Doctor of Philosophy
Planetary Science and Exploration
McCausland, Phil J. A.
Flemming, Roberta L.
Minerals in planetary bodies record accumulated strain from deformation events such as hypervelocity impacts but potentially also parent body processes such as gravitational compaction. This work investigates olivine microstructural features that may distinguish between different impact levels versus parent body plastic deformation regimes. Achondritic meteorites from the ureilite parent body, the Moon, Mars as well as terrestrial basalt and mantle xenolith have been investigated using micro-XRD, EBSD, micro-XRF and other methods to study their parent body deformation history. A Matlab® method, unit segment length (USL), is herein developed to visualize the crystal boundary geometry and quantify the apparent subboundary density in 2D EBSD maps for single crystals. For differently shocked achondrite meteorites and non-shocked terrestrial rocks, USL in olivine grains is shown to increase with greater shock pressure, indicating the increased subboundary density and decreased subdomain size. Moreover, the scattered twist boundaries in olivine are found to increase with higher shock state, distinct from long edge boundaries that are known to be dominant in high-temperature and low-pressure deformation. By combining USL with mineralogical and isotopic evidence, the research studied the petrogenesis of achondrite meteorites in their parent body. XRD and EBSD observations reconstructed the petrogenesis of monomict ureilites in a small sized body with a possible post-impact deformation. Petrological, EPMA and in situ SIMS O isotopes investigation of newly discovered aluminous spinel with olivine grains in a polymict ureilite provided further thermal and geochemical constraints on the ureilite parent body evolution. Mineralogical and geochemical endeavors using XRD, XRF, and EPMA in lunar polymict breccia revealed a complex crustal process that mixed noritic-troctolitic clasts and surficial feldspathic material with plutonic basaltic material via impact events. Finally, XRD and EBSD revealed olivine was deformed differently in the heavily shocked Martian basalt and dunite. Olivine in Martian basalt was mechanically deformed into 1-5 micron neoblastic crystallites while maintaining the original host crystal boundaries, and the olivine in the Martian dunite exhibited extensively developed subboundary density but has not recrystallized. Mineralogical evidence combined with microstructural features investigated in this work helps to decode the achondritic parent body shock history from parent body deformation.
Summary for Lay Audience
This work primarily used olivine, one of the dominant rock forming minerals, as a deformation recorder to study their microstructure development in response to high-strain rate hypervelocity impact versus low-strain rate regional deformation. Achondrite meteorites delivered from the ureilite parent body (UPB), Lunar, and Mars were examined to understand their parent body deformation history. Terrestrial samples composed of Hawaiian olivine cumulates, xenocrysts in Hawaiian basalts, and kimberlites were also studied as the Earth analogue. The work utilized non-destructive analytical instrument of XRD and EBSD to study crystal structure, combining with XRF, EPMA, and in situ SIMS to provide a thorough petrogenesis investigation on the selected meteorites and terrestrial samples. A new Matlab® method, unit segment length (USL) was developed to visualize the crystal boundary geometry and quantify the apparent subboundary density in 2D EBSD maps for single crystals. USL in olivine grains is found to increase with increase of deformation pressure. In addition, the amount of scattered twist subdomain boundaries in olivine was also observed to increase with higher shock state. The USL method is validated by studying well-categorized terrestrial samples and the published literature data. By reconstructing the microstructures in ureilite meteorites, the UPB is found to be a rocky body with size smaller than 300 km in radius with a possible parent deformation after the impact. A newly discovered aluminous spinel along with other olivine grains in a polymict ureilite provided further temperature constraints on the UPB evolution. Lunar felspathic breccia displays a complex surficial process, including crustal crystallization, re-melting, and impacts, that mixed magnesium-rich clasts, surficial feldspathic material, and deeply buried gabbroic material. Finally, heavily shocked Martian basalt and dunite showed differently deformation history based on olivine microstructure development. Martian shergottite has olivine that were deformed into small crystallites with original grain boundary still identifiable. Moreover, relict clasts were found to retain the pre-shock deformation features. Martian dunite has stained olivine caused by nano iron with high density of dislocation but not yet recrystallized. The work established a tangible way for the future research on probing the evolutionary history of meteorites and their parent bodies.
Li, Yaozhu, "Deformation and Evolution of Achondrite Parent Bodies" (2023). Electronic Thesis and Dissertation Repository. 9716.
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Available for download on Sunday, December 31, 2023