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The Behaviour of Iodine and Xenon in the First Asteroids

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The Behaviour of Iodine and Xenon in the First Asteroids THE BEHAVIOUR OF IODINE AND XENON IN THE FIRST ASTEROIDS A thesis submitted to the University of Manchester for the degree of Doctor of Philosophy in the Faculty of Engineering and Physical Sciences 2012 Jennifer Louise Claydon School of Earth, Atmospheric and Environmental Sciences Table of Contents TABLE OF CONTENTS LIST OF FIGURES .......................................................................................................... 5 LIST OF TABLES ............................................................................................................ 7 ABSTRACT ...................................................................................................................... 8 DECLARATION .............................................................................................................. 9 COPYRIGHT STATEMENT ......................................................................................... 10 ACKNOWLEGEMENTS ............................................................................................... 11 CHAPTER 1. INTRODUCTION. .................................................................................. 12 1.1. Introduction .......................................................................................................... 12 1.2. Meteorites ............................................................................................................. 13 1.2.1.Chondrites ...................................................................................................... 13 1.2.2. Melted asteroids ............................................................................................ 21 1.2.3. Timescale of meteorite formation and evolution .......................................... 25 1.3. Noble Gas Cosmochemistry ................................................................................. 28 1.3.1. Solar .............................................................................................................. 28 1.3.2. In situ ............................................................................................................. 30 1.3.3. “Planetary” .................................................................................................... 30 1.3.4. Exotic ............................................................................................................ 32 1.4. The I-Xe chronometer .......................................................................................... 32 1.4.1. Development and early use. .......................................................................... 32 1.4.2. Verification of the I-Xe system as a chronometer ........................................ 36 1.4.3. Closure to xenon loss .................................................................................... 37 1.4.4. Irradiation standards ...................................................................................... 40 1.4.5. Absolute ages ................................................................................................ 42 1.4.6. Recent use ..................................................................................................... 43 1.5. Thesis Aim and Objectives .................................................................................. 44 CHAPTER 2. I–Xe ANALYSIS. .................................................................................... 46 2.1. The RELAX mass spectrometer........................................................................... 46 2.1.1. Resonance ionisation and detection of xenon ............................................... 46 2.1.2. Sample concentrator ...................................................................................... 49 2.1.3. Mass resolution ............................................................................................. 50 2.1.4. Air calibrations .............................................................................................. 51 2.1.5. Data acquisition and reduction ...................................................................... 53 2.1.6. Instrument sensitivity .................................................................................... 55 2.1.7. Operating Procedure...................................................................................... 60 2 Table of Contents 2.1.8. Xenon components ........................................................................................ 61 2.2. Irradiation ............................................................................................................. 66 CHAPTER 3. CONSTRAINING THE THERMAL HISTORY OF THE R- CHONDRITE PARENT BODY ..................................................................................... 69 3.1. Introduction .......................................................................................................... 69 3.2. Sample descriptions and preparation ................................................................... 70 3.3. Xe components ..................................................................................................... 71 3.3.1. Unirradiated samples ..................................................................................... 71 3.3.2. Irradiated samples ......................................................................................... 77 3.4. I-Xe results ........................................................................................................... 83 3.4.1. Correlation between 129Xe* and iodine in irradiated samples ...................... 83 3.4.2. I-Xe ages derived from isochrons ................................................................. 87 3.5. Discussion ............................................................................................................ 89 3.5.1. Link between I-Xe age and metamorphism? ................................................ 89 3.5.2. “Old” age of NWA 830 ................................................................................. 90 3.5.3. I-Xe ages compared to reported ages of R-chondrites .................................. 92 3.5.4. I-Xe ages of R-chondrites compared with other chondrites.......................... 94 3.6. Summary .............................................................................................................. 96 CHAPTER 4. IODINE AND XENON IN THE FIRST BASALTS .............................. 98 4.1. Introduction .......................................................................................................... 98 4.2. Results .................................................................................................................. 99 4.2.1. Xenon components in eucritic meteorites ..................................................... 99 4.2.2. 129Xe* and iodine in eucritic meteorites...................................................... 110 4.2.3. Chronological meaning? ............................................................................. 120 4.3. Discussion .......................................................................................................... 122 4.3.1. Comparison of absolute ages of Ibitira ....................................................... 122 4.3.2. Xe in the anomalous eucrite Ibitira vs. “nomalous” eucrites ...................... 123 4.3.3. Xe in the anomalous eucrite Bunburra Rockhole vs. “nomalous” eucrites 124 4.3.4. Extended Xe-loss on Vesta?........................................................................ 125 4.5. Summary ............................................................................................................ 127 CHAPTER 5. VOLATILE MOVEMENTS IN THE UNIQUE CRUSTAL ACHONDRITE GRA 06129 ........................................................................................ 129 5.1. Introduction ........................................................................................................ 129 5.2. Sample description ............................................................................................. 129 5.3. Results of I–Xe analysis ..................................................................................... 131 3 Table of Contents 5.3.1. Fissiogenic xenon ........................................................................................ 132 5.3.2. Large amounts of excess 128Xe* ................................................................. 136 5.3.3. Distinct releases .......................................................................................... 136 5.3.4. I–Xe “Age”/Initial iodine ratio.................................................................... 137 5.3.5. Trace–element chemistry ............................................................................ 137 5.4. Discussion .......................................................................................................... 141 5.4.1. Origin of the excess 129Xe* ......................................................................... 141 5.4.2. Origin of the Q–Xe ..................................................................................... 143 5.4.3. Terrestrial atmosphere rather than Q–Xe? .................................................. 144 5.4.4. Implications for I–Xe dating ....................................................................... 144 5.5. Conclusions and implications of this work ........................................................ 145 5.5.1. Summary of results ..................................................................................... 145 5.5.2. Order
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