Phosphate (U-Th)/He Thermochronology of Zagami and ALHA77005 Martian Meteorites

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Phosphate (U-Th)/He Thermochronology of Zagami and ALHA77005 Martian Meteorites
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Farah,Annette Emily
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Degree:
Master's ( M.S.)
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University of Florida
Degree Disciplines:
Geology, Geological Sciences
Committee Chair:
Min, Kyoungwon Kyle
Committee Members:
Mueller, Paul A
Foster, David A

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Subjects / Keywords:
alha77005 -- diffusion -- fractional -- helium -- mars -- martian -- merrillite -- meteorite -- phosphate -- shock -- thermal -- thermochronology -- thorium -- uranium -- zagami
Geological Sciences -- Dissertations, Academic -- UF
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Geology thesis, M.S.
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theses   ( marcgt )
government publication (state, provincial, terriorial, dependent)   ( marcgt )
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Electronic Thesis or Dissertation

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Abstract:
Martian meteorites bring a wealth of information pertaining to the evolution of the Martian lithosphere and atmosphere, as well as the ejection of meteorites from Mars and their delivery to Earth. Zagami and ALHA77005 are two Martian meteorites (shergottites) with similar compositions and crystallization ages, yet different thermal histories. To better constrain the shock history of Zagami and ALHA77005, I applied (U-Th)/He thermochronology to multiple aggregates of merrillite (phosphate) from both Martian meteorites. After identifying phosphate aggregates using SEM (Scanning Electron Microscopy), the samples were divided into groups consisting of 5-20 grains per group based on their size. The (U-Th)/He data obtained from these groups yield ages that range between 19.8 Ma and 202.4 Ma for Zagami, and 5.9 Ma to 78.2 Ma for ALHA77005. For Zagami, the ages from groups of larger aggregates (~111-202 Ma) are older than the ages from groups of smaller ones (~20-200 Ma). To understand the age distribution, the textures of phosphate aggregates were investigated using SEM, BSE (Back-Scattered Electron) imaging, and compositional (chemistry) maps. The phosphate aggregates used for (U-Th)/He dating have similar 2D surface areas of phosphate, but the larger aggregates have thicker attached, non-phosphatic phases. The observed textures and age-size relationship suggest that the large phosphate aggregates experienced the least amount of alpha recoil loss and yield the most reliable ages. To estimate the peak shock temperatures reached by these meteorites during their ejection from Mars, a simple volume diffusion model was applied. The most reliable ages of Zagami and ALHA77005 are converted to helium fractional losses of 0.17 and 0.94, respectively, with an assumption that the ejection-related shock is completely responsible for the observed He loss. The diffusion domain radii of Zagami and ALHA77005 were estimated at 0.05-6 microns and 5-15 microns, respectively, through detailed image analysis of 56 phosphate grains in two thin sections. For the volume diffusion modeling, these helium fractional loss and diffusion domain estimates, as well as other physical parameters related to the parent meteoroids were combined with general He diffusion properties in merrillite. For Zagami, average peak shock temperatures of 213 and 407 degrees Celsius were obtained at a diffusion domain radius of 0.05 and 6 microns, respectively. The results are comparable to, or slightly higher, than previous values of 220 +/- 50 degrees Celsius (Nyquist et al., 2001) when converted using the average temperature of space (-70 degrees Celsius) at the time of ejection from Mars. However, the new estimates are apparently higher than the converted estimates of 70 +/- 5 degrees Celsius (Fritz et al., 2005). For ALHA77005, the models produced average peak temperatures of 520 and 615 degrees Celsius given a diffusion domain size of 5 and 15 microns, respectively. These peak shock temperatures are consistent with previous suggestions of 450-600 degrees Celsius (Nyquist et al., 2001) and 800 +/- 200 degrees Celsius (Fritz et al., 2005).
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Statement of Responsibility:
by Annette Emily Farah.
Thesis:
Thesis (M.S.)--University of Florida, 2011.
Local:
Adviser: Min, Kyoungwon Kyle.

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1 PHOSPHATE (U TH)/HE THERMOCHRONOLOGY OF ZAGAMI AND ALHA77005 MARTIAN METEORITES By ANNETTE EMILY FARAH 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 2011

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2 2011 Annette Emily Farah

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3 To my family for their love and support

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4 ACKNOWLEDGMENTS I extend a heartfelt thank you to Kyoungwon Kyle Min, my advisor, for his trust, patience and guidance while I worked with him throughout the past three years. I would also like to t hank my committee members Paul Mueller and David Foster, for their advice and support. Thank you to Ann Heatherington, George Kamenov, Ray Thomas, and Dow Van Arnam for their help with equipment and software for my research. Thank you to Sergio Restrepo and Antonios Marsellos for their assistance and kind words and to Annie Wintzer for her work and contribution to this project. I am especially grateful to Bo Gustaf son from the Department of Astronomy and professors and students from the Department of Geological Sciences at the University of Florida for their cooperation and support towards the completion of my project. I offer a s pecial thank you to the Ladies of t he Office (Nita Fahm, Susan Lukowe, Pamela Haines, and Carrie Williams) for their help with countless things and taming of the copy machine Th is research was performed by the generous support of the Korean Institute of Geoscience and Mineral Resources (KI GAM). I would like to thank KIGAM for supplying a sample of Zagami and to the National Aeronautics and Space Administration (NASA) for providing a sample and thin section of ALHA77005. I would also like to acknowledge the curators from the Smithsonian I nstitution, Washington, D.C. and the American Museum of Natural History, New York, NY for supplying Zagami thin sections USNM6545 4 and 4709 1, respectively. I also recognize and greatly appreciate the assistance of Peter W. Reiners and Stefan Nicolescu f rom the University of Arizona Department of Geosciences for their expertise and handling of my first set of samples

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5 TABLE OF CONTENTS page ACKNOWLEDGMENTS ................................ ................................ ................................ .. 4 LIST OF TABLES ................................ ................................ ................................ ............ 7 LIST OF FIGURES ................................ ................................ ................................ .......... 8 ABSTRACT ................................ ................................ ................................ ................... 10 CHAPTER 1 INTRO DUCTION ................................ ................................ ................................ .... 12 Background of Methodology ................................ ................................ ................... 12 Purposes of This Study ................................ ................................ ........................... 15 2 BACKGROUND ................................ ................................ ................................ ...... 16 Types of Meteorites ................................ ................................ ................................ 16 Martian Meteorites ................................ ................................ ................................ .. 17 T wo Minerals Unique in Meteorites: Maskelynite and Merrillite .............................. 21 Martian Meteorites of This Study ................................ ................................ ............ 22 Zagami ................................ ................................ ................................ ............. 22 ALHA77005 ................................ ................................ ................................ ...... 24 3 (U TH)/HE THERMOCHRONOLOGY ................................ ................................ ..... 30 4 ANALYTICAL METHODS ................................ ................................ ....................... 33 5 (U TH)/HE RESULTS ................................ ................................ ............................. 59 Cosmogenic 4 He Correction ................................ ................................ ................... 59 (U Th)/He Ages ................................ ................................ ................................ ....... 59 6 DISCUSSION ................................ ................................ ................................ ......... 66 Alpha Recoil ................................ ................................ ................................ ........... 66 Diffusion Behavior ................................ ................................ ................................ ... 69 Diffusion Domain ................................ ................................ .............................. 70 Diffusion Parameters ................................ ................................ ........................ 72 Fractional Loss ................................ ................................ ................................ ....... 73 Peak Shock Temperatures ................................ ................................ ..................... 76 Zagami ................................ ................................ ................................ ............. 77 ALHA77005 ................................ ................................ ................................ ...... 79 Post shock versus peak shock temperatures ................................ ................... 81

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6 Future Studies ................................ ................................ ................................ ........ 82 7 CONCLUSIONS ................................ ................................ ................................ ..... 87 APPENDICES A H: ZAGAMI AND ALHA77005 IMAGES ................................ ............. 90 A SEM CHEMICAL MAPS FOR ZAGAMI PHOSPHATE AGGREGATES ................. 91 B SEM CHEMICAL MAPS FOR ALHA77005 PHOSPHATE AGGREGATES .......... 128 C TRACED ZAGAMI MERRILLITE PHOSPHATES ................................ ................. 149 D TRA CED ALHA77005 MERRILLITE PHOSPHATES ................................ ........... 161 E ZAGAMI MICROFRACTURES ................................ ................................ ............. 179 F ALHA77005 MICROFRACTURES ................................ ................................ ........ 184 G ZAGAMI THERMAL MODELS ................................ ................................ .............. 187 H ALHA77005 THERMAL MODELS ................................ ................................ ........ 198 LIST OF REFERENCES ................................ ................................ ............................. 207 BIOGRAPHICAL SKETCH ................................ ................................ .......................... 218

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7 LIST OF TABLES Table page 2 1 Mineralogical composition for Zagami. ................................ ............................... 27 2 2 Radiometric ages for Zagami and ALHA77005. ................................ ................. 28 4 1 List of 80 out of 165 Zagami aggregates. ................................ ........................... 37 4 2 List of 85 out of 165 Zagami aggregates ................................ ........................... 38 4 3 List of 56 out of 83 ALHA77005 aggregates ................................ ...................... 39 4 4 List of 27 out of 83 ALHA77005 aggregates. ................................ ...................... 40 5 1 (U Th)/He results, fractional loss, and age calculations. ................................ ..... 62 5 2 Aggrega te fraction data and cosmogenic contribution calculations. ................... 63 5 3 Average ages and fractional loss calculations per grouped fractions. ................ 64 6 1 Zagami shock temperatures from thermal modeling ................................ ........... 83 6 2 ALHA77005 shock temperatures from thermal modeling. ................................ .. 84

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8 LIST OF FIGURES Figure page 2 1 Two pieces of Zagami. ................................ ................................ ....................... 29 2 2 ALHA77005 in situ on Antarctica ice. ................................ ................................ 29 4 1 Zagami rock chip sample ................................ ................................ .................... 41 4 2 ALHA77005 samples. ................................ ................................ ......................... 41 4 3 Zagami single phosphate aggregate. ................................ ................................ 42 4 4 ALHA77005 single phosphate aggregate. ................................ .......................... 42 4 5 Zagami aggregates for group Z103 113. ................................ ............................ 43 4 6 ALHA77005 aggregates for group A41 56 ................................ ......................... 44 4 7 Zagami aggregates for group Z01 20. ................................ ................................ 45 4 8 Zagami BSE image fld005_i mg02 and histogram. ................................ .............. 46 4 9 ALHA77005 BSE image fld003_img01 and histogram. ................................ ...... 47 4 10 SEM composite image of thin section Zagami 47 09 1. ................................ ....... 48 4 11 SEM composite image of thin section ALHA77005 120. ................................ .... 49 4 12 Zagami aggregates for group Z21 40. ................................ ................................ 50 4 13 Zagami aggregates for group Z41 60. ................................ ................................ 51 4 14 Zagami aggregates f or group Z61 80. ................................ ................................ 52 4 15 Zagami aggregates for group Z81 92. ................................ ................................ 53 4 16 Zagami aggregates for group Z93 102. ................................ .............................. 53 4 17 Zagami aggregates for grou p Z114 123. ................................ ............................ 54 4 18 Zagami aggregates for group Z124 134. ................................ ............................ 54 4 19 Zagami aggregates for group ZAG01. ................................ ................................ 55 4 20 Zagami aggregates for group ZAG234 ................................ ............................... 55 4 21 Zagami aggregates for group ZAG05. ................................ ................................ 5 5

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9 4 22 ALHA77005 aggregates for group A01 20. ................................ ........................ 56 4 23 ALHA77005 aggregates for group A21 40. ................................ ........................ 57 4 24 ALHA77005 aggregates for group AHp123. ................................ ....................... 58 4 25 ALHA77005 aggregates for group AHp45. ................................ ......................... 58 5 1 Helium 4 versus Age plots for Zagami and ALHA77005. ................................ ... 65 6 1 Thermal Modeling for Zagami ................................ ................................ ............. 85 6 2 Thermal Modeling for ALHA77005 ................................ ................................ ..... 86

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10 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 PHOSPHATE (U TH)/HE THERMOCHRONOLOGY OF ZAGAMI AND ALHA77005 MARTIAN METEORITES By Annette Emily Farah August 2011 Chair: Kyoungwon Kyle Min Major: Geology Martian meteorites bring a wealth of information pertaining to the evolution of the Martian lithosphere and atmosphere, as well as the ejection of meteorites from Mars and their delivery to Earth. Zagami and ALHA77005 are two Martian meteorites (shergottites) with similar composition s and crystallization ages, yet different thermal histories. To better constrain the shock history of Zagami and ALHA77005, I app lied (U Th)/He thermochronology to multiple aggre gates of merrillite (phosphate) from both Martian meteorites. After identifying phosphate aggregates using SEM ( Scanning Electron Microscopy ) the samples were divided into groups consisting of 5 20 grains per group based on their size. The (U Th)/He dat a obtained from these groups yield ages that range between 19.8 Ma and 202.4 Ma for Zagami, and 5.9 Ma to 78.2 Ma for ALHA77005. For Zagami, the ages from groups of larger aggregates (~111 202 Ma ) are older than the ages from groups of smaller ones (~20 2 00 Ma). To understand the age distribution, the textures of phosphate aggregates were investigated using SEM BSE (Back Scattered Electron) imaging and compositional (chemistry) maps The phosphate aggregates used for (U Th)/He dating have similar 2D su rface areas of phosphate, but the larger aggregates have thicker attached non phosphatic phases.

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11 The observed textures and age size relationship suggest that the large phosphate aggregates experienced the least amount of alpha recoil loss and yield the m ost reliable ages. To estimate the peak shock temperatures reached by these meteorites during the ir ejection from Mars, a simple volume diffusion model was applied. The most reliable ages of Zagami and ALHA77005 are converted to helium fractional losses ( f He ) of 0.17 and 0.94, respectively, with an assumption that the ejection related shock is completely responsible for the observed He loss The diffusion domain radii, a of Zagami and ALHA77005 were estimated a t 0.05 6 m, and 5 15 m, respectively through detailed image analysis of 56 phosphate grains in two thin sections For the volume diffusion modeling, these estimates ( f He a ) as well as other physical parameters related to the parent meteoroids were combined wit h general He diffusion properties in merrillite For Zagami, average peak shock temperatures of 213 C were obtained at a diffusion domain radius The results are comparable to, or slightly higher, than previou s values of 220 50 et al. 2001) when converted using the average temperature of space ( 70 at the time of ejection from Mars However the new estimates are apparently higher than the converted estimates of 70 5 C (Fritz et al. 2005). For ALHA77005 the models produced average peak temperatures of 520 C respectively These peak shock temperatures are consistent with previous suggestions of 450 600 et al. 2001) and 800 200 C (Fritz et al. 2005)

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12 CHAPTER 1 INTRODUCTION Martian meteorites are known to have experienced shock metamorphism during their ejection from Mars. The ejection dynamics of Martian meteorites have been studied through petrologic investigation s (McSween, 1984, 1985; Meyer, 2003), shock experiments (St ffler et al. 1986; St ffler, 2000; El Goresy et al. 2000a, 2000b), numerical simulations (Melosh, 1985; Hartmann and Neukum, 2001; Head et al. 2002; Artemieva and Ivanov, 2004), cosmic ray expo sure ages (Eugster et al. 1997; Eugster, 2003) and thermochronologic constraints (Weiss et al. 2002; Shuster and Weiss, 2005; Min and Reiners, 2007). Ejection processes and delivery mechanisms of meteorites from Mars, through space, to Earth have also b een discussed (Melosh, 1985, 1995; Gladman, 1997; Nyquist et al. 2001; Fritz et al. 2005). To better constrain the timing of such events and understand their dynamic processes, (U Th)/He thermochronology was applied to phosphates from rock chip samples of Martian meteorites Zagami and ALHA77005. Background of Methodology Thermal histories of meteorites provide a basis for further comprehending the origin and heat radiation of the early solar system as performed through various studies including interste llar grains and Pb isotope ages from chondrites (Alexander, 2001; Hutchison et al. 2001; Hutchison, 2004), accretion processes of the planets through Rb Sr and U Pb systems (Hutchison, 2004) as well as shock impact dynamics (Melosh, 1985) and evidence of early hypervelocity impacts (Shu et al. 2001; Hutchison, 2004). The most widely used thermochronometers are K/Ar and 40 Ar/ 39 Ar which can constrain cooling histories in the range of ~550 C down to ~200 C (McDougall et al. 1999).

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13 These methods have been applied to a range of meteorites including chondrites (Turner et al. 1978; Bogard et al. 1987; Trieloff et al. 2003), lunar samples (Turner et al. 1971), and Martian meteorites (Ash et al. 1996; Turner et al. 1997; Bogard and Park, 2008; Bogard, 2009 ; Bogard et al. 2009; Cassata et al. 2010) mainly to document cooling from formation or shock metamorphism. In contrast, (U Th)/He methods have been widely applied to terrestrial samples to constrain thermal histories below ~200 C which are directly re lated to exhumation histories (Reiners and Ehlers, 2005). With relatively low closure temperatures of ~70 C for apatite (Farley 2002; Wolf et al. 1996), ~190 C for zircon (Reiners 2005), and ~200 C for titanite (Reiners et al. 1999) at typical grain d imensions and terrestrial cooling rates, the (U Th)/He methods can provide important information on the relatively low temperature thermal histories of terrestrial and extraterrestrial samples. Another important applicati on of the (U Th)/He thermochronom eter is to constrain transient thermal events such as wildfire (Mitchell and Reiners 2003) or shock metamorphism on meteorites (Min 2005). In fire prone or wildfire areas, (U Th)/He methods applied to apatite and zircon minerals can determine thermal hi stories of exposed bedrocks (Mitchell and Reiners, 2003) or detached rock fragments (Reiners et al. 2007). Evidence of shock registered in meteorites and impact craters can be further supported through the study of their (U Th)/He systems (Min and Reiner s, 2007) Alth ough the (U Th)/He thermochronometer has been widely applied to terrestrial samples for their low temperature thermal histories, its applications to meteorites are limited probably due to the traditional concept that 4 He can easily diffuse ou t of the system resulting in unreasonably young (U Th)/He ages. This concept is mainly from

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14 whole rock data requiring more thorough examination. Single and multi grain analysis, contrary to the whole rock method, focuses on selected minerals of similar physical dimensions, producing more consistent (U Th)/He ages. The contribution of radiogenic 4 He to the total measured 4 He is much larger for mineral based methods than for whole rock dating, thus yielding significantly more precise ages (Min, 2005a, 200 5b). Some examples of (U Th)/He dating applied to meteorites include (1) whole rock age calculations based on published U, Th and 4 He concentrations of ordinary chondrites (Watson and Wang, 1991), (2) single grain phosphate ages on Acapulco, an achondriti c meteorite with a chondritic texture (Min et al. 2003), (3) single grain phosphate data of Martian meteorite Los Angeles (Min et al. 2004) and ALH 84001 (Min and Reiners, 2007), (4) He loss data for whole rock and mineral separates from a suite of Marti an meteorites (Schwenzer et al. 2008), and (5) single grain phosphate ages from St. Severin LL6 chondrite (Min et al. 2011). The (U Th)/He method s can be efficiently applied to Martian meteorites because the He concentration in the Martian atmosphere i s low, causing only a limited amount of 4 He to be injected into Martian meteorites during shock events. However such c ontamination of atmospheric gas has been a serious issue for 40 Ar/ 39 Ar dating for Martian meteorites (Bogard and Garrison, 1999). Multip le radiogenic and cosmogenic isotope systems of Martian meteorites can provide a chronological sequence of events on Mars. For exampl e ALH 84001 formed at ~4.5 Ga ( from Sm/Nd system: Jagoutz et al. 1994; Nyquist et al. 1995), experienced a severe shock at ~4.0 Ga ( from 40 Ar/ 39 Ar system: Treiman, 1995), and ejected from Mars at ~15 Ma ( from cosmogenic isotope systems: Treiman, 1998 ). Furthermore, implications such as timing of impact

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15 metamorphism and maximum shock temperatures can be evaluated to better understand their thermal histories. Purposes of This Study The goals of this research are to (1) improve fundamental aspects of the (U Th)/He thermochronometer in application to shocked meteorites and (2) constrain temperature conditions of shock metamorp hism for two Martian meteorites, Zagami and ALHA77005. For these goals, I performed (U Th)/He analysis on a few hundred phosphate grains, estimated their diffusion domain and generated thermal models to infer thermal histories. For thermal modeling, I e xamined the sensitivity of the final results and compared them with previously reported parameters

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16 CHAPTER 2 BACKGROUND Types of Meteorites Meteorites are classified into iron, stony, or stony iron groups based on their metallic iron nickel (Fe Ni) and silicate (silicon oxygen, Si O) contents. Iron meteorites are made primarily of iron nickel metal alloy; stony meteorites consist primarily of silicate minerals; and stony iron meteorites contain roughly a mixture of equal proportions of iron nickel metal and silicates. A brief explanation of each classification follows. Iron meteorites contain two major minerals: kamacite (nickel poor iron of 5 15% Ni ) and taenite (nickel rich iron >15 20% Ni). Trace amounts of other elements are also present in iron meteorites measuring below 500 ppm (Shirley and Fairbridge, 1997). I ron meteorites are further classified in two ways: the appearance of a Widmannsttten pattern and concentrations of gallium (Ga), germanium (Ge), and iridium (Ir). The presence or absen ce of a Widmannsttten pattern is used as an older structural classification method depending on varying amounts of nickel; a meteorite with no pattern and low nickel content is identified as a hexahedrite, and one with no pattern and high nickel content i s known as an ataxite. However, a meteorite with an average to high amount of nickel (kamacite) separated by varying lamellae of taenite will exhibit a Widmannsttten pattern. The second classification method is based on trace element concentrations of N i, Ga, Ge, and Ir. The amount of these trace elements and other diagnostic features, present in the meteorite determines further class divisions, i.e. IIAB, IVB etc. For example iron meteorites from group IVB are Ir rich and Ga and Ge poor (less vola tile) while group IIAB is Ga and Ge rich (more

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17 volatile) with a negative correlation between Ir and Ni reflecting fractional crystallization (Hutchison, 2004). Both classification schemes relate to the chemistry of iron meteorites and whether an element prefers to occur as a metal (siderophile elements) versus a silicate (lithophile elements) or sulfide (chalcophile elements). Stony meteorites are the most common class of meteorites and consist principally of silicate minerals and are divided into chondri tes and achondrites depending on the presence of chondrules (near spherical masses of silicates: Hutchison, 2004). Chondrites have chemical compositions closely resembling the atmosphere of the Sun and exhibit textures indicating no melting has occurred since their formation (Hutchison, 2004). Achondrites lack chondrules and are generally different in chemical composition than chondrites. However, a group of rare meteorites, known as primitive achondrites, have chondritic mineralogies and resemble chond ritic chemical compositions suggesting minimal melting (Hutchison, 2004), yet are free of chondrules. Stony iron meteorites are the least common type of meteorites and consist roughly of a mixture of equal proportions of iron nickel metal and rock (silic ates) commonly referred as siderolites Stony iron meteorites composed of a continuous matrix of Fe Ni metal with crystal fragments of olivine are pallasites. And those composed of brecciated mixtures of metal plus olivine and recrystallized silicates a re mesosiderites. Martian Meteorites Martian meteorites are achondritic stony igneous rocks originally categorized into one of three classes: Shergottite, Nakhlite, and Chassignite (SNC). The SNC meteorites are named after their discovery from locations in Shergotty (in the Gaya district of Bihar, India) Nakhla ( Nakhla region of Abu Hommos in Alexandria, Egypt)

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18 and Chassigny (Chassigny, Haute Marne in France). They are categorized based on their petrologic and chemical characteristics : shergottites a re basalts and lherzolites, nakhlites are olivine clinopyroxenites, and chassignites are dunites (McSween, 1985, 1994; Hutchison, 2004). Currently there are 24 shergottites, 7 nakhlites, and 2 chassignites known One other Martian meteorite, ALH 84001, i s an orthopyroxenite with its own category. Basaltic shergottites are composed primarily of pyroxene and maskelynite (shocked plagioclase) with accessory olivine, sulfides, phosphates and oxides (McSween, 1985, 1994; Hutchison, 2004). The app earance of fi ne grain textures and aligned pyroxene crystals suggest s that these rocks have magma tic composition s and formed from quickly cooled lava (McSween, 1994; Treiman et al. 2000). Lherzolitic shergottites are rich in olivine and composed of chromite surrounde d by orthopyroxene crystals with interstitial maskelynite, phosphate and other accessory oxides (McSween, 1985; Treiman et al. 2000). These rocks originate from a plutonic environment (McSween, 1994) based on early formation of (olivine cumulate) crystal s (Hutchison, 2004) which slowly settled and cooled in a deep magma chamber (Treiman et al. 2000). Early Sm Nd studies produced an isochron age of ~ 1.3 Ga (Wooden et al. 1982; Shih et al. 1982) interpreted as the time of formation. Recent isotopic d ata provide a younger age of ~180 Ma as the time of shock metamorphism from an explosive event. Nakhlites are cumulate rocks from basaltic magma consisting of magnesian augite with less abundant Fe rich olivine (McSween, 1994; Treiman, 2005). Inclusions in olivine and augites suggest trapped magma, and deposits of iddingsite and salt

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19 minerals indicate the presence of water (Treiman, 2005). These rocks may have formed from a lava flow, shallow intrusion or subsurface sill (Hutchison, 2004; Treiman, 2005) Chassignites are predominant ly olivine cumulates abundant in Mg for NWA 2737 and Fe for Chassigny (Meyer, 2003; Beck et al. 2006). Both exhibit similar petrographic and chemical compositions to suggest their parental melts are related, although the me lt from NWA 2737 is less evolved (Beck et al. 2006). Following these types of identifications, Martian meteorites continue to be scrutinized and re examined. While many Martian meteorites fall within the existing SNC groups, others exhibit characterist ics where the SNC classification may no longer apply. ALH 84001 and ALHA77005 are two examples where their association to other Martian meteorites may not exactly fit the SNC classification. For example, ALH 84001 was originally classified as a diogenite for its orthopyroxenite cumulate composition with minor chromite, Na rich plagioclase, and Fe Mg Ca carbonates (MacPherson, 1985; Mittlefehldt, 1994) S ubsequently ALH 84001 was categorized into the HED (howardites, eucrites, and diogenites) group most likely originating from the asteroid Vesta. However later mineralogic and petrologic studies showed that the existence of these carbonates, along with the presence of pyrite, Na rich plagioclase (and maskelynite), and an enrichment in Fe 3+ distinguish i t from diogenites (Mittlefehldt, 1994). Furthermore, other features in ALH 84001 such as oxygen isotopic composition (Clayton, 1993), maskelynite composition, and shock textures closely resemble those of Martian meteorites, albeit not necessarily belongin g to any one of the SNC groups (Mittlefehldt, 1994; Swindle, 1995). Therefore, this one meteorite is classified into a

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20 separate group of Orthopyroxenite and contributes another addition to the group: SNCO. Another example is ALHA77005, the parent body o f one of the samples used for this research. ALHA77005 is a member of the shergottite group. Mineral and chemical compositions in ALHA77005 are similar to the other shergottites, however its cumulate texture and plutonic origin make it noticeably differe nt from the basaltic shergottites Since its crystallization and cosmic ray exposure ages are comparable to basaltic shergottites, ALHA77005 was classified a lherzolitic shergottite. SNCO are believed to be the only Martian meteorites. While grouping M artian meteorites emphasizes their similarities, it may also ignore qualities that make them different i.e. ALHA77005 as a shergottite In this report, I use the SNC nomenclature when discussing the parent bodies of my samples to remain consistent with t heir definitions from books and published articles. Following the naming convention, the rock chip samples in this research are from two shergottites: basaltic shergottite Zagami and lherzolitic shergottite ALHA77005. However after the introduction of th is report, I will refer to them as Martian meteorites. In general, Martian meteorites closely resemble terrestrial igneous rocks. Their crystallization ages are significantly younger (~180 Ma to 1250 Ma) than most other meteorites. The only exception is ALH 84001 with a very old crystallization age of ~ 4.5 Ga (Jagoutz et al. 1994). Martian meteorites share similar properties that are highly distinctive from other meteorites in terms of their mineral chemistry, redox state, and radiometric ages (McSween 1985). They consist of mafic and ultramafic cumulate rocks with petrologic features such as poikilitic texture in olivine and chromite and

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21 glassy rims around plagioclase crystals (McSween, 1985). Such characteristics suggest relatively rapid formation from a large parent body (McSween, 1985, 1994). Their oxygen isotope composition (Clayton and Mayeda, 1996), trapped gases (Bogard and Johnson, 1983) matching the Martian atmosphere, and young formation ages (Treiman et al. 2000) indicate their origin t o be from Mars (McSween, 1985). To date, 99 Martian meteorite pieces have been discovered (Meteoritical Bulletin Database from the Meteoritical Society: http://www.lpi.usra.edu/meteor/index.php ) incl website: http://www2.jpl.nasa.gov/snc/ ; Baalke, 2006) Many of these meteorites were discovered as broken pieces of a single stone and naturally paired through petrologic and chemical investigations. Two Minerals Unique in Meteorites: Maskelynite and Merrillite Maskelynite and merrillite are two minerals frequently identified in extraterrestrial samples. Maskelynite has been naturall y detected on terrestrial samples collected from impact craters (Lambert and Grieve, 1984) showing that these Earth rocks experienced severe shock leading to the formation of the mineral (Milton and Carli, 1963). Petrologic evidence has presented maskelyn ite as a glassy form of plagioclase resulting from strong shock metamorphism below pressures of 45 GPa (Stffler, 2000; Nyquist et al. 2001; Fritz et al. 2005). Pressures in excess of 45 GPa would cause plagioclase to recrystallize (Fritz et al. 2005b) and experience shock induced fusion (Stffler, 2000) thereby exhibiting flow structures and vesiculation consistent with normal glass instead of diaplectic glass (shocked isotropic plagioclase) associated with maskelynite (Stffler, 1988). Excessive pres sure would require extreme post shock temperatures causing

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22 plagioclase to mix with melt (Stffler, 1988, 2000). Maskelynite is considered a main characteristic in all lherzolitic shergottites. Another mineral commonly found in extraterrestrial rocks is merrillite. The name was assigned to hydrogen free whitlockite to differentiate it as a separate mineral from terrestrial whitlockite (Jolliff, 2006). The chemica l composition of whitlockite is Ca 18 (Mg,Fe) 2 (PO 4 ) 12 [PO 3 (OH)] 2 and that of merrillite is Ca 18 Na 2 Mg 2 (PO 4 ) 14 (Jolliff 2006; Hughes et al. be used to describe phosphates having this chemical composition. Merrillite can be distinguished from whitlockite by two important che mical signatures: (1) lack of (OH) and (2) existence of Na (Hughes et al. 2008). Merrillite occurs only in extraterrestrial material, although synthetic merrillite has been grown for crystallographic studies (Hughes, 2008). Martian Meteorites of This Stu dy Zagami Zagami was discovered immediately after landing on Earth by a farmer on October 3, 1962 at Zagami, Katsina Province, Nigeria (Meyer, 2003). The sample weighed approximately 18.1 kg with a suggested minimum radius of ~11.3 cm (Schwenzer et al. 2 008). Figure 2 1 shows a portion of Zagami as acquired by a private collector (Haag, 2010). consisting mostly of pyroxenes (pigeonite and augite) and maskelynite. Approximately 20% of the rock is comprised of a dark mottled lithology (DML) with heterogeneously distributed iron oxide enriched pyroxenes, maskelynite, and melt pockets. Accessory minerals of sulfides, oxides, merrillite and other phosphates are also identified ( Vis tisen et al. 1992; McCoy et al. 1993 ; Wang et al. 1999 ) Zagami consists of fine grained

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23 (average grain dimension of 0.24 mm ) and coarse grained (0.36 mm) portions (Meyer, 2003). Table 2 1 lists mineralogic modes of the two Zagami lithologies and thei r grain size portions (McCoy et al. 1991, 1999; Meyer, 2003). The grain size variations of Zagami suggest a complex series of crystallization processes (Nyquist et al. 2006) possibly involving a two stage magmatic history (McCoy et al. 1991). The first stage can be represented by the (1) presence of homogeneous Mg rich, amphibole bearing melt inclusions in pyroxene crystals and (2) development of fine lamellae in pyroxene indicating slow cooling within a magma chamber (McCoy et al. 1991; Nyquist et al. 2001). The second stage can be characterized by (1) Fe rich rims in pyroxene crystals and (2) overall fine grained textures in Zagami, both suggesting rapid cooling in a moderately thick lava flow (McCoy et al. 1991; McCoy et al. 1999; Langenhorst and Poirier, 2000a). Zagami is given a cooling rate < 0.5 C /hr (McCoy et al. 1992; McCoy et al. 1999). In summary, the two stage magmatic history starts with an initial slow crystallization within a magma chamber followed by a rapid cooling during a magma flow near the surface (McCoy et al. 1999). Shock conditions were estimated to include pressures of 31 2 GPa (Stffler et al. 1986) using refractive index data of maskelynite (Binns, 1967) and a post shock temperature of 220 50 C (Stffler, 2000; Nyq uist et al. 2001). From analyses of black glassy veins, the appearance of high pressure phases (Malavergne et al. 2001) such as stishovite and KAlSi 3 O 8 hollandite (Langenhorst and Poirier, 2000a, b) suggest a minimum pressure and temperature of ~30 GPa and 2400 2500 C (Langenhorst and Poirier, 2000b) respectively. Furthermore, Langenhorst and Poirier (2000a, b)

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24 conclude d the black glassy veins indicate rapid shear melting during shock. From petrographic studies and refractive index data, Fritz et al. (2005) deduced shock pressures of 29.5 0.5 GPa and post shock temperature of 70 5 C Zagami yielded relatively consistent radiometric ages (Table 2 2). Borg et al. (2003) determined U Th Pb ages from whole rock samples and mineral separ ates, and concluded that the parent body magma formed during the early stage of planet formation (~4550 Ma) followed by differentiation at a much younger period (163 4 Ma). Bouvier et al. (2005) reported a Sm Nd mineral isochron age of 155 Ma and Lu Hf isochron age of 185 Ma from analyses performed on Zagami whole rock, maskelynite and pyroxene separates. Whole rock Pb isotope analyses applied to Zagami resulted in an older isochron age of 4.048 0.017 Ga, while 207 Pb/ 206 Pb and 204 Pb/ 206 Pb compositions of other shergottites including Zagami resulted in a crystallization age of 4.0 Ga (Bouvier et al. 2005). Nyquist et al. (2006) reported Rb Sr isochron ages of 166 12 Ma for coarse grained and 177 9 Ma for fine grained lithologies of Zagami. ALHA77 005 ALHA77005 (Figure 2 2) was discovered in Victoria Land, Allan Hills of Antarctica in 1977 (Yanai, 1981; Meyers, 2003; Harvey, 2003) weighing approximately 0.483 kg and measuring 9.5 cm 7.5 cm 5.25 cm (Score et al. 1981; Nishiizumi et al. 1986) ALHA77005 is classified as a lherzolitic shergottite with a gabbroic, olivine rich composition. It mainly consists of olivine (~52%), low Ca pyroxene (~26%), high Ca pyroxene (~11%), maskelynite (~8 9%), and other accessory minerals (~2% ) including chro mite, ilmenite, merrillite, and troilite (McSween et al. 1979a, b; Ma et al. 1981; Mason, 1981; Meyer, 2003 ). The mode of m errillite is only ~0.4% (Treiman et al.

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25 1994). The olivine is anhedral and commonly occurs in poikilitic texture with pyroxene ( Shearer et al. 2009). Veins and melt pockets, commonly found in olivine crystals, show wide compositional variations for P 2 O 5 (0 8 wt%), SiO 2 (35 71 wt%), and REE (e.g., Nd = 0.1 to 26 ppm) suggesting a variable contribution of merrillite within the imp act melt (Edmunson et al. 2005). Trace element patterns analyzed for maskelynite and plagioclase suggest fractional crystallization occurred in a closed system environment (Wadhwa et al. 1994). Petrographic features observed in ALHA77005 have been consi dered particularly interesting with respect to shock metamorphism. Most of the olivine in ALHA77005 has a yellowish brown discoloration exhibiting homogeneous chemical compositions within each grain as a result of re equilibration upon cooling (Ikeda 199 4) and shock induced oxidation (Beech et al. 2008). From textural observations in olivine, pyroxene and maskelynite, McSween et al. ( 1980 ) suggested shock pressures of 35 50 GPa. Based on re evaluation of the textures, Fritz et al. (2005) suggested pe ak shock pressures of ~45 55 GPa with post shock temperatures of 800 200 C (Fritz et al. 2005). Shock experiments combined with observed refractive indices for maskelynite suggest a peak shock pressure of 43 2 GPa (Stffler 2000; Nyquist et al. 200 1) and post shock temperature of 450 et al. 2001). Such high pressure and temperature conditions of shock can also be assumed from evidence of partial recrystallization (Fritz et al. 2005) along the rim of shock induced plagioclase melt p ockets (Beech et al. 200 8). These observations suggest lasting for 0.5 h as illustrated from annealing experiments (Ostertag, 1982; Beech et al.

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26 2008). Strongly vesiculated plagioclase melt suggests shock pre ssures were in excess of 45 GPa (Fritz et al. 2005). Even higher shock pressure conditions of 45 80 GPa were proposed based on deformation features in olivine and pyroxene, skeletal crystals of pigeonite with residual glass, and common occurrence of melt veins (Boctor et al. 1999). Based on these shock features, ALHA77005 has been considered the most heavily shocked Martian meteorite ( McSween and Stffler, 1980; Ikeda, 1994; Nyquist et al. 2001; Fritz et al. 2005 ). Radiometric ages of this meteori te are scatter ed (Table 2 2 ). Among these, modern Rb Sr and Sm Nd studies indicate a preferred crystallization age of 179 5 Ma (Nyquist et al. 2001; Schwenzer et al. 2008). The apparently older K Ar and 40 Ar/ 39 Ar ages are likely caused by (1) incorpo ration of a sufficient amount of unsupported 40 Ar from the Martian atmosphere or mantle and (2) excessive production of cosmogenic 36 Ar and 40 Ar (Nyquist et al. 2001; Bogard and Garrison, 1999).

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27 Table 2 1. Mineralogical composition for Zagami Normal Z agami Dark mottled Lithology Fine grain Coarse grain Fine grain Coarse grain Pyroxene 74 78 80 71 77 80 Maskelynite 18 21 10 22 9 14 Oxides 2 2 1 3 Sulfides < 1 < 1 < 1 < 1 Phosphates 1 1 1 2 Mineralogical composition (in vol. %) for Normal Zagami and Dark mottled lithologies of Zagami. Sources: McCoy et al. 1991; McCoy et al. 1999; and Meyer 2010.

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28 Table 2 2. Radiometric ages for Zagami and ALHA77005 Radiometric Ages (Ma) 40 Ar/ 39 Ar K Ar Rb Sr Sm Nd 232 Th 208 Pb 238 U 206 Pb Preferred Age Zagami 209 [1] 166 6 [3] 166 12 [3] 229 8 [6] 156 6 [3] 177 3 [7] 242 [2] 180 4 [4] 180 37 [5] 230 5 [6] 183 6 CG [5] 186 5 FG [5] ALHA77005 1100 100 [8] 1330 130 [9] 154 6 [10] 135 40 [10] 179 5 [7] 3500 [2] 185 11 [11] 1 73 6 [11] 185 12 [4] CG=coarse grained and FG=fine grained [1] Bogard and Park (2008). [2] Bogard and Garrison (1999). [3] Borg et al. (2005). [4] Shih et al. (1982). [5] Nyquist et al. (1995) [6] Chen and Wasserburg (1986). [7] Nyquist et a l. (2001). [8] Schaeffer et al. (1981). [9] Miura et al. (1995). [10] Jagoutz (1989). [11] Borg et al. (2002).

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29 Figure 2 1. Two pieces of Zagami with a combined weight of 2,794 grams (~6.2 lbs) and measuring 158 x 165 x 70 mm Source: Haag ( 1991). Fi gure 2 2. ALHA77005 in situ on Antarctica ice. Source: Yanai (1981).

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30 CHAPTER 3 (U TH)/HE THERMOCHRONOL OGY (U Th)/He thermochronology relies on the accumulation of radiogenic 4 He produced from radioactive decay of parent isotopes 238 U, 235 U, and 232 Th. Although 147 decay producing 4 He atoms, its contribution to the total radiogenic 4 He is less significant than U and Th for most samples. U and Th follow a decays) producing multiple inter mediate daughter isotopes. The decay from one intermediate isotope to the next is performed through the release of alpha or beta particles, and in some cases gamma particles. For instance, the parent isotope 238 U has 8 alpha decay steps in its decay chain with a final step to the stable isotope 206 Pb. During the alpha decay, one alpha particle ( 4 He nucleus: 2 protons and 2 neutrons) is emitted from the nucleus of the parent atom. The produced He nucleus couples with electrons becoming a 4 He atom. A sim ilar process occurs for 235 U and 232 Th with 7 and 6 alpha decays, respectively. In contrast, 147 Sm yields only one alpha particle during its decay to stable 143 Nd. In various types of meteorites, U and Th are mainly concentrated in phosphate minerals such as merrillite and apatite. Once the U Th Sm 4 He abundances are determined from a sample, an age can be determined from the following equation: 4 He = 8 238 U [exp(t 238 ) 1] + 7 235 U [exp(t 235 ) 1] + 6 232 Th [exp(t 232 ) 1] + 147 Sm [exp(t 147 ) 1]. If the produced 4 He atoms are completely preserved in the system, the resulting (U Th)/He age should indicate the timing of crystallization. However, 4 H e can easily diffuse out of the system because of (1) its nature as a noble gas being inert and not bound in crystal structures, and (2) its small atomic size allowing it to pass through

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31 crystal structures. To account for the amount of remaining 4 He prese nt in a system, the temperatures. Because the diffusion rate increases with temperature, (U Th)/He ages represent the timing when the mineral passed through its closure temperature. This T c ( McDougall and Harrison 1999) The closure temperature can be expressed by the following equation (Dodson, 1973): T c = R / [ E ln ( A D o / a 2 ) ], where R is the gas constant, E D diminishes, a is the diffusion domain size, A is the geometry factor, and D o is the pre exponential factor (Dodson, 1973). Following t his equation, the closure temperature is sensitive to various factors such as the diffusion domain size, crystal geometry, and cooling rate. To estimate the closure temperature of terrestrial apatite, Farley (2000) performed high precision stepped heating experiments on Durango apatite which is the best understood standard for (U Th)/He dating. From these experiments, Farley (2000) calculated a closure temperature of approximately 68C assuming a cooling rate of 10C/Ma for a diffusion domain radius of 10 0 m. If a rock is exposed to sufficiently high temperatures, the diffusion rate of daughter elements would be larger than the production rate of radioactive decay, resulting in essentially no accumulation of the daughter s (open system). If a rock is at s ufficiently low temperatures, the system almost completely retains the daughter products (closed system). Between these two end member conditions, the thermochronometer would experience a gradual transition from one to the other, resulting in partial rete ntion of the

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32 daughters ( McDougall and Harrison, 1999) The temperature range of such a transition (or converted depth range at assumed geothermal gradient) is known as a partial retention zone (PRZ) ( McDougall and Harrison, 1999; Reiners, 2002 ). In a tra nsition from an open system to a closed system, an apparent age can be extrapolated showing a point in time when the daughter product begins to accumulate at a constant rate. Other U and Th carriers such as titanite and zircon function as good (U Th)/He thermochronometers characterized by their helium diffusion properties and well defined closure temperatures (Reiners and Farley, 1999; Reiners, 2005). As a result, a wider range of low temperature (40 C 240 C ) thermal histories of terrestrial rocks were constrained using (U Th)/He as a thermochronometer (Reiners, 2002; Stockli, 2005). For meteorites, low temperature thermal histories are poorly constrained mainly due to the lack of He diffusion data for extraterrestrial merrillite, a major reservoir of U and Th in meteorites. However, a recent study on He diffusion properties in merrillite and apatite (Min et al. 2011) may provide a basis for more reliable constraints on low temperature thermal histories for meteorites. According to the study (Min et al. 2011), a closure temperature for merrillite is calculated as ~104C (at an assumed cooling rate of 2C/Ma and a diffusion domain radius of 59 m), which is significantly higher than that of apatite. These new diffusion properties are used in this stu dy for thermal modeling of Zagami and ALHA77005.

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33 CHAPTER 4 ANALYTICAL METHODS Thin sections and rock fragments were examined with optical and stereo microscopes and a scanning electron microscope (SEM). To obtain precise (U Th)/He ages, phosphate bearin g aggregates were separated from meteorite rock fragments, degassed using a helium extraction line dissolved and analyzed using inductively coupled plasma mass spectrometry (ICP MS) for U Th Sm 4 He measurements. Below is a detailed account of the methods Small rock chips of Zagami (~15 mm 10 mm 2 mm, Figure 4 1 ) and ALHA77005 (~9 mm 7 mm 2 mm, 0.27 gram, Figure 4 2 ) were carefully crushed, and the resulting fragments were sieved. The final separates were scanned using SEM in order to identify ph osphate bearing separates Since these separates are pieces of the rock (or meteorite), they consist of other minerals, including phosphate grains. Throughout this report, I will refer to these phosphate bearing separates as aggregates or phosphate aggre gates. A total of 248 phosphate aggregates were selected: 165 from Zagami and 83 from ALHA77005 (Tables 4 1 4 2 4 3 and 4 4 ) Each phosphate aggregate was scanned to reveal the chemical composition of each sample. The chemical data from the scans are displayed as 2 dimensional images (or chemical maps) and used to determine morphological relationships between phosphates and other phases included in the same aggregate. Chemistry maps and results from energy dispersive spectroscopy (EDS) spot analyses for two phosphate aggregates are shown in Figure 4 3 for Zagami and Figure 4 4 for ALHA77005. The aggregates from Figures 4 3 and 4 4 measure between 150 250 m (Figure 4 5 ) and 63 4 6 ), respectively. Chemistry maps for all

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34 selected aggre gates in this study are found in Appendix A for Zagami and Appendix B for ALHA77005. Regional and spot analyses from chemical mapping detect high P and Ca peaks strongly associated with phosphates, and moderate signals of S, O, Al, and Mg suggest the exis tence of other phases (e.g., silicate) in these aggregates (Figures 4 3 and 4 4 ). From the presence of Na with Mg, together with high signals of P and Ca, these phosphates are identified as merrillite [Ca 18 Na 2 Mg 2 (PO 4 ) 14 ]. From the analyses of the 248 phosphate aggregates, the majority of the identified phosphates are classified as merrillite. After petrographic examinations, one to twenty phosphate aggregates were wrapped in Pt or Nb tubes and subsequently sealed to avoid any sample loss during analyt ical procedures. A total of 17 packets were arranged: twelve for Zagami and five for ALHA77005. Three Zagami samples (ZAG01, ZAG234, ZAG05: Table 4 2 ) and two ALHA77005 samples (AHp123, AHp45: Table 4 4 ) were wrapped in Pt tubes. The remaining nine Zaga mi samples (Z01 20, Z21 40, Z41 60, Z61 80, Z81 92, Z93 102, Z103 113, Z114 123, Z124 134 Tables 4 1 and 4 2 ) and three ALHA77005 (A01 20, A21 40, A41 56 Table 4 3 ) were wrapped in Nb tubes. Size fractions, number of aggregates, and names corresponding to each packet are listed in Tables 4 1 and 4 2 for Zagami and Table s 4 3 and 4 4 for ALHA77005. As indicated in these t ables, phosphate aggregates were grouped based on their sizes and measured based on their overall 4 He and U Th Sm concentrations (Table 5 1 ) The sample packets were loaded in a stainless steel planchette and degassed under high vacuum using a diode laser. The extracted gas was purified using a cryotrap and NP 10 getter for 3 Zagami (ZAG01, ZAG234, ZAG05: Table 4 2 ) and 2

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35 ALHA77005 (AHp 123, AHp45: Table 4 4 ) packets. For all the remaining samples, only NP 10 getter was used. The 4 He/ 3 He ratios of blanks with and without the cryogenic trap are essentially identical. Extracted gas from each of the 17 packets was then spiked with 3 He and analyzed using a quadrupole mass spectrometer. Each of the 17 packets was dissolved in 5% nitric acid, spiked, and its U Th Sm abundances were determined using ICP MS. O riginally each phosphate aggregate was to be analyz e d separately (single grain inst ead of multi grain analysis ) to gain high spatial resolution. However, preliminary 4 He analyses for 33 ALHA77005 single aggregates at the University of Arizona yielded very low signals, commonly less than 150% of procedural blanks. Based on these prelimi nary data, the analy ses in this study consist of groups of multi ple grains (aggregates) T herefore the resulting ages represent pooled ages of the gr oups of aggregates. Phosphorus chemistry maps for grouped aggregates Z103 113, A41 56, and Z01 20 are show n in Figures 4 5 4 6 and 4 7 respectively For the remaining grouped aggregates, phosphor us chemistry maps are shown in F igures 4 12 to 4 21 for Zagami and Figures 4 22 to 4 25 for ALHA77005. Each figure represents one packet of samples used for (U Th )/He analysis. For three more packets (ZAG 0 5, ZAG234 and ZAG 0 1 Figures 4 19 4 20 and 4 21 ), chemistry mapping was not performed and only b ack scattered electron ( BSE) images are displayed. From these chemistry maps, the two dimensional distr ibution of phosphate grains was deduced in each aggregate. It is noteworthy that aggregates of different sizes may have similar dimensions as the phosphate grains. In this case, the large aggregates have other phases attached to

PAGE 36

36 phosphates, thus more efficiently c apturing radiogenic 4 He atoms which were energetic immediately after radioactive decay. Two Zagami and one ALHA77005 thin sections were examined under a stereo microscope for basic petrography and fracture free areas. BSE images and chemical scans were p roduced using SEM For examples of BSE images of merrillite from thin sections refer to Figure 4 8 for Zagami and Figure 4 9 for ALHA77005. For composite views of full thin section scans, refer to Figures 4 10 and 4 11 for Zagami and ALHA77005, respecti vely. More BSE images of merrillite are presented in Appendix C for Zagami and Appendix D for ALHA77005.

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37 Table 4 1. List of 80 out of 165 Zagami aggregates used in this study ZAGAMI 75 125 150 packets Z01 20 06 grn01 26 grn01 14 grn01 27 grn01 15 grn01 30 grn01 16 grn01 31 grn01 18 grn01 33 grn01 18 grn02 33 grn02 22 grn01 33 grn03 22 grn02 36 grn01 22 grn03 38 grn01 23 grn01 41 grn01 Z21 40 42 grn01 52 grn02 43 grn01 54 grn01 45 grn01 55 grn01 45 grn02 58 grn01 46 grn01 58 grn02 47 grn01 60 grn01 47 grn02 64 grn01 48 grn01 65 grn01 51 grn01 65 grn02 52 grn01 65 grn03 Z41 60 67 grn01 20 grn01 44 grn01 68 grn01 25 grn01 51 grn01 68 grn02 25 grn02 51 grn02 69 grn01 28 grn01 51 grn03 71 grn01 29 grn01 64 grn01 72 grn01 39 grn01 64 grn02 72 grn02 39 grn02 Z61 80 63 grn01 75 grn01 4 grn01 63 grn02 75 grn02 4 grn02 63 grn03 86 grn01 5 grn01 66 grn01 86 grn02 8 grn01 66 grn02 106 grn01 68 grn01 112 grn01 68 grn02 116 grn01 72 grn01 117 grn01 Each group (Z01 20, Z21 40, Z41 60, and Z61 80) co nsists of agg regates listed in each corresponding row by their fraction size.

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38 Table 4 2. List of 85 out of 165 Zagami aggregates used in this study ZAGAMI 150 125 packets packets Z81 92 14 grn01 20 grn01 ZAG01 Zag0011 001 15 grn01 21 grn01 Zag0022 001 16 grn01 21 grn02 Zag0023 002 17 grn01 21 grn03 Zag0026 001 17 grn02 22 grn01 Zag0038 002 18 grn01 23 grn01 ZAG234 Zag0040 001 Z93 102 24 grn01 31 grn01 Zag0041 001 27 grn01 31 grn02 Zag0042 001 28 grn01 31 grn03 Zag0046 001 30 grn01 31 grn04 Zag0047 001 30 grn02 32 grn01 Zag0047 002 Zag0047 00 3 Z103 113 37 grn01 42 grn02 Zag0047 004 37 grn02 43 grn01 Zag0049 001 39 grn01 43 grn02 Zag0052 001 41 grn01 45 grn01 Zag0059 001 41 grn02 45 grn02 Zag0059 002 42 grn01 Zag0059 003 Zag0060 002 Z114 123 46 grn01 52 grn01 Zag0060 001 47 grn01 52 grn02 48 grn01 52 grn03 ZAG05 Zag0004 001 48 grn02 54 grn01 Zag0011 002 50 grn01 54 grn02 Zag0022 001 Zag0022 002 Z124 134 56 grn01 Zag0065 001 56 grn02 Z ag0069 001 57 grn01 Zag0070 001 58 grn01 Zag0083 001 58 grn02 Zag0083 002 58 grn03 Zag0093 001 59 grn01 Zag0100 001 59 grn02 61 grn01 61 grn02 61 grn03 E ach group (Z81 92, Z93 102, 103 113, Z114 123, Z124 134, ZAG01, ZAG234, and ZAG05) consists of aggregates listed in each corresponding row by their fraction size.

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39 Table 4 3. List of 56 out of 83 ALHA77005 aggregates used in this study ALHA77005 63 packets A01 20 02 grn01 15 grn02 03 grn01 15 grn03 03 grn02 16 grn01 04 grn01 16 grn02 04 grn02 17 grn01 08 grn01 17 grn02 08 grn02 18 grn01 09 grn01 19 grn01 11 grn01 20 grn01 15 grn01 20 grn02 A21 40 21 grn01 29 grn03 22 grn01 39 grn01 22 grn02 39 grn02 23 grn01 42 grn01 27 grn01 45 grn01 27 grn02 46 grn01 28 grn01 46 grn02 28 grn02 47 grn01 29 grn01 51 grn01 29 grn02 51 grn02 A41 56 49 grn01 64 grn01 32 grn01 51 grn01 53 grn01 66 grn01 36 grn01 55 grn01 59 grn01 66 grn02 37 grn01 55 grn02 60 grn01 66 grn03 44 grn01 56 grn01 Each group (A01 20, A21 40, and A41 56) consists of aggregates listed in each corresponding row by their fraction size.

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40 Table 4 4. List of 27 out of 83 ALHA77005 aggregates used in this study ALHA77005 63 150 packets A Hp123 Cu0010130 01 Cu0020039 01 Cu0030008 01 Cu0010046 02 Cu0020006 01 Cu0030012 01 Cu0010042 01 Cu0030005 01 Cu0030020 01 Cu0010027 03 Cu0030006 01 Cu0030022 01 Cu0010030 02 Cu0030007 01 Cu0030023 01 AHp45 Cu0030030 0 1 Cu0030048 01 Cu0030032 01 Cu0030052 01 Cu0030039 01 Cu0030060 01 Cu0030040 01 Cu0030062 01 Cu0030045 01 Cu0030067 02 Cu0030046 01 Cu0030075 01 Each group (AHp123 and AHp45) consists of aggregates listed in each corresp onding row by their fraction size.

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41 Figure 4 1 Zagami rock chip sample used for this research. Figure 4 2

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42 Figure 4 3 Zagami single phosphate aggregate. SEM chemical maps ( top left ) illustrate very good P, Ca, and O coverage with fair amounts of Al, Mg, Si and very low C. EDS spot analyses (bottom left ) show very high P and Ca peaks with moderate Mg, Na, and Si signals. Figure 4 4 ALHA77005 single phosphate aggregate. SEM chemical maps (top right ) illustrate very good P and Ca coverage with fair amounts of O, Mg, Si and very low C and Al. EDS spot analyses (bottom right ) show very high P and Ca peaks with moderate Mg and Na signals.

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43 Figure 4 5 Zagami aggregates display ing phosphorus SEM scans for group Z103 113 measuring 150 250 m. Most larger fractions illustrate partial phosphorus coverage relative to smaller aggregates. Counting from top left to right, the eighth image is chemically mapped in Figure 4 3

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44 Figure 4 6 ALHA77005 aggregates displaying phosphorus SEM scans for group A41 56 measuring 63 m for the bottom eight aggregates (black bold borders). Counting from top left to right, the fourth image is chemically mapped in Fi gure 4 4

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45 Figure 4 7 Zagami aggregates displaying phosphorus SEM scans for group Z01 20 measuring 75 125 m. Note that these smaller fractions have almost complete phosphorus coverage radially extending to the rim of the grain when compared to the lar ger fractions.

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46 Figure 4 8 Zagami BSE image fld005_img02 and histogram. Blue traces on thin section of merrillite (top) mark fracture free areas (FFA). Radius distribution (bottom histogram) is defined by FFAs from the merrillite thin section. Left axis represents the raw frequency of each radius (blue solid bars), right axis illustrates the weighted frequency (pale blue bars with borders) of the larger FFAs. Thin section (Zagami 4709 1) supplied by the American Museum of Natural History, New York, NY.

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47 Figure 4 9 ALHA77005 BSE image fld003_ img 01 and histogram. Red traces on thin section of merrillite (top) mark fracture free areas (FFA). Radius distribution (bottom histogram) is defined by FFAs from the merrillite thin section. Left axis re presents the raw frequency of each radius (red solid bars), right axis illustrates the weighted frequency (pale red bars with borders) of the larger FFAs. Thin section (ALHA77005 120) supplied by NASA.

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48 Figure 4 10. SEM composite image of thin section Z agami 4709 1 Green areas are merrillite (phosphate) minerals

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49 Figure 4 11 SEM composite image of thin section ALHA77005 120. Green areas are merrillite (phosphate) minerals.

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50 Figure 4 12 Zagami aggregates displaying phosphorus SEM scans for group Z 21 40 measuring 75 125 m.

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51 Figure 4 13 Zagami aggregates displaying phosphorus SEM scans for group Z41 60 measuring 75 125 m for the top seven aggregates and 125 bottom thirteen aggregates (black bold borders).

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52 Figure 4 14 Zagami aggregates displaying pho sphorus SEM scans for group Z61 80 measuring 125 150 m for the top sixteen aggregates and 150 the bottom four aggregates (black bold borders).

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53 Figure 4 15 Zagami aggregates displaying phosphorus SEM scans for group Z81 92 measuring 150 250 m. Figure 4 16 Zagami aggregates displaying phosphorus SEM scans for group Z93 102 measuring 150 250 m.

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54 Figure 4 17 Zagami aggregates displaying phosphorus SEM scans for group Z114 123 measuring 150 250 m. Figure 4 18 Zagami aggregates display ing phosphorus SEM scans for group Z124 134 measuring 150 250 m.

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55 Figure 4 19 Zagami aggregates displaying SEM scans for group ZAG01 measuring 125 150 m. Figure 4 20 Zagami aggregates displaying SEM scans for group ZAG234 measuring 125 150 m. Fi gure 4 21 Zagami aggregates displaying SEM scans for group ZAG05 measuring 125 150 m.

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56 Figure 4 22 ALHA77005 aggregates displaying phosphorus SEM scans for group A01 20 measuring 63 150 m.

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57 Figure 4 23 ALHA77005 aggregates displaying phosphorus SE M scans for group A21 40 measuring 63 150 m.

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58 Figure 4 24 ALHA77005 aggregates displaying phosphorus SEM scans for group AHp123 measuring 63 180 m for the bottom ten (black bold borders). Figure 4 25 ALHA7 7005 aggregates displaying phosphorus SEM scans for group AHp45 measuring 150 180 m.

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59 CHAPTER 5 (U TH)/H E RESULTS Cosmogenic 4 He Correction To accurately calculate (U Th)/He ages, radiogenic 4 He abundances need to be estimated. Because 4 He can be also p roduced through interactions with cosmic rays, cosmogenic 4 He need s to be subtracted from the measured 4 He. The common way to correct this effect is based on the production rate of cosmogenic 4 He calculated from an isotopic ratio of 4 He/ 3 He (Paneth and Re asbeck, 1952; Shuster and Farley, 2003; Min, 2005). The following parameters are used for cosmogenic 4 He correction: (1) the estimated mass of each sample (using density values from Schwenzer et al. 2008), (2) the cosmogenic 4 He production rate of 8.05 x 10 8 (Eugster, 1988; Lorenzeti et al. 2003), and (3) exposure ages of 3.2 Ma and 2.7 Ma for ALHA77005 and Zagami, respectively ( Eugster et al. 1997; Schwenzer et al. 2008). For Zagami, the contribution of cosmogenic 4 He to the total 4 He is gener ally in the range of 0.46% 6.0 8%. For ALHA77005, however, the cosmogenic 4 He contr ibution is in the range of 10.36 % 44.02 % (Tables 5 1 and 5 2 ) (U Th)/He Ages The twelve Zagami packets yielded widely scattered (U Th)/He ages ranging from 19.8 Ma to 202.4 Ma with an average of 107.3 Ma and a standard deviation of 61.0 Ma (Table 5 1 and Figure 5 1 ). Among these twelve packets, five contained large aggregate fractions measuring between 1 50 250 m (Z81 92, Z93 102, Z103 113, Z114 123, Z124 134 Table 4 2 ) res ulting in ages ranging from 110.7 Ma to 202.4 Ma (average = 146.6 Ma, standard deviation = 34.8 Ma Table 5 3 ). The average age corresponds to a fractional loss ( f He ) of 0.17 (Table 5 3 ) when a reset of the helium

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60 system was assumed at 177 Ma (summarized in Nyquist et al. 2001; Schwenzer et al. 2008) These ages are fairly consistent with a calculated (U Th)/He age of 11 3 Ma by using U and Th concentrations analyzed by Schwenzer et al (2008) from whole rock samples of Zagami. F ive groups consist of m ixed aggregate s measuring between 75 250 m: Z41 60 measures between 75 150 m and Z61 80 measures between 125 250 m; ZAG01, ZAG234, and ZAG05 each consists of aggregates measuring between 125 150 m (Tables 4 1 and 4 2 ) These five mixed aggregate group s yielded ages in the range of 19.8 Ma to 93.6 Ma (average = 51.2 Ma, standard deviation = 31.4 Ma, f He = 0.71 Table 5 3 ). The remaining two groups consist of aggregate s measuring between 75 125 m (Z01 20, Z21 40 Table 4 1 ) with ages ranging from 99.6 Ma and 199.9 Ma ( average = 149.7 Ma standard deviation = 70.9 Ma, f He = 0.15 Table 5 3 ). For ALHA77005, four out of the five groups yielded tightly clustered (U Th)/He ages ranging from 5.9 Ma to 17.9 Ma, with the fifth one producing an old er age of 78 .2 Ma (Table 5 1 5 3 and Figure 5 1 ). A fractional loss ( f He ) of 0.94 (Table 5 3) corresponds to the younger ages (5.9 17.9 Ma) when the helium system was reset at an assumed crystallization age of 179 Ma (summarized in Nyquist et al. 2001; Schwenzer e t al. 2008) The estimated fractional loss is consistent with a whole rock value of 94 3% calculated by Schwenzer et al (2008) using their U Th data combined with the mean He concentration from Schultz and Franke (2004). Schwenzer et al (2008) appli ed this same U Th data combined with He concentration, both obtained from a whole rock sample (43.2 mg) and calculated a fractional loss of 100 30%.

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61 No apparent correlation is observed between the ages and aggr egate sizes for the ALHA77005 groups For Zagami, aggregates from the five large groups (150 250 m) show a positive correlation between its (U Th)/He ages and size of phosphate aggregate. The scattered age distribution may suggest either (1) differentiated sampling of multiple diffusion domains (Min and Reiners, 2007) or (2) heterogeneous heating du ring or immediately after shock impact (Cassata et al. 2010).

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62 Table 5 1. (U Th)/He results, f ractional l oss, and a ge c alculations Packet name U (mg) Th (mg) Sm (mg) 4 He (fmol) 4 He per aggregate [fmol] f He (%) Age (Ma) ZAGAMI ZAG234* 3.57 E 08 2.11E 07 2.82E 07 9.17 0.61 90.44 19.77 Z41 60* 9.59E 08 2.98E 07 1.09E 06 27.39 1.37 84.74 30.20 Z61 80* 6.99E 08 3.14E 07 1.39E 06 29.74 1.49 80.37 37.73 ZAG05 7.61E 10 8.01E 08 1.97E 07 8.05 0.73 59.53 74.53 ZAG01 6.31E 10 3.85E 08 1.39E 08 4. 31 0.86 48.72 93.61 Z01 20 3.38E 08 2.03E 07 8.19E 07 44.74 2.24 45.49 99.56 Z103 113 5.31E 08 3.16E 07 1.58E 06 78.03 7.09 39.17 110.73 Z124 134 5.31E 08 2.36E 07 1.13E 06 75.84 6.89 30.27 126.48 Z114 123 3.92E 08 1.78E 07 9.18E 07 64.30 6.43 20.63 14 3.44 Z81 92 5.94E 08 3.04E 07 1.41E 06 108.32 9.03 17.09 149.65 Z21 40* 2.19E 08 1.58E 07 8.33E 07 65.64 3.28 11.53 199.86 Z93 102* 2.75E 08 1.47E 07 6.53E 07 69.68 6.97 13.03 202.44 ALHA77005 A41 56 1.91E 08 6.05E 08 4.87E 07 1.08 0.07 98.52 5.88 AHp123 1.16E 08 1.69E 07 1.70E 07 1.99 0.13 97.84 7.10 A21 40 1.84E 09 2.47E 08 2.71E 07 0.64 0.03 93.58 14.77 AHp45 3.82E 09 1.30E 07 3.07E 07 3.37 0.28 91.86 17.85 A01 20* 2.89E 09 2.40E 08 2.82E 07 3.77 0.19 58.30 78.19 Ages consist of corrected radiogenic, 4 He(rad), values from cosmogenic contribution. f He is helium fractional loss since crystallization. Crystallization ages: ALHA77005 = 179 Ma and Zagami = 177 Ma (Nyquist et al. 2001; Schwenzer et al. 2008). Expo sure ages: ALHA77005 = 3.2 Ma and Zagami = 2.7 Ma (Eugster et al. 1997; Schwenzer et al. 2008). Outliers, marked with (*), yielded ages one standard deviation away from the mean age. Table sorted by age.

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63 Table 5 2. Aggregate fraction data and cosmog enic contribution calculations Packet name Aggregate Fractions Aggregates per packet 4 He(cos) [fmol] 4 He(rad) (fmol) 4 He(cos / cos+ rad) Age (Ma) ZAGAMI ZAG234* 125 150 15 0.5940 9.1711 0.0608 19.77 Z41 60* 75 125, 25 150 20 0.5861 27.3870 0.0210 30.20 Z61 80* 125 150, 150 250 20 1.0282 29.7444 0.0334 37.73 ZAG05 125 150 11 0.4356 8.0537 0.0513 74.53 ZAG01 125 150 5 0.1980 4.3124 0.0439 93.61 Z01 20 75 125 20 0.3046 44.7360 0.0068 99.56 Z103 113 150 250 11 1.3404 78.0348 0.0169 110.73 Z124 134 150 250 11 1.3404 75.8411 0.0174 126.48 Z114 123 150 250 10 1.2186 64.2967 0.0186 143.44 Z81 92 150 250 12 1.4623 108.3196 0.0133 149.65 Z21 40* 75 125 20 0.3046 65.6377 0.0046 199.86 Z93 102* 150 250 10 1.2186 69.6782 0.0172 202.44 A LHA77005 A41 56 63 150, >180 16 0.8491 1.0796 0.4402 5.88 AHp123 63 150, 150 180 15 0.8375 1.9865 0.2966 7.10 A21 40 63 150 20 0.4361 0.6403 0.4052 14.77 AHp45 150 180 12 0.9731 3.3741 0.2239 17.85 A01 20* 63 150 20 0.4361 3.7749 0.1036 78.19 4 He(cos): amount of cosmogenic 4 He per packet. 4 He(rad): [measured amount of 4 He per packet] [ 4 He(cos) per packet]. 4 He(cos / cos+ rad): ratio of cosmogenic contribution. Ages consist of corrected radiogenic, 4 He(rad), values from cosmogenic contributio n. Outliers, marked with (*), yielded ages one standard deviation away from the mean age. Table sorted by age.

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64 Table 5 3 Average a ges and f ractional l oss c alculations per g rouped f ractions Ages (Ma) and f He per Fraction Average Standard Deviation ZAGAMI All Fractions Age 107.33 60.95 f He 39.47 34.51 Excluding Outliers (*) Age 114.00 27.34 f He 35.76 15.44 Smaller Fractions Age 149.71 70.92 f He 15.47 40.32 Mixed Fractions Age 51.17 31.44 f He 71.22 17 .78 Larger Fractions Age 146.55 34.75 f He 17.32 19.78 ALHA77005 All Fractions Age 24.76 30.29 f He 86.24 16.85 Excluding Outlier (*) Age 11.40 5.83 f He 93.67 3.25 f He : helium fractional loss since crystallizatio n. Smaller, mixed, and larger fractions measure between 75 125 m, 75 250 m, and 150 250 m, respectively.

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65 Figure 5 1 Helium 4 versus Age plots for Zagami and A LHA77005 phosphate aggregates. Data points with error bars are labeled by sample gro up name. Number in parenthesis is the number of aggregates in each group. ZAGAMI ALHA7700 5

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66 CHAPTER 6 DISCUSSION Alpha Recoil Alpha particles ( 4 He atoms) are emitted from the decay of parent nuclides and tend to move away from its initial position before coming to rest This motion occurs from the kinetic energy created during the decay process. The moving or recoil distance is generally between 10 30 m for common target minerals (Farley et al. 1996), but depends on density and chemical composition of the host mineral as well as the parent nuclides For apatite and zircon, the recoil distance is known to be approximately 20 m (Farley et al. 1996). A portion of alpha particles may be ejected from the host mineral resulting in too young apparent ages. The most commo n way to correct for this effect is based on the morphology of the grain and the distribution of parent nuclides in the grain (Farley et al. 1996; Hourigan et al. 2005). Such alpha recoil correction is routinely done for terrestrial samples, in many cas es with an assumption that U and Th are homogeneously distributed in the grains. For meteorites, however, it is almost impossible to retrieve phosphate grains without modification of their original morphologies T hey are small, cracked, and irregularly shaped hampering the application of the morphology based alpha recoil correction for these samples. By not applying the alpha recoil correction the procedure assumes that the extracted phosphate grains are from inner parts of originally large r grains (Mi n et al. 2003), or the phosphate aggregates can efficiently capture the recoiled alphas within the attached phases (Min et al. 2004 ; Min and Reiners, 2007). The former approach is based on morphological modifications (abrasion of marginal parts of phos phate grains) during sample preparation procedures, as well as the

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67 reasonably good (U Th)/He ages without alpha recoil correction. The latter approach is from the observation of the final phosphate aggregates and from the fact that the alpha recoil uncorr ected ages are consistent with other isotopic systems (Min et al. 2004 ; Min et al. 2011). As alpha atoms are ejected into these attached phases, the majority of ejected alphas may remain in these neighboring grains requiring minimal alpha recoil correct ion and efficiently capturing radiogenic 4 He atoms. Nevertheless, these neighboring phases may also lose ejected alphas or contain trapped alpha particles from another system. How much is lost or retained is a direct consequence of the density and chemic al structures of the attached phases. In this study, I followed the second approach which requires collection of phosphate aggregates to avoid alpha recoil correction. According to SEM examination on aggregates from Zagami and ALHA77005, the dimensions of phosphates themselves are similar for the different size groups, but the large group s (150 250 m) contain more attached phases around phosphates increasing the size of the aggregate. Therefore, the aggregates in the smaller group s have higher phosphate to aggregate volume ratios than the larger aggregates (Figures 4 5 4 6 and 4 7 ). Such morphol ogical differences in phosphate aggregates are likely related to the (U Th)/He ages. The (U Th)/He age s of Zagami show a tendency to increase as a function of size of phosphate aggregate (Figure 5 1 ). Although this relationship does not hold for all size fractions, it is obvious that the largest size fractions (150 250 m; samples Z81 92, Z93 102, Z103 113, Z114 123, and Z124 134 Table 4 2 ) yielded consistently higher (U Th)/He ages than the remaining samples The most plausible explanation of such age size relationship s is that the attached phases in

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68 the largest group s are thick enough (> ~ 40 m) to preserve most of the recoiled alphas, whereas the smaller samples have thinner (< ~20 um) attached phases and too thin a veneer to completely capture recoiled alphas yielding apparently younger (U Th)/He ages (Figure 4 7 ). Because the small aggregate group s may have experienced incomplete shielding of recoiled alphas at different degrees depending on the thickness and morphology of the attached phases, they show younger and more scattered ages tha n the large group samples Therefore, the ages from the larger Zagami aggregates (147 35 Ma Table 5 3 ) are believed to be more reliable than the remaining aggregates (79 62 Ma). Comparing these ages to previous studies, whole rock (U Th)/He age of ~113 Ma calculated from da ta by Schwenzer et al. (2008) corresponds to the lower limit estimates from the large phosphate aggregates. An alternative explanation for the age size relationship is that these larger Zagami aggregates have larger diffusion domains, t hus less sensitive to thermal disturbance and yielding older ages. However, for shocked meteorites, diffusion domains are believed to be much smaller than the grain itself because of many fractures developed inside the grain. Therefore, it is necessary to examine these fr actures in phosphate grains to constrain the diffusion domain size, which affects the resulting (U Th)/He ages. ALHA77005 s hows no consistent relationship between size and age (Figure 5 1 ). In reviewing the thickness and morphology of the attached phases in these groups, the majority of the phases had a thickness > ~ and a few of them were < ~ regardless of their fraction size (Figures 4 6 4 22 to 4 25 ). In fact, sample A41 56 produced the youngest (U Th)/He age of 5.9 Ma (Table 5 1 ) with ag gregates measuring between 63 150 m and >180 m (Table 4 3 ) and most attached phases having a

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69 thickness > ~ However sample A01 20 measu ring between 63 150 m (Table 4 3 ) and with most of the attached phases also exhibiting a thickness > ~ produced an age of 78.2 Ma much older than A41 56 (Table 5 1 ) but relatively young compared to its crystallization age (179 Ma) Based on these observations and (U Th)/He age results (Figure 5 1 ), if the attached phases are efficiently capturing recoiled alphas, then these samples must have experienced an extensive loss of helium to yield young ages. Since these ages are a result of only five samples or data points as seen in Figure 5 1 m ore analyses are required to determine a precise distribution of ( U Th)/He ages for ALHA77005. Diffusion Behavior The high diffusivity of helium at relatively low temperatures allows for (U Th)/He dating to act as a powerful tool in studying shallow crustal exhumation or transient thermal events of various samples. Th e volume diffusion can be best described by the Arrhenius equation which relates the temp erature to diffusion properties : D/a 2 = D o /a 2 exp( E a /RT ) where D is the diffusion coefficient, a is the diffusion domain radius, E a is the activation energy, R is the gas constant, and T is the temperature (McDougall and Harrison, 1999). From previous studies, Arrhenius plots showed a consistent linear relationship between log( D ) and 1/ T and the regressed line defines the diffusion parameters ( E a and D o ) from its slo pe and y intercept (Zeitler et al. 1987; Farley, 2000). However models implemented on Durango fluorapatite showed a change in the rate of diffusion a t temperatures greater than 300 et al. 1996; Farley, 2000) digressing from the linear regression model. Although physical or chemical changes such as the loss of a volatile did not demonstrate a change in diffusion behavior, Wolf et al. (1996) concluded that the presence of mu ltiple diffusion domains and subtle changes in crystal

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70 structure within the mineral may be responsible for variations in helium diffusivity. According to a range of studies on (U Th)/He systems, variations of helium retentivity suggest that a number of fa ctors may be involved: (1) radiation damage induced by the ejection of high energy alpha particles (as explained in the previous section) may modify the diffusion properties, (2) the system may be experien cing a complex thermal history and (3) the presenc e of structural defects such as cracks may act as fast pathways enhancing the rate of diffusive loss. Diffusion Domain In order to constrain the physical dimensions of diffusion domains, 22 phosphate grains in two thin sections e American Museum of Natural ) were carefully examined u sing SEM and an optical microscope Phosphates present in Zag ami and ALHA77005 samples exhibit numerous fractures acting as good pathways for helium diffusion, thus the diffusion domain dimensions are likely smaller than the grain size of phosphates. According to our detailed image processing of multiple BSE images all of the analyzed phosphate grains in Zagami contain numerous fractures. ALHA77005 also exhibits fractures but the population is much lower than in Zagami. To quantitatively estimate the diffusion domain size, I defined fracture free areas (FFA) repr esent ed by areas bounded by visible fractures. FFAs were determined by tracing fractures for each of the 56 (22 from Zagami and 34 from ALHA77005) total phosphate grains examined from thin sections. Afterwards, the two dimensional area of each FFA was converted to a hypothetical circle with the same surface area, and the radius of this circle was calculated. This

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71 on these calculations, it is clear that smaller FFAs a re more dominant than larger ones. Because the contributions of the smaller FFAs to the final (U Th)/He age are less significant than those of the larger ones, the data were weighted based on the surface areas of each FFA. Figures 4 8 and 4 9 are two ex amples (out of the 56 total phosphate grains) for Zagami and ALHA77005, respectively, showing traced fractures in one phosphate grain (top) and their corresponding histogram (bottom). The histograms in Figure 4 8 (blue solid bars) and Figure 4 9 (red soli d bars) show the distribution of diffusion domain radii defined from the respective traced fractures as illustrated on the top images of each figure. The weighted data from Figures 4 8 (pale blue bars with borders) and 4 9 (pa le red bars with borders) yie ld an esti mated diffusion domain radius of ~6 for Zagami for ALHA77005 respectively. Traced fractures for each of the 56 phosphate grains with their corresponding histograms are found in Appendices C and D for Zagami and ALHA77005, respectively. From measurements of 3861 FFAs from the 22 phosphates in Zagami, the most representative average diffusion domain radius falls in the range of 4 6 m (Appendices C and G) From measurements of 1130 FFAs from 34 phosphates in ALHA77005, the most representative average diffusion domain radius f alls in the range of 9 15 m (Appendices D and H) It is clear that FFAs for ALHA77005 are apparently larger than those for Zagami. The estimated diffusion domain radii represent the maximum diffusion domain sizes for these phosphates because of the po tential existence of microfractures within the FFAs. Although these microfractures (blurry,

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72 fairly indistinguishable fractures) were more noticeable when we increased the magnification in the SEM, they remained indistinct and too faint to trace (Appendice s E and F) Zagami evidently contained far more microfractures (Appendix E) reducing the practical diffusion domain radius by a factor of one (or two) orders of magnitude. Therefore, these are conservative estimates of diffusion domain radii, and the t rue P eak shock temperatures at each of these estimates are shown in Figure 6 1 At a t emperature is estimated at ~213 More on peak shock temperatures is discussed later in this chapter. In contrast, ALHA77005 had far fewer microfractures (Appendix F) where many of the FFAs are ALHA77005, five are imperfections (distinct from microfractures) that may also contribu te to helium diffusion. Therefore the 6 2 a peak shock t emperature is estimated at ~525 a Diffusion Parameters During shock metamorphism, an increa se in temperature occurs instantaneously followed by rapid cooling to ambient temperature in space Diffusive loss of helium from phosphates varies as a function of diffusion domain size and thermal history related to the shock event. By understanding diffusion properties of extraterrestrial phosphates, it is possible to constrain the maximum temperature experienced during shock metamorphism. For terrestrial apatites, their He diffusion behavior is relatively well understood through stepp ed heating experiments for (1) 4 He

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73 from untreated apatites (Zeitler et al. 1987; Farley, 2000; Wolf et al. 1996), (2) 4 He and 3 He produced by proton bombardment (Shuster and Farley, 2003) and (3) 4 He artificially implanted along the c axis and perpendicu lar direction of apatite (Cherniak et al. 2009). However diffusion properties for extraterrestrial merrillite are poorly understood because they have (1) generally a small size, therefore having a small amount of radiogenic 4 He, and (2) irregular shapes often with many cracks hampering precise determination of diffusion parameters. Nevertheless, 3 He/ 4 He diffusion experiments were conducted on large apatite and merrillite crystals from an unshocked chondrite (Min et al. 2011). From the linear relationsh ip in the Arrhenius plot, Min et al. (2011) determined diffusion parameters for merrillite: E a = 135.1 2.5 kJ/mol and ln( D o /a 2 ) = 5.73 0.37 s 1 These values were used in this study for thermal modeling to show a relationship between helium fractional loss ( f He ) and maximum shock temperature based on diffusion domain size (Figures 6 1 and 6 2 ; Appendices G and H ). Fractional Loss The age equation allows for calculati ng the time over which helium has accumulated within the sample. If the timing of initi al inception of radiogenic 4 He accumulation is known, it would be possible to estimate the effect of secondary thermal events. For example, from the known crystallization ages of Martian meteorites and the abundances of parent nuclides of U Th Sm, the exp ected radiogenic 4 He can be calculated. If the measured 4 He is lower than the expected value, it means the meteorite experienced post crystallization degassing. Fractional loss ( f He ) which simply represents the ratio between the measured to the expected 4 He, provides important constraints on the nature of thermal events experienced by the meteorites.

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74 Other neighboring phases in the aggregates have very low U and Th, thus their contributions are expected to be very minor. If there are some radiogenic 4 He in the neighboring phases, most likely in silicates of pyroxene and olivine, they would cause slow He diffusion However, because this amount of He diffusion is expected to be very small, I concentrated on phosphate s only for thermal modeling Fraction al losses of 4 He were calculated using preferred crystallization ages of 177 Ma for Zagami and 179 Ma for ALHA77005 (Nyquist et al. 2001; Schwenzer et al. 2008). For example, if no helium fractional loss ( f He = 0) occurred for a certain sample, the (U T h)/He age should be the same as the crystallization age. If a complete loss ( f He = 1) of 4 He occurred during the ejection related shock event, the resulting (U Th)/He age should be identical to its cosmic ray exposure age. One of the five sample groups of ALHA77005 yielded a low f He of 0.57 (Table s 5 1 and 5 3 ) The remaining four groups of ALHA77005 produced f He values greater than 0.9 (average f He = 0.94) and are consistent with calculated losses by Schwenzer et al (2008). The twelve Zagami sample g roups have f He ranging from 0.9 (for the youngest age at 19.8 Ma) to 0.15 for the oldest age at 202 Ma (Table 5 1 ). The average fractional loss for all twelve groups is f He = 0.39 (39 35%) and has a larger error when compared with whole rock loss value s of 36 6% and 56 18% (Schwenzer et al ., 2008). A breakdown of these losses by fraction size for Zagami are as follows (Table 5 3): (1) t he five large groups (150 250 m) have an average f He = 0.17 (17 20%) (2) the five mixed groups (75 250 m) have an average f He = 0.71 (71 18%) and (3) the two remaining groups (75 125 m) have an average f He = 0.15 (15 40%)

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75 Two samples of Zagami yielded ages older than its crystallization age, thus producing fractional loss values less than zero. This may (1) suggest the existence of an unidentified source of 4 He or (2) offer some evidence that the crystallization age of Zagami may be close to or slightly greater than 200 M a. If the former is true, the calculated f He values are likely to have uncertainties at least the level of offset (by 15%). If the latter is correct, this is consistent with previous studies on U Th Pb radiometric ages of approximately 229 230 Ma (Chen and Wasserburg 1986; Nyquist et al. 2001) and 39 Ar 40 Ar age of about 242 Ma (Bogard and Garrison 1999). Bogard and Park (2008) calculated a radiometric 39 Ar 40 Ar age of ~209 Ma and proposed that excess 40 Ar may have contributed to ages older than prev iously reported. In this case, the calculated f He overestimated the true values requiring systematic modification. The larger f He values for ALHA77005 than Zagami suggest that either (1) the shock temperature condition s of ALHA77005 are higher or (2) t he diffusion domain size of phosphates in ALHA77005 is smaller. According to our preliminary studies on the textures of merrillite, the dimensions of fracture free areas are not much different for these two meteorites. Therefore, the shock temperature of ALHA77005 is expected to be much higher than Zagami, which is qualitatively consistent with previous studies (Nyquist et al. 2001; Fritz et al. 2005). The relationship between helium fractional loss and maximum shock temperatures can be established wi th an assumption of volume diffusion and represented through thermal modeling (Figures 6 1 and 6 2 ; Appendices G and H ). An assumed parent body radius ( A ) calculated from previous estimates, ambient temperature of Mars ( T s ), and depth of the sample from t he surface of the parent body ( d ) are applied to the

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76 model at varying temperatures until helium fractional loss ( f He ) reaches a value of 1. For fractional loss values close to or equal to 1 (complete helium loss), f He becomes less dependent on the radius of the body ( A ) and diffusion domain size ( a ) and an initial temperature based on diffusion parameters can be inferred from the model. Given f He and diffusion domain values, a maximum peak shock temperature can also be implied. Peak Shock Temperatures A shock event causes an instantaneous temperature increase to a peak shock temperature followed by conductive cooling. During this thermal event, a portion of radiogenic 4 He ( accumulated since crystallization ) diffuses out of the grain, and the degree of d iffusive loss (or fractional loss) can be used to estimate peak shock temperatures and temperature variations at different conditions P eak shock temperatures in this study are calculated and modeled by using helium diffusion parameters of merrillite, amb ient temperatures on Mars, and parameters related to the parent body of Zagami and ALHA77005 Specifically from Zagami and ALHA77005, their respective helium fractional losses ( f He ) and estimated maximum and minimum diffusion domain size s are also applied in the models The helium diffusion parameters of m errillite used in this study are E a = 135.1 2.5 kJ/mol ( 32.28 kcal/mol) and D o = 0.01 cm 2 /s (Min et al. 2011). A mbient surface temperatures for Mars include very a cold ( T s = nd hot ( T s = et al. 2006) temperature, and the average space temperature at Mars distance from the Sun T s = 70 C (Butler, 1966). Remaining parameters used to calculate peak shock temperatures are described in the sections below for each Martian meteorite, Zagami and ALHA77005

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77 T hermal modeling created from these parameters are displayed in Figures 6 1 and 6 2 and Appendices G and H. These models illustrate maximum (or peak) shock temperatures as a function of helium fractional loss an d diffusion domain size. From the models f or Zagami with f He = 0.17, peak shock temperatures are calculated at 213 C for a = 0.05 m and 407 C for a = 6 m. And for ALHA77005 with f He = 0.94, peak shock temperatures are calculated at 520 C for a = 5 m and 615 C for a = 15 m. Zagami In addition to diffusion properties and ambient temperatures, t he following parame ters related to the parent meteoroid (body) and diffusion domain size of Zagami were also applied : radius of parent body ( A ) = 23 cm (Eugster et al. 2002), 25 cm (Schnabel et al. 2001), and 35 cm (Artemieva and Ivanov 2004); diffusion domain radii a = d = 15 cm and 18 cm. Calculating the depth from parent body surface specifies the location of my sample with respect to the Zagami parent meteorite. The rock chip sample for this study was retrieved approximately 5 to 8 cm (personal communication with Michael Farmer who provided the Zagami sample to KIGAM ) below the fusion crust of the recovered Zagami meteorite. Because the parent body should have experienced physical ablation during its entry in to Mars is expected to be larger. The depth from parent body surface was then calculated by using the approximate minimum radius (Schwenzer et al. 2008) of the recovered Zagami meteo rite and estimating a minimum parent body radius of 23 cm. The samples were retrieved at a depth approximately 15 to 18 cm below the surface of the pre atmospheric parent body of Zagami before undergoing ablation as it passes through the atmosphere of Ear th.

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78 According to the thermal modeling for Zagami phosphate aggregates with f He = 0.17, average peak shock temperatures of approximately 213 C and 407 are calculated for a diffusion domain size of a = a = (from FFA data) respectively. T hese shock temperatures may be sensitive to other factors such as parent body size ( A ), depth from surface of parent body ( d ), and ambient temperatur es ( T s ) of Mars. More specifically the following was also determined f rom thermal modeling (Table 6 1 ) : (1a) for a d = 15 cm, the calculated shock temperatures deviate by 6 (211 C to 217 C) for A = 23, 25, and 35 cm at T s 70 a nd 148 (1b) At a d = 18 cm, the calculated shock (207 C to 213 C) for the same values as A and T s (2 a ) At a d (405 C to 414 C) (2b) And at a d = 18 cm, the s hock temperatures deviate by 10 (397 C to 407 C) for the same A and T s values as (1). From these models, peak shock temperatures for Zagami are fairly robust against extreme temperatures and minimally affected b y the body size and sample surface depth of the parent meteoroid. Post shock temperatures describe the temperature increase produced from a shock wave as it propagates through a material. Immediately after a shock event, the temperature of Martian rocks (ambient temperature) increases by the post shock shock temperature s + ambient Mars temperature s are used for comparison with peak shock temperatures in this study. For Zagami, p revious estimates of po st shock temperature are used from Nyquist et al. (2001) and Fritz et al. (2005).

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79 P eak shock temperatures from this study ( ) are evidently higher than those from Fritz et al. (2005) at 70 5 However from the models, a p ost shock temperature of 70 5 et al. 2005) may be achieved with a diffusion domain size, a < Therefore, Zagami would need to have innumerable amounts of fractures, far more than the ones seen in this study to reach a diffusion domai n size less than 0.0005 m. If future analyses show that Zagami may possess this many fractures for a < 0.0005 m, then a p ost shock temperature of 70 5 et al. 2005) may be likely for Zagami. In contrast, the Zagami peak shock temperature of 21 3 C falls within range of post shock temperature 220 50 C (Nyquist et al. 2001) only when the ambient surface temperature of Mars reaches from the models, this same temperature of 220 50 C (Nyquist et al. 2001) may also be achieved wit h an ambient temperature in space of 70 C and a = 0.005 m. This diffusion domain size is one order of magnitude smaller than the suggested a This setting is likely if further studies demonstrate the diffusion domain size may be less than or equal 0.005 m for Zagami. ALHA77005 T o create thermal models for ALHA77005 the same diffusion properties ( E a = 135.1 2.5 kJ/mol D o = 0.01 cm 2 /s : Min et al. 2011) and ambient temperatures ( T s = ; T s et al. 2006 ; T s = 70 C: Butler, 1966) used for Zagami are applied for ALHA77005 In addition, t he following parameters are used for ALHA77005 thermal modeling : radius of parent body ( A ) = 25 cm (Artemieva and Ivanov, 2004; Fritz et al. 2005), 30 cm (A rtemieva and Ivanov, 2004; Fritz et al. 2005),

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80 and 35 cm (Artemieva and Ivanov, 2004); sample depth from parent body surface, d = 24 cm and 28 cm; and diffusion domain radii a 50 m. In calculating the depth from parent body surface for ALHA77005, the same steps were followed as described above for Zagami. No current data is available to report from where in the recovered meteorite the rock chip samples were r etrieved. Based on dimensions of the recovered meteorite, 9.5 cm 7.5 cm 5.25 cm (Nishiizumi et al. 1986), the rock chip samples in this study may have been retrieved no more than possibly 2 to 3 cm from the fusion crust of ALHA77005. Selecting a par ent body radius of 30 cm, the rock chip samples may have been retrieved approximately 24 to 28 cm from the surface of the parent body of ALHA77005 before passing through the atmosphere of Earth. From the thermal model ing for ALHA77005 phosphate aggregate s with f He = 0.94, average peak shock temperatures are estimated at a (from FFA data) respectively. Similar to Zagami, these shock temperatures may be affected by parent body size ( A ), depth from surface of parent body ( d ), and ambient temperatures ( T s ) of Mars. From the se models the following was determined (Table 6 2 ) : (1) for a hock temperatures deviate by 17 (516 C to 533 C) and 22 (609 C to 631 C) respectively for d = 24 cm, A = 25, 30, and 35 cm, and T s 148 For a (509 C to 520 C) and 13 (601 C to 614 C) respectively, for d = 28 cm, A = 30 and 35 cm, and T s value s same as in (1). Based on these values, peak shock temperatures for ALHA77005 are slightly

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81 more sensitive than Zagami but somewhat resilient against extreme temperatures and slightly affected by the depth from surface and body size of the parent meteoroi d. In comparing ALHA77005 results with previous estimates from Nyquist et al. (2001), the following was determined (1) B oth peak shock temperatures ( 615 ) fall well within range of p ost shock temperature 450 600 ( Nyquist et al. 2001 ) when the ambient surface temperature of Mars is 22 (2) For an average temperature in space the peak shock temperature of 520 also fal ls withi n this range (450 600 et al. 2001). When comparing these peak shock temperatures with values from Fritz et al. (200 5), (1) the peak shock temperature of 615 falls within p ost shock temperature 800 200 et al. 2 005) during an average temperature in space of (2) For an ambient surface temperature of temperatures fall w ithin this same range 800 200 et al. 2005). In contrast to Zagami, peak shock temperatures for ALHA7700 5 are more favorable with previous values from Nyquist et al. (2001) and Fritz et al. (2005). Post shock versus peak shock temperatures Post shock temperatures describe the temperature increase produced from a shock wave as it propagates through a material To calculate post shock temperatures, a thermodynamic equation of state (EOS) is applied using a Hugoniot model relating particle velocity, shock wave velocity, and energy released during decompression after a shock event ( Stffler et al ., 1988 ; Sharp e t al ., 2006 ) Experimental EOS has been derived for terrestrial geologic materials and used on meteorites with similar composition to determine their post shock temperatures ( Stffler et al ., 1986; Artemieva and Ivanov, 2004 ; Fritz et al ., 2005 ) In cont rast, p eak shock temperatures in this study

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82 are calculated through thermal modeling using several factors related directly to the meteorite: diffusion properties from a meteorite mineral (merrillite), characteristics from Martian meteorite sa mples, and par ameters from the parent planet, Mars. In this case, peak shock temperatures offer an absolute temperature (versus relative temperature) for the ejected meteorite upon impact Future Stud ies Future (U Th)/He studies on Martian meteorites are required to c omplement the results from this study and subsequently obtain discrete peak shock temperatures. Items to consider: (a) select phosphate aggregates with thicker attached phases measuring between 40 m to 50 m to evaluate grain size control on maximum temperatures (b) analyze aggregates with mineral phases other than phosphates such as pyroxene or plagioclase bearing aggregates to determine and compare their U, Th, Sm, and He abundances with phosph ate aggregates, and (c) separate more ALHA77005 phosphate samples to better define (U Th)/He ages with those from this study Other important studies may include analyzing whole rock samples to compare with these results applying the use of SEM with a st ronger magnification to constrain diffusion domain size and possibly determining a method to p erform single grain analyses on merrillite crystals from meteorites

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83 Table 6 1. Zagami shock temperatures from thermal modeling simulations at d = 15 cm and d = 18 cm. d =15 cm T s = 22 o C T s = 70 o C T s = 148 o C A 23 214 410 216 413 217 414 25 213 409 215 411 216 413 35 211 405 213 407 215 409 avg 212.67 408 214.67 410.33 216 412 std dev 1.53 2.65 1.53 3.06 1 2.65 d =18 cm T s = 22 o C T s = 70 o C T s = 148 o C A 23 211 404 213 406 213 407 25 211 404 212 404 213 406 35 207 397 209 399 210 400 avg 209.67 401.67 211.33 403 212 404.33 std dev 2.31 4.04 2.08 3.61 1 .73 3.79 d = depth from surface of parent body. T s = ambient surface temperature of Mars. A = parent body size. Overall shock temperature averages: ~213C at 0.05 m and ~407C at 6 m.

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84 Table 6 2. ALHA77005 shock temperatures from thermal modeling simu lations at d = 24 cm and d = 28 cm. d =24cm T s = 22 o C T s = 70 o C T s = 148 o C A 25 531 628 532 630 533 631 30 520 616 522 617 524 619 35 516 609 517 611 519 613 avg 522.33 617.67 523.67 619.33 525.33 621 std dev 7.77 9.61 7.64 9.71 7.09 9.17 d =28cm T s = 22 o C T s = 70 o C T s = 148 o C A 5 30 517 610 518 612 520 614 35 509 601 511 603 512 604 avg 513 605.5 514.5 607.5 516 609 std dev 5.66 6.36 4.95 6.36 5.66 7.07 d = depth from surface of parent body. T s = ambient surface temperature of Mars. A = parent body size. Overall shock temperature averages: ~520C at 5 m and ~615C at 15 m.

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85 Figure 6 1 Thermal Modeling for Zagami. T s = 70C, d = 15 cm, and A = 23 cm. Diffusion properties of merrillite: E a = 32.28 kcal/mol, ln( D o /a 2 ) = 5.73/s, an d D o = 0.01 cm 2 /s. Diffusion properties of apatite (Ap): E a = 32.9 kcal/mol, ln( D o /a 2 ) = 13.44/s, and D o = 50 cm 2 /s (Farley, 2000) Post shock temperature calculated by Nyquist et al (2001) is displayed as a converted peak shock temperature range (light gray box with blue border). Each curve represents the diffusion profile per diffusion domain size (refer to legend for range of radii). For this study, a = 0.05 m (gray dotted line) and a = 6 m (black dotted line).

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86 Figure 6 2 Thermal Modeling for ALHA77005. T s = 70C d = 24 cm, and A = 30 cm. Diffusion properties of merrillite: E a = 32.28 kcal/mol, ln( D o /a 2 ) = 5.73/s, and D o = 0.01 cm 2 /s. Diffusion properties of apatite (Ap): E a = 32.9 kcal/mol, ln( D o /a 2 ) = 13.44/s, and D o = 50 cm 2 /s (Farley, 2000) Post shock temperature calculated by Nyquist et al (2001) is displayed as a converted peak shock temperature range (light gray box with blue border). Each curve represents the diffusion profile per diffusion domain size (refer to legend for rang e of radii). For this study, a = 5 m (gray dotted line) and a = 15 m (black dotted line).

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87 CHAPTER 7 CONCLUSIONS (1) S e mi quantitative chemical analyse s using EDS (Energy Dispersive Spectroscopy) suggest high P and Ca concentrations along with moderate Mg and Na signals. These chemical analyses are consistent f or almost the entire population of phosphates identified in Zagami and ALHA 77005 Martian meteorites. The results indicate that the phosphates in these meteorites are exclusively merrillite. The detection of merrillite in these phosphates is consistent with the composition of these meteorites from previous studies for Zagami ( Vistisen et al. 1992; McCoy et al. 1993; Wang et al. 1999; Meyer, 2003 ) and ALHA77005 ( McSween et al. 1979a, b; Mason, 1981; Treiman et al. 1994; Meyer, 2003). ( 2 ) (U Th)/He ages were obtained from multiple phosphate aggregates in Zagami and ALHA77005. Because single grain analysis did not yield enough 4 He, multi ple grain (5 20 grains per packet) analysis was performed The ages of all size groups are in the range of 19.8 Ma 202.4 Ma for Zagami and 5.9 Ma 7 8.2 Ma for ALHA77005 The widely scattered ages are probably due to different de grees of alpha recoil loss. ( 3 ) For Zagami, the (U Th)/He ages from the five lar ge phosphate aggregate groups (~111 202 Ma; 150 matically older than the small and mixed groups (~20 200 Ma; 75 the chemical maps of the individual aggregates obtained using SEM the 2 D areas of phosphate grains are almost identical for large and small aggregates, and the aggregate size difference is mainly from the attached phases. P hosphate aggregates from the five large groups typically contain thick layers (> 40 m) of attached phases, efficiently capturing recoi led alphas within the aggregates. In contrast, most of the aggregates from the small groups have relatively

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88 thin layers (< 20 m in many cases) of other phases around phosphate grains, potentially allowing partial recoil loss from the system. Based on th e textural observations and age size relationship, I conclude that the uncorrected (U Th)/He ages from the five large group s ( 150 250 m, ~111 202 Ma, average = 147 35 Ma) are more reliable than those from the remaining groups ( 75 250 m, ~20 200 Ma, ave rage = 79 62 Ma). ( 4 ) For ALHA77005, areas of phosphate grains observed with the SEM are similar between large and small aggregates The m ajor size differences within these areas in ALHA77005 phosphate grains may come from attached phases, just like in Z agami. However unlike Zagami, ALHA77005 produced young ages regardless of the size of the aggregate showing no immediate relationship between age, thickness of attac hed phases, and fraction size. ( 5 ) Based on detailed examination of thin sections, numerou s fractures are identified in the phosphate crystals. To constrain the diffusion domain size, I analyzed fracture patterns in 56 phosphate crystals and defined 4991 fracture free areas (FFAs). The distribution of FFAs shows weighted average radii of 6 m and 15 m for Zagami and ALHA7005, respectively. However, these estimates only provide the upper limits of diffusion domain radius because these FFAs contain microfractures which can be only observed at high SEM magnifications. Based on the distributio n of such microfractures, I conclude that the practical diffusion domain radii are in the range of 0.05 6 m for Zagami and 5 15 m for ALHA77005. ( 6 ) Peak shock temperatures in this study are determined from thermal modeling using helium diffusion p roperties of merrillite ambient temperatures of Mars, and

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89 parameters related to the parent meteoroid of Zagami and ALHA77005. F ractional losses ( f He ) and estimated maximum and minimum diffusion domain sizes, a for Zagami and ALHA77005 are also considere d for these models From thermal modeling for Zagami with f He = 0.17 peak shoc k temperatures are calculated to be 213 C for a = 0.05 m and 407 C for a = 6 m. For ALHA77005 with f He = 0.94 peak shoc k temperatures are calculated to be 520 C for a = 5 m and 615 C for a = 15 m. At these temperatures Zagami and ALHA77005 show very little variation at different ambient conditions and regardless of the size of the meteoroid Based on these models peak shock temperatures for ALHA77005 are slightly more sensitive than Zagami but both are fairly robust against extreme ambient temperatures, parent body size and depth from surface of the parent meteoroid. To complement reported post shock temperatures based on shock conditions, peak shock temperatures fo r Martian meteorites after ejection offer absolute temperatures built directly on their attributes.

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90 APPENDICES A H : ZAGAMI AND ALHA770 05 IMAGES The images in the following appendices can be viewed individually and at a larger scale. Requests can be made by contacting Kyoungwon Kyle Min at kmin@ufl.edu or Annette E mily Farah at afarah@ufl.edu

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91 APPENDIX A SEM CHEMICAL MAPS FO R ZAGAMI PHOSPHATE A GGREGATES

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128 APPENDIX B SEM CHEMICAL MAPS FO R ALHA77005 PHOSPHAT E AGGREGATES

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149 APPENDIX C TRACED ZAGAMI MERRIL LITE PHOSPHATES AND DIFFUSION DOMAIN HIS TOGRAMS The following BSE images are from t hin section Zagami 4709 1 supplied by the American Museum of Natural History, New York, NY. Each thin section image that follows in this appendix shows white traces on the mineral merrillite. The traces mark fracture free areas (FFA) identified in each mineral Each histogram shows the radius distribution defined by FFAs from the merrillite thin section. The left axi s of each histogram represents the raw frequency of each radius (blue solid bars), and the right axis illustrates the weighted frequency (pale blue bars with borders) of the larger FFAs

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150 Zagami BSE image fld001_img01 and histogram. 215 Fracture free areas. Estimated mean radius: 3 m Estimated weighted mean radius: 5 m Zagami BSE image fld001_img02 and histogram. 164 Fracture free areas. Estimated mean radius: 3 m Estimated weighted mean radius: 5 m

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151 Zagami BSE image fld001_img03 and histogram. 102 Fracture free areas. Estimated mean radius: 4 m Estimated weighted mean radius: 7 m Zagami BSE image fld001_img04 and histogram. 221 Fracture free areas. Estimated mean radius: 3 m Est imated weighted mean radius: 6 m

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152 Zagami BSE image fld001_img05 and histogram. 341 Fracture free areas. Estimated mean radius: 2 m Estimated weighted mean radius: 3 m Zagami BSE image fld002_img01 and histogram. 136 Fracture free areas. Estimated mean radius: 2 m Estimated weighted mean radius: 3 m

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153 Zagami BSE image fld002_img02 and histogram. 69 Fracture free areas. Estimated mean radius: 3 m Estimated weighted mean radius: 8 m Zagami BSE image fld002_img03 and histogram. 272 Fracture free areas. Estimated mean radius: 2 m Estimated weighted mean radius: 4 m

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154 Zagami BSE image fld002_img04 and histogram. 189 Fracture free areas. Estimated mean radius: 2 m Estimated weighted mean radius: 3 m Zagami BSE image fld002_img05 and histogram. 105 Fract ure free areas. Estimated mean radius: 5 m Estimated weighted mean radius: 7 m

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155 Zagami BSE image fld003_img01 and histogram. 99 Fracture free areas. Estimated mean radius: 3 m Estimated weighted mean radius: 5 m Zagami BSE image fld003_img02 and histo gram. 177 Fracture free areas. Estimated mean radius: 4 m Estimated weighted mean radius: 5 m

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156 Zagami BSE image fld005_img01 and histogram. 123 Fracture free areas. Estimated mean radius: 4 m Estimated weighted mean radius: 7 m Zagami BSE image fld005 _img02 and histogram. 280 Fracture free areas. Estimated mean radius: 4 m Estimated weighted mean radius: 6 m

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157 Zagami BSE image fld005_img03 and histogram. 235 Fracture free areas. Estimated mean radius: 3 m Estimated weighted mean radius: 5 m Zagami BSE image fld005_img04 and histogram. 379 Fracture free areas. Estimated mean radius: 4 m Estimated weighted mean radius: 5 m

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158 Zagami BSE image fld005_img05 and histogram. 150 Fracture free areas. Estimated mean radius: 5 m Estimated weighted mean radiu s: 10 m Zagami BSE image fld006_img01 and histogram. 77 Fracture free areas. Estimated mean radius: 7 m Estimated weighted mean radius: 13 m

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159 Zagami BSE image fld006_img02 and histogram. 186 Fracture free areas. Estimated mean radius: 5 m Estimated we ighted mean radius: 8 m Zagami BSE image fld006_img03 and histogram. 124 Fracture free areas. Estimated mean radius: 6 m Estimated weighted mean radius: 9 m

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160 Zagami BSE image fld006_img04 and histogram. 120 Fracture free areas. Estimated mean radius: 5 m Estimated weighted mean radius: 7 m Zagami BSE image fld006_img05 and histogram. 97 Fracture free areas. Estimated mean radius: 5 m Estimated weighted mean radius: 7 m

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161 APPENDIX D TRACED ALHA77005 MER RILLITE PHOSPHATES A ND DIFFUSION DOMAIN HISTOGRAMS The following BSE images are from t hin section ALHA77005 120 supplied by the National Aeronautics and Space Administration (NASA) Each thin section image that follows in this append ix shows red traces on the mineral merrillite. The traces mark fracture free areas (FFA) identified in each mineral Each histogram shows the radius distribution defined by FFAs from the merrillite thin section. The left axis of each histogram represent s the raw frequency of each radius (red solid bars), and the right axis illustrates the weighted frequency (pale red bars with borders) of the larger FFAs.

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162 ALHA77005 BSE image fld001_img01 and histogram. 62 Fracture free areas. Estimat ed mean radius: 8 m Estimated weighted mean radius: 14 m ALHA77005 BSE image fld002_img01 and histogram. 36 Fracture free areas. Estimated mean radius: 15 m Estimated weighted mean radius: 24 m

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163 ALHA77005 BSE image fld003_img01 and histogram. 39 Fract ure free areas. Estimated mean radius: 7 m Estimated weighted mean radius: 22 m ALHA77005 BSE image fld003_img02 and histogram. 111 Fracture free areas. Estimated mean radius: 7 m Estimated weighted mean radius: 15 m

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164 ALHA77005 BSE image fld004_img01 and histogram. 35 Fracture free areas. Estimated mean radius: 6 m Estimated weighted mean radius: 8 m ALHA77005 BSE image fld007_img01 and histogram. 26 Fracture free areas. Estimated mean radius: 9 m Estimated weighted mean radius: 13 m

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165 ALHA77005 BS E image fld007_img02 and histogram. 21 Fracture free areas. Estimated mean radius: 9 m Estimated weighted mean radius: 15 m ALHA77005 BSE image fld008_img01 and histogram. 38 Fracture free areas. Estimated mean radius: 13 m Estimated weighted mean radi us: 23 m

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166 ALHA77005 BSE image fld008_img02 and histogram. 42 Fracture free areas. Estimated mean radius: 12 m Estimated weighted mean radius: 21 m ALHA77005 BSE image fld008_img03 and histogram. 11 Fracture free areas. Estimated mean radius: 13 m Esti mated weighted mean radius: 30 m

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167 ALHA77005 BSE image fld009_img01 and histogram. 54 Fracture free areas. Estimated mean radius: 11 m Estimated weighted mean radius: 27 m ALHA77005 BSE image fld009_img02 and histogram. 43 Fracture free areas. Estimated mean radius: 9 m Estimated weighted mean radius: 15 m

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168 ALHA77005 BSE image fld009_img03 and histogram. 71 Fracture free areas. Estimated mean radius: 6 m Estimated weighted mean radius: 12 m ALHA77005 BSE image fld009_img04 and histogram. 42 Fracture free areas. Estimated mean radius: 9 m Estimated weighted mean radius: 14 m

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169 ALHA77005 BSE image fld009_img05 and histogram. 21 Fracture free areas. Estimated mean radius: 9 m Estimated weighted mean radius: 12 m ALHA77005 BSE image fld010_img01 and histogram. 40 Fracture free areas. Estimated mean radius: 6 m Estimated weighted mean radius: 8 m

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170 ALHA77005 BSE image fld010_img02 and histogram. 31 Fracture free areas. Estimated mean radius: 5 m Estimated weighted mean radius: 8 m ALHA77005 BSE ima ge fld010_img03 and histogram. 12 Fracture free areas. Estimated mean radius: 7 m Estimated weighted mean radius: 10 m

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171 ALHA77005 BSE image fld011_img01 and histogram. 18 Fracture free areas. Estimated mean radius: 8 m Estimated weighted mean radius: 11 m ALHA77005 BSE image fld011_img02 and histogram. 34 Fracture free areas. Estimated mean radius: 7 m Estimated weighted mean radius: 11 m

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172 ALHA77005 BSE image fld011_img03 and histogram. 7 Fracture free areas. Estimated mean radius: 9 m Estimated wei ghted mean radius: 12 m ALHA77005 BSE image fld012_img01 and histogram. 29 Fracture free areas. Estimated mean radius: 8 m Estimated weighted mean radius: 12 m

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173 ALHA77005 BSE image fld013_img01 and histogram. 8 Fracture free areas. Estimated mean radiu s: 9 m Estimated weighted mean radius: 23 m ALHA77005 BSE image fld013_img02 and histogram. 22 Fracture free areas. Estimated mean radius: 7 m Estimated weighted mean radius: 10 m

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174 ALHA77005 BSE image fld015_img01 and histogram. 22 Fracture free areas Estimated mean radius: 13 m Estimated weighted mean radius: 18 m ALHA77005 BSE image fld015_img02 and histogram. 81 Fracture free areas. Estimated mean radius: 8 m Estimated weighted mean radius: 14 m

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175 ALHA77005 BSE image fld015_img03 and histogram. 44 Fracture free areas. Estimated mean radius: 6 m Estimated weighted mean radius: 11 m ALHA77005 BSE image fld016_img01 and histogram. 17 Fracture free areas. Estimated mean radius: 6 m Estimated weighted mean radius: 11 m

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176 ALHA77005 BSE image fld01 6_img02 and histogram. 22 Fracture free areas. Estimated mean radius: 10 m Estimated weighted mean radius: 19 m ALHA77005 BSE image fld017_img01 and histogram. 12 Fracture free areas. Estimated mean radius: 12 m Estimated weighted mean radius: 24 m

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177 A LHA77005 BSE image fld017_img02 and histogram. 11 Fracture free areas. Estimated mean radius: 14 m Estimated weighted mean radius: 19 m ALHA77005 BSE image fld017_img03 and histogram. 12 Fracture free areas. Estimated mean radius: 9 m Estimated weighte d mean radius: 13 m

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178 ALHA77005 BSE image fld022_img01 and histogram. 23 Fracture free areas. Estimated mean radius: 6 m Estimated weighted mean radius: 8 m ALHA77005 BSE image fld022_img02 and histogram. 33 Fracture free areas. Estimated mean radius: 7 m Estimated weighted mean radius: 13 m

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179 APPENDIX E ZAGAMI MICROFRACTURE S The following images are close up views of merrillite (phosphate) minerals. White traces on each image represent fracture free areas (FFA) from each merrillite. All histograms show a radius distribution defined by FFAs fo r each respective merrillite. Left axis represents the raw frequency of each radius (blue solid bars), and the right axis illustrates the weighted frequency (pale blue bars with borders) of the larger FFAs. All insets display a close up view of evident m icrofractures. The appearance of microfractures suggests rapid pathways for helium diffusion. All images were retrieved from thin section (Zagami 4709 1) supplied by the American Museum of Natural History, New York, NY.

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184 APPENDIX F ALHA 77005 MICROFRACTURES The following images are close up views of merrillite minerals. Red traces on each image represent fracture free areas (FFA) from each merrillite. All histograms show a radius distribution defined by FFAs for each respective merrilli te. Left axis represents the raw frequency of each radius (red solid bars), and the right axis illustrates the weighted frequency (pale red bars with borders) of the larger FFAs. The appearance of microfractures suggests rapid pathways for helium diffusi on. All images were retrieved from thin section (ALHA77005 120) supplied by NASA.

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187 APPENDIX G ZAGAMI THERMAL MODEL S The following thermal modeling simulations for Zagami show the relationship between helium fractional loss and maximum shock tem perature at varying diffusion domain radii Each curve represents the diffusion profile per diffusion domain size (refer to the legend of each graph for the range of radii). One comparison is also made with apatite assuming a radius of 50 m (Ap50 m, bl ack solid line) and d iffusion properties of apatite: E a = 32.9 kcal/mol, ln( D o /a 2 ) = 13.44/s, and D o = 50 cm 2 /s (Farley, 2000) Diffusion domain sizes are calculated from fracture free area (FFA) data of Zagami phosphates (merrillite) from thin section (Zagami 4709 1). The light gray box represents the post shock temperature 220 50 C (Nyquist et al. 2001) converted to a peak shock temperature range by applying ambient temperatures of Mars Two tables follow the simulations at the end of this append ix listing all the shock temperatures as calculated from the thermal modeling of Zagami Diffusion properties of merrillite: E a = 32.28 kcal/mol, ln( D o /a 2 ) = 5.73/s, and D o = 0.01 cm 2 /s. Number of F FAs = 3861 (from 22 phosphates). Maximum a verage d iffu sion d omain s ize = 3.82 m Maximum a verage w eighted d iffusion d omain s ize = 6.27 m (black dotted line) Diffusion d omain s ize e stimate from microfractures = 0.05 m (light gray dotted line) Overall d iffusion d omain s ize r ange for Zagami: 0.05 m to 6 m

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197 Shock temperatures from thermal modeling at d = 15 cm and d = 18 cm. d = depth from surface of parent body. T s = ambient temperature of Mars. A = parent body size. Overall shock temperature averages: ~2 13C at 0.05 m and ~407C at 6 m. d =15 cm T s = 22 o C T s = 70 o C T s = 148 o C A 23 214 410 216 413 217 414 25 213 409 215 411 216 413 35 211 405 213 407 215 409 avg 212.67 408 214.67 410.33 216 412 std dev 1.53 2.65 1.53 3.06 1 2.65 d =18 cm T s = 22 o C T s = 70 o C T s = 148 o C A 0. 23 211 404 213 406 213 407 25 211 404 212 404 213 406 35 207 397 209 399 210 400 avg 209.67 401.67 211.33 403 212 404.33 std dev 2.31 4.04 2.08 3.61 1.73 3.79

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198 APPENDIX H ALHA77005 THERMAL MO DELS The fol lowing thermal modeling simulations for ALHA77005 show the relationship between helium fractional loss and maximum shock temperature at varying diffusion domain radii Each curve represents the diffusion profile per diffusion domain size (refer to the leg end of each graph for the range of radii). One comparison is also made with apatite assuming a radius of 50 m (Ap50 m, black solid line) and d iffusion properties of apatite: E a = 32.9 kcal/mol, ln( D o /a 2 ) = 13.44/s, and D o = 50 cm 2 /s (Farley, 2000) Dif fusion domain sizes are calculated from fracture free area (FFA) data of ALHA77005 phosphates (merrillite) from thin section (ALHA77005 120). The light gray box represents the post shock temperature 450 600 C (Nyquist et al. 2001) converted to a peak sh ock temperature range by applying ambient temperatures of Mars Two tables follow the simulations at the end of this appendix listing all the shock temperatures calculated from the thermal modeling of ALHA77005 Diffusion properties of merrillite: E a = 32.28 kcal/mol, ln( D o /a 2 ) = 5.73/s, and D o = 0.01 cm 2 /s. Number of F FAs = 1130 (from 34 phosphates). Maximum a verage d iffusion d omain s ize = 8.82 m Maximum a verage w eighted d iffusion d omain s ize = 15.29 m (black dotted line) Diffusion d omain s ize e st imate from microfractures = 5 m (light gray dotted line) Overall d iffusion d omain s ize r ange for ALHA77005: 5 m to 15 m

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206 Shock temperatures from thermal modeling at d = 24 cm and d = 28 cm. d = depth from surface of parent body. T s = ambient temperature of Mars. A = parent body size. Overall shock temperature averages: ~520C at 5 m and ~615C at 15 m. d =24cm T s = 22 o C T s = 70 o C T s = 148 o C A 25 531 628 532 630 533 6 31 30 520 616 522 617 524 619 35 516 609 517 611 519 613 avg 522.33 617.67 523.67 619.33 525.33 621 std dev 7.77 9.61 7.64 9.71 7.09 9.17 d =28cm T s = 22 o C T s = 70 o C T s = 148 o C A 30 517 610 518 612 520 614 35 509 601 511 603 512 604 avg 513 605.5 514.5 607.5 516 609 std dev 5.66 6.36 4.95 6.36 5.66 7.07

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218 BIOGRAPHICAL SKETCH Annette Emily Farah rece ived a Bachelor of Science degree in geolo gy, Bachelor of Arts degree in m athematics and a minor in physics from the University of South Florida (Tampa, FL) in 2003. After graduation Annette worked as a Web Content Developer in 2003 and as a Geologist in 2006. During this time, she saved enough money for her move to Gainesville and attend graduate school in August 2008. She will receive a Master of Science degree in geology from the University of Florida in August 2011. Annette will continue to follow her interests in the sciences by enrolling in a PhD program or pursuing a career in the industry.