Magmatic thermal histories and emplacement mechanisms of Martian shergottite meteorites

Eckley, Scott Allan
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Shergottites are meteorites originating from Mars with a broadly basaltic composition. Derived from partial melting of the mantle and filtered through the crust, shergottites provide a window into magma generation and evolution on Mars. Using X-ray computed tomography (CT), we investigated a suite of shergottites to infer details about their petrogenesis, especially their thermal histories and emplacement environments. In a novel survey of 3D olivine morphologies, sizes, and core Fe-Mg content in five olivine-bearing shergottites, we identified the common occurrence of Mg-rich olivine megacrysts with morphologies indicative of rapid crystal growth, requiring large degrees of undercooling and possibly fast cooling. This method allows us to interpret crystallization sequences and thermal histories. Results indicate that undercooling in two geochemically enriched shergottites occurs during initial magma pooling in the lower crust, a process that has not been documented in Earth basalts. Whereas, in two geochemically depleted samples, undercooling occurs later in the crystallization sequence but before the final eruption and solidification. We demonstrate the conditions needed for rapid growth are common for mafic magmatism on Mars, and that these magmas experience complex thermal histories characterized by discrete episodes of large undercooling at depth. Because shergottites were ejected from the Martian surface during impact events, their geologic contexts are unknown. We measured 3D petrofabrics and crystal size distributions in eight shergottites that span the known lithologic categories (i.e., basaltic, olivine-phyric, gabbroic, and poikiltic) to constrain their emplacement mechanisms. To contextualize the petrofabric results, we also conducted the same analyses on terrestrial igneous rocks from a variety of intrusive and extrusive environments. When interpreted in the context of the analogue study, shergottite petrofabric analyses combined with crystal size distribution analyses indicate that all samples (gabbroic, poikilitic, basaltic, and olivine-phyric) in this study were emplaced in the subsurface at various depths. This study helps to decipher emplacement styles, which is important for understanding potential interactions with crustal material, subsolidus cooling histories, shock-metamorphic processes, and potential exposure at the surface.