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Oxygen Isotope Systematics of Ordinary Chondrite Chondrules: Insights Into the Inner Solar System Planetary Reservoir

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Oxygen Isotope Systematics of Ordinary Chondrite Chondrules: Insights Into the Inner Solar System Planetary Reservoir Oxygen isotope systematics of ordinary chondrite chondrules: insights into the inner solar system planetary reservoir Leonardo Baeza February 2019 A thesis submitted for the degree of Master of Philosophy of the Australian National University © Copyright by Leonardo Baeza 2019 ii DECLARATION This thesis consists of a study carried out at the Australian National University. I certify that this thesis is my own work except where otherwise acknowledged and has not been accepted for the award of any other degree or diploma at any university. Leonardo Baeza iii iv Acknowledgements In these few lines I would like to thank all the people that have made this thesis possible and all the people who have supported me during this enjoyable process. In particular, my parents, who gave me the opportunity to explore different arts from the very beginning. To my friends in Chile and Australia, family, RSES administrative and technical staff, friends from work, uni, particularly Morgan, Geoff, Suzette, Gianluca, Antonio, Pat, Melanie, Joëlle, Lianne, Michael, James, Johanna, Mari, and Rose. I am especially grateful to Trevor who gave me the opportunity to study these ancient rocks and has been of great support, both on personal and academic levels. To Janaina for her help, feedback, and great vibes, to Guil for his help at the very beginning of my MPhil, and Yuri, for his scientific support and criticism. Also thankful to the two Peters and Dave from the SHRIMP group, and Bob for EPMA assistance. In addition, I would also like to thank the Chilean Government and its educational institution, CONICYT, for granting me with their international scholarship scheme, Becas Chile. Finally, the most important ones, my wife Tutú, who made this crazy idea of playing with meteorites a family project, and my children, Vasco and Mila, who remind me every day about the essential things in life. v vi ABSTRACT Chondrules are fundamental objects from which important insights in the conditions of the solar protoplanetary disk can be inferred. In this regard, oxygen isotope composition of chondrules have played a key role tracking the physicochemical environment under which their formation occurred. It has been proposed that chondrite asteroids accreted specific chondrule groups or ‘chondrule populations’ in terms of oxygen isotopes but this hypothesis has never been statistically assessed. Systematic in situ measurements of oxygen isotope composition of chondrule olivine from ordinary chondrites (H, L, and LL groups) by SHRIMP-SI and a robust statistical evaluation allowed us to conclude for the first time that the different ordinary chondrite parent bodies sampled the same main population of chondrules in their accretion location. This population is characterised by 17 a mean of Δ O = 0.74 ± 0.10‰ (σ95%) with a variability of 0.53‰ (σ95%). Ordinary chondrite chondrules were then formed in the same gaseous oxygen isotope reservoir or region of the protoplanetary disk. This population is formed by chondrules with different chemical composition (Type I and Type II) indicating that their formation region was chemically differentiated and form a chemical continuum rather than two separated classes. Our results together with literature data lead us to propose an oxygen isotope gradient of the protoplanetary disk ambient gas under which these and other astrophysical objects (e.g. refractory inclusions) were formed, 16O-richer closer to the Sun and 16O- poorer at longer radial distances. Ultimately, we suggest a change in the paradigm related to the oxygen isotope reservoirs of the solar system, particularly the so-called planetary reservoir. It is proposed that the gaseous oxygen isotope planetary reservoir is a discrete reservoir enriched in SiOgas molecules and other rock-forming elements present since the earliest stages of the solar system evolution possibly reflecting the average oxygen isotope composition of the primordial dust of the solar nebula. The mean composition of 18 17 the planetary reservoir is δ O = 4.64 ± 0.15‰ (σ95%) and δ O = 3.03 ± 0.10‰ (σ95%), with variabilities of 0.97‰ and 0.52‰ (σ95%), respectively. vii TABLE OF CONTENTS Acknowledgements ....................................................................................................................... v Abstract ........................................................................................................................................ vi 1. INTRODUCTION ................................................................................................................. 1 1.1. Problem presentation ..................................................................................................... 2 2. Background............................................................................................................................ 8 2.1. Oxygen Isotopes ............................................................................................................ 8 2.2. Chondrites .................................................................................................................... 13 2.3. Refractory inclusions ................................................................................................... 14 2.4. Chondrules ................................................................................................................... 15 2.5. Matrix .......................................................................................................................... 20 2.6. Ordinary chondrites ..................................................................................................... 20 3. SAMPLES AND ANALYTICAL METHODS................................................................... 25 3.1. Samples ........................................................................................................................ 25 3.2. Optical Microscopy and Scanning Electron Microscopy ............................................ 29 3.3. Electron Microprobe Analysis ..................................................................................... 29 3.4. Sensitive High Resolution Ion Microprobe Stable Isotopes (SHRIMP-SI) ................ 30 3.5. Data Reduction ............................................................................................................ 36 4. RESULTS ............................................................................................................................ 40 4.1. Matrix effect ................................................................................................................ 40 4.2. Chondrule petrography and olivine chemical compositions ....................................... 42 4.3. Oxygen Isotopes .......................................................................................................... 54 4.3.1. Data analysis ........................................................................................................ 54 4.3.2. Oxygen isotope compositions .............................................................................. 58 4.3.3. Oxygen isotope composition and chondrule olivine Type .................................. 67 5. DISCUSSION ...................................................................................................................... 73 5.1. Matrix Effect ............................................................................................................... 73 5.2. Chondrule size ............................................................................................................. 74 5.3. Sample reclassification ................................................................................................ 76 5.4. Ordinary chondrites chondrule main population ......................................................... 78 5.5. ‘Statistical outliers’: insights into inner solar system spatially separated oxygen isotope gaseous reservoirs ..................................................................................................... 100 5.6. 16O-rich chondrule olivine: low mixing of dust from distinct oxygen reservoirs ...... 109 viii 5.7. Ordinary chondrites bulk composition: insights into terrestrial planet formation .... 122 5.8. Type I vs. Type II chondrules: physicochemical conditions of the ambient gas ...... 129 5.9. The oxygen isotope planetary reservoir of the inner solar system ............................ 138 6. Conclusions ....................................................................................................................... 149 References ................................................................................................................................. 153 Appendix ................................................................................................................................... 172 ix LIST OF FIGURES Figure 1.1 Oxygen isotope compositions of bulk individual chondrules from ordinary chondrites ....................................................................................................................................................... 5 Figure 2.1. Oxygen isotope composition of major chondrite classes and ranges of bulk chondrules ....................................................................................................................................................... 9 Figure 2.2. Oxygen isotope composition of the solar system materials ...................................... 10 Figure 2.3. Photodissociation and self-shielding model in a molecular
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