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A Proposed Origin and Timeline for Plagioclase and Pyroxene Bearing Clasts in Martian Meteorite NWA 7034 Msc Thesis

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A Proposed Origin and Timeline for Plagioclase and Pyroxene Bearing Clasts in Martian Meteorite NWA 7034 Msc Thesis A proposed Origin and Timeline for Plagioclase and Pyroxene bearing Clasts in Martian Meteorite NWA 7034 MSc Thesis Line Schug Wageningen University and Research 30.03.2019 ABSTRACT NWA 7034 is a unique Martian meteorite. It is one of the few pieces of Martian crust that has made its way to Earth. It has been classified as a breccia, meaning it is a clast composed of older clasts, bound by a matrix. This made it possible to study the compositions and textures of multiple generations of rocks, enriching our knowledge about the processes that defined rock formation on the surface of Mars over billions of years. Based on compositional differences of feldspars and pyroxenes, and textural differences of basaltic clasts, it is likely that NWA 7034 underwent at least two generations of brecciation, resulting in a diverse assembly of clasts. High pressure shock textures, such as fractures, twinning in pyroxene, quenched pyroxene and plagioclase glass, were observed in older generation clasts and to a lesser extent in the youngest generation clasts. The distribution of shock textures indicate that the source rock underwent at least three impacts prior to impacting on Earth. Using spectral analysis, the compositions of feldspars and pyroxenes was obtained. Pyroxenes were magnesic and occurred both as ortho- and clinopyroxene, implying different cooling rates and therefore multiple heating events. Formation temperatures of 1210-<500˚C ± 50˚C in older generation clasts, and 1225-600˚C ± 50˚C in the youngest generation clasts were inferred from their composition. Both alkali and plagioclase feldspars were found. The plagioclase was sodic, denoting a formation temperature of 1260-1150˚C ± 25˚C in older generation clasts, and 1245-1150˚C ± 25˚C in the youngest generation clasts. Unless part of a gabbroic rock, alkali feldspar occurred as perthite, implying a formation temperature of ≤1000˚C ± 50˚C, and pressure <0.5 GPa. In gabbroic rocks, its composition suggests a crystallization temperature of ≤1070˚C ± 50˚C. Bulk rock composition and general mineralogical assemblage match mineral composition of Gusev crater, which has been suggested to be situated above an alkali volcanic source. Furthermore, a cumulate clast was found in NWA 7034 whose composition matches a Noachian-age (~4.1 – ~3.7 Ga) layered intrusion that was exposed following the meteorite impact that formed Gusev crater during the Hesperian period (~3.7 – ~3.0 Ga). Petrology and molar composition of plagioclase and pyroxene in the subsamples closely match the clasts analysed at Gale crater. It is likely that rocks sourced from both these regions ended up in NWA 7034, and that its ejection location was in the zone that encompasses impact ejecta from both Gale and Gusev crater. Overall, the petrologic evidence suggests that NWA 7034’s formation and alteration history began with alkaline rocks formed in the Pre-Noachian (~>4.4 Ga), followed by accumulation, brecciation and alteration occurring throughout the Amazonian period (~3.0 Ga – present) until its ejection from Mars and subsequent impact on Earth (~7.3 Ma). ACKNOWLEDGEMENTS I would like to thank Stephen Mojzsis and Nigel Kelly for their guidance and inspiration, the amazing opportunity to study NWA 7034, and inviting me to be there in person to watch and participate first-hand how data was being obtained in the laboratories in Colorado. I would also like to thank Benny Guralnik for supervising the project and for Cathelijne Stoof and Jakob Wallinga for taking over after Benny Guralnik’s departure from the University. I am grateful to everyone who has believed in me and encouraged me to go on. Last but not least, I would like to thank my family for their support and allowing me to pursue a scientific career. Thank you for taking the time to read this thesis. I hope it will be an informative and enjoyable experience for you. Line Schug TABLE OF CONTENTS Abstract ..........................................................................................................................................................2 Acknowledgements ........................................................................................................................................3 Table of Contents ...........................................................................................................................................4 Table of Figures ..............................................................................................................................................6 Table of Tables ................................................................................................................................................8 Table of Info Boxes .........................................................................................................................................8 1 Introduction ............................................................................................................................................1 Background .....................................................................................................................................1 What makes NWA 7034 special? ....................................................................................................4 Research Motivation ......................................................................................................................6 Research questions .........................................................................................................................6 2 Methods .................................................................................................................................................8 Sample preparation ........................................................................................................................9 Instrumental analysis................................................................................................................... 10 Interpretation of results .............................................................................................................. 13 Assumptions ................................................................................................................................ 15 3 Results ................................................................................................................................................. 16 Feldspar phenocrysts ................................................................................................................... 19 Pyroxene phenocrysts ................................................................................................................. 31 Gabbroic clasts ............................................................................................................................ 47 3.3.1 Norite-pyroxenite ................................................................................................................ 48 3.3.2 Gabbronorite ....................................................................................................................... 51 3.3.3 Apatite-plagioclase(-ilmenite) ............................................................................................. 60 3.3.4 Gabbro ................................................................................................................................. 64 Peridotites ................................................................................................................................... 68 Spherules ..................................................................................................................................... 70 4 Discussion ............................................................................................................................................ 82 Genesis of minerals and clasts .................................................................................................... 82 4.1.1 Feldspar phenocrysts ........................................................................................................... 84 4.1.2 Pyroxene phenocrysts ......................................................................................................... 87 4.1.3 Gabbroic clasts .................................................................................................................... 91 4.1.4 Peridotites ........................................................................................................................... 95 4.1.5 Spherules ............................................................................................................................. 96 4.1.6 Vein ...................................................................................................................................... 98 Quantifying impact events .......................................................................................................... 99 Source rock origin on Mars ........................................................................................................ 101 Comparison to other studies ..................................................................................................... 103 5 Conclusion ......................................................................................................................................... 104 Bibliography ..............................................................................................................................................
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