PETROGENESIS OF ENRICHED AND INTERMEDIATE POIKILITIC SHERGOTTITES: FROM MAGMATIC SOURCE TO EMPLACEMENT

Martian poikilitic shergottite meteorites likely represent a significant lithology of the martian crust, as gabbroic hypabyssal rocks. The poikilitic shergottites, comprising >20% of all martian meteorites, have had limited work conducted on them, leaving potential gaps in the petrologic history of Mars. Here, to better understand the magmatic evolution and emplacement of these rocks, we present bulk-rock data, in situ major element data, oxygen fugacity (ƒO₂) values, phosphorus (P) maps, and quantitative textural analyses, on the most comprehensive suite (11 samples) of poikilitic shergottites yet, including three that have not been studied before [Northwest Africa 11065, NWA 11043, and NWA 10961].

All poikilitic shergottites show a ƒO₂ increase, ranging from 1.2 to 2.9 log units, from the poikilitic texture (early stage crystallization) to the non-poikilitic texture (late stage crystallization). Past modeling has shown that a ƒO₂ increase greater than ~1 log unit cannot be attributed to auto-oxidation alone, requiring external contributions, such as degassing, and/or assimilation of oxidized crust. In addition, early stage ƒO₂ calculated for light rare earth element intermediate rocks of this study (from the Fayalite-Magnetite-Quartz –FMQ– buffer -3.3 to -4.1) all overlap with previously reported ƒO₂ values of depleted shergottites, such as Tissint (FMQ ~ -3.5), implying ƒO₂ is not linked to martian magmatic sources as previously suggested. Phosphorus mapping of poikilitic and non-poikilitic olivine grains was conducted to explore crystal growth rate variations, as P is highly resistant to diffusion, preserving P patterns throughout growth. Phosphorus patterns of poikilitic grains show P poor centers, surrounded by fine oscillatory bands. Non-poikilitic grains display more complex P patterns with fine bands at grain centers, continuing to grain boundaries. These patterns support steady equilibrium growth during early stage crystallization, followed by disequilibrium growth during ascent. Quantitative textural analyses for seven enriched and three intermediate poikilitic shergottites are all in close resemblance. These results suggest that, despite disparate geochemical classifications (thus likely disparate sources), they were emplaced into the martian crust by similar mechanisms.