GEOCHEMICAL SIGNATURES OF MARTIAN SHOCKED CALCIUM PHOSPHATE MINERALS: IMPACTS AND IMPLICATIONS

Calcium phosphates (apatite, merrillite) are often targeted for Martian research as these minerals greatly aid our understanding of geologic processes and planetary evolution. This is due to the fact that phosphates can accommodate a wide-scope of trace elements (REEs, actinides) while apatite in particular can incorporate volatile species (F, Cl, OH). However, care must be taken when examining these phosphates, as all Martian meteorites have been subjected to some degree of shock which may cause extensive chemical heterogeneities. As such, the original objective of these works was to investigate the effect of shock metamorphism on elemental distributions for those phosphate minerals contained in the Martian meteorites ALH 84001 and Tissint. However, upon collecting data, a recently discovered (2021) phosphate phase was found to occur in the Martian meteorite Tissint. This mineral, keplerite, is the Na-deficient counterpart to the most abundant meteoric phosphate: merrillite. The mineral discoverers, Britvin et al., 2021, assert that keplerite occurs in extreme Na-depleted, High-T, environments. Here we show analytical results for Tissint which demonstrate keplerite (a novel find) coexisting with Na-bearing feldspar. Discovery of keplerite in Tissint shortly after its first publication suggests that the mineral is more common, and exists in more environments, than currently assumed. Further, ALH 84001 results revealed the presence of very distinct merrillite overgrowths on apatite grains, which have not been well documented in previous works. Therefore, the new objective of these works shall be a comparative study between two Martian meteorite samples which show evidence of novel (presence of keplerite in Tissint) and distinctive (presence of merrillite overgrowths on apatite in ALH 84001) phosphate phases: these phosphates, and their associated phases, are evaluated to understand their occurrence, formation, and geologic significance in Martian magma systems.