Apatite Formation in Shergottite NWA 7397: Implications for the Mapping of Martian Meteorites

Abstract Martian meteorites are currently our only existing samples from Mars. They are divided into two primary types, the shergottites and the nakhlite-chassignite types. The shergottites are by far the most abundant of the Martian meteorites. Apatite in particular is the only volatile bearing phase in these and thus is crucial for understanding volatile cycles on Mars. The primary goal of the study is to understand the effects of shock metamorphism on the volatile content of apatite. In particular, looking at intergrown apatite-merrillite grains to observe the Cl variation within apatite and to determine if the merrillite is in fact merrillite or if its tuite. Here we used various chemical analyses to accurately map the mineralogy of shergottite NWA 7397 to learn more about volatile content and shock effects to constrain its petrogenesis. For NWA 7397, what we focused on was the phosphates. Mainly on the calcium phosphates, apatite ( Ca 5 [ PO 4 ] 3 [ F, CL, OH ]) and merrillite ( Ca 9 Na ( Fe, Mg )( PO 4 ) 7 ). We can identify apatite based on its volatile components ( F and Cl ) as part of its crystal structure which merrillite lacks. Apatite is, however, the only volatile bearing phase in shergottites, making it our only was to constrain water and Cl content of martian magmas and by extension martian mantle reservoirs. However, the volatile contents can be effected by shock ejection when the rock was blasted off the martian surface. This means we need to understand the effects of shock before using apatite to estimate magma volatile content. From our research so far we have been able to identify apatite and merrillite, but no tuite. This will require further analysis to identify tuite, if any is present.