


S17



j 16


160

150


140


130

120


110


120 F


110 -


100 -

N


7 7.5 8 8.5 9 9.5 7 7.5 8 8.5 9 9.5


MgO (wt.%)


MgO (wt.%)


Figure 5-8. Continued


0

x
o z
o
0
o
E o



x x x

x
0


x
08 x
o x
x


*T


x
x
0 x



0 %0 0


0




-ib



o0, x x x xx
X x
x
x


0
b x



xo x

0 Xx
0 x x x

0


I I I



















































Sy Y

o ox
o x
0 0
x x
o o
0
0






0 0







0 x
0


x x
0o x
o


8 -


7 7.5 8 8.5
MgO (wt.%)


5.5


5 x


4.5 x


4 0
4
0 0 '0

S3.5 -


3 0


2.5


2


o
00


X

o
o


9 9.5 7 7.5 8 8.5
MgO (wt.%)


Figure 5-8. Continued


6



5



C4







2



1

15


14


0






0 Ux
00 0
0o 0 x xx
S x x x


9 9.5






















1.5


1.4


1.3


31.2


1.1


1


0.9

1.2


1.1


0.8


0.7


1.4


1.3 .,-
O 0

1.2 x
:o 0 0
x C a
1.1 *
0 x x x x
1 -


0.9


0.8
7 7.5 8 8.5 9
MgO (wt.%)


5.5 k


4.5 k


3 5.5


o
0 0
O x


x

S-

x 1


9.5 7 7.5 8 8.5
MgO (wt.%)


Figure 5-8. Continued


o x



0

0

0
Sx xx
Sx X X X


6I

0

o c o
0


O x0 X X X
8 x x
x


0


0 lb

g o
00
0o



- x > xx
x


9 9.5






















0.6




0.55




S0.5

E


0.45




0.4

0.7


0.65


0.6


~0.55


S0.5


0.45


0.4


0.35

0.35


0.3


0.25


0.2


0.15


0.1


So Dive 3963
o Dive 3968
4 o Dive 3970
JZ o0 x Dive 3974





0 0
0 x< x x
0 x X
x










0








0 o
o
xmo x

















x
x xx x X











x




x


0 x
0



S o x



0
o


7.5 8 8.5 9 9.5 7
MgO (wt.%)


7.5 8 8.5 9 9.5
MgO (wt.%)


0
o 0


x x o x
xx x
0 *


n n4A


7.J
7


0 o0
0
0 e

x
oo x x x
X x

x c


x





x
x
08 x
0
0x
80
0 e O
0 0. x x x 0
0 0 X>
0 0
O~ox>


Figure 5-8. Continued


n n









Flow Units With Fronts, Dives 3963 and 3974

The second and third flow units from dive 3963 (3963-3,6 and 3963-4,5,8) have

greater abundances in incompatible elements than the first flow unit (3963-7,9) and flow

front (3963-10,11) closest to the ridge axis (Figures 5-4 and 5-8). Most trace element

concentrations from each flow unit and the flow front are each analytically identical.

However unlike with the major elements, where only MgO contents for the first and

second flow units varied outside of analytical uncertainty, several trace element

concentrations from each flow unit likewise vary. Those that follow trends presumably

related to fractional crystallization include La and Nd for the flow front, Ni, La, and Th

for the second flow unit, and La, Nd, and Th for the third flow unit. Also, sample 3974-

4i from the second flow unit appears to have lower Nd, Dy, Yb, and Zr abundances than

the rest of the lavas in that flow unit.

Overall, samples from dive 3974 vary between the most primitive samples from

dive 3963 and the more evolved samples from dives 3968 and 3970, creating a general

trend of increasing incompatible element concentrations in samples with lower MgO

contents. Within each flow unit, variation in most trace element abundances is within

analytical uncertainty. A major exception involves sample 3974-5 from the middle flow

unit, which has higher abundances of some trace elements that either follow expected

fractionation trends (Sc, V, Ga, Y) or do not (Co, Rb, Sr, Nb, Th, and U), in relation to

the other lavas from this flow unit (3974-6 and 3974-9) (Figures 5-5 and 5-8). In

combination with lava 3974-6, both exhibit lower concentrations of Ni and higher

concentrations of Zr and the light REE relative to 3974-9 (Figures 5-5 and 5-8).









Channels, Dive 3968

Most of the trace element concentrations of the sheet and lobate flows that

comprise the two channels and surrounding volcanic terrain are analytically identical. Of

the lavas sampled within and near the southern channel (3968-1 to 5), all but 3968-5

(sampled from the margin) exhibit slightly lower abundances outside of analytically

uncertainty of Co, Y, U, and most REE relative to the rest of the samples from dive 3968

(Figures 5-6 and 5-8). However, these differences may be explained by slight differences

in instrument precision as a result of these samples being run at different times.

Off-Axis Mounds, Dive 3970

The northern pillow mound (3970-4,5,6) has higher abundances of incompatible

elements relative to the southern pillow mound (3970-9,10,11), with the lavas comprising

the surrounding terrain lying between these two extremes (3970-1,2,3,7,8) (Figures 5-7

and 5-8). As with the major element concentrations of the southern pillow mound, the

trace element concentrations are analytically identical for most trace elements if sample

3970-10-2 is excluded. On the other hand, 3970-4 from the northern pillow mound

consistently has higher incompatible elemental abundances outside of analytical

uncertainty relative to the other pillow lavas from this mound.

Summary

Overall, the flow fronts near 9o50' N display the greatest variation in both major

and trace element concentrations relative to the other samples analyzed. On the other

hand, the off-axis mounds and channels near 9030'N have, in general, more evolved

compositions and lower abundances of the most incompatible elements relative to the

least incompatible elements. As a whole, both the major and trace element variations for

each dive mimic apparent fractional crystallization trends. While only MgO contents for






87


some of the flow units and pillow mounds vary outside of analytical uncertainty,

variations in the more analytically precise trace element concentrations occur in all cases

for at least of few elements.














CHAPTER 6
PETROGENESIS

The petrogenetic history of lavas erupted from the 9-10 N segment of the EPR

is controlled by various factors, including the composition of the mantle source, style and

extent of melting at the source, mixing with other melts, and the manner of crystallization

[Langmuir et al., 1992]. Most chemical variation observed in axial MORB can be

explained by fractional crystallization and mixing of magmas in the axial magma

chamber [Batiza and Niu, 1992; Langmuir et al., 1992]. The major and trace element

concentrations of the flow units and off-axis mounds were modeled to better understand

and distinguish between these processes.

Major Element Models

PETROLOG 2.1 was used to model the fractional crystallization of a melt by

utilizing pseudo-liquidus temperatures [Nathan and Vankirk, 1978; Nielsen and Dungan,

1983; Ariskin et al., 1986]. This technique allows calculations of liquidus temperatures

for each phase of interest in a range of melt compositions using mineral-melt equilibria

models. If the phase is metastable in a given melt composition, then a pseudo-liquidus

temperature is calculated. Once pseudo-liquidus temperatures have been calculated for

selected minerals capable of crystallizing from a user-provided melt composition, the

temperatures are compared. The mineral first on the liquidus is the one with the highest

calculated temperature. This mineral is removed from the melt and the process continues

as additional phases are added to the fractionating assemblage until a user-defined

amount of crystallization is attained. Incremental fractional crystallization in 1 wt %









increments results in a series of melt compositions that form a liquid line of descent

(LLD), the pathway taken as the melt evolves due to the separation of minerals from the

melt in which they formed.

Before simulating the crystallization of a melt, the user must input data and select

calculation parameters, including the major element composition of a parental melt.

Parental compositions were estimated from the most primitive (highest wt.% MgO)

basaltic samples collected from the flow units and pillow mounds (Tables 6-1 to 6-3).

The channels investigated during dive 3968 were not evaluated for their petrogenetic

evolution due to their lack of chemical variation, which suggests that they did not

experience any crystallization. In the calculations, olivine, plagioclase, and

clinopyroxene were selected as potential crystallizing phases because these minerals are

present in the lavas themselves. Other invariant calculation parameters include setting

the oxidation state at the QFM f02 buffer [Borisov and .shikin, 1990], making crystal-

liquid equilibrium calculations after each 0.01 wt.% of the minerals are subtracted from

the melt, and terminating calculations once 50% crystallization has been completed.

Water content (0.0 and 0.2 wt.%) and pressure (0.5 and 1 kbar) were varied to better

approximate the range of conditions under which the magmas crystallized. Only a few

empirical models account for water in the basalt system: an olivine-melt model developed

by Falloon andDanyushevsky [2000] and plagioclase- and clinopyroxene-melt models by

Danyushevsky [2001].

Flow Units, Dives 3963 and 3974

The range of compositions of samples collected from the overlapping flow units

are well constrained by numerous LLD calculations generated using samples 3963-9,

3963-11, 3974-11, and 3973-2 as parental magma compositions (Figure 6-1). The