Deformation and Evolution of Achondrite Parent Bodies
Abstract
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.