CONSTRAINING MARTIAN MAGMATISM AND SHERGOTITTE FORMATION THROUGH MELT INCLUSION ANALYSES OF A POIKILITIC SHERGOTITTE SUITE

Martian poikilitic shergottite meteorites are cumulate rocks that can be studied to understand magma evolution on Mars originating from the crust-mantle boundary up to shallow depths. Poikilitic shergottites consist of more than 20% of the total martian meteorite collection and are characterized as gabbroic or ultramafic potentially representing a major lithology in the martian crust. Although poikilitic shergottites are abundant, we have not fully constrained their parental magma compositions nor their link to other shergottites. Through comprehensive melt inclusion (trapped melt pockets) analyses, we aim at gaining a better understanding of poikilitic shergottite formation and sources, allowing us to constrain petrological links between the shergottite subgroups and better understand the evolution of the martian interior.

In our study, we examined a suite of five meteorites including: Allan Hills (ALHA) 77005, northwest Africa (NWA) 11065, NWA 7755, NWA 10618, and NWA 11043 by conducting melt inclusion analyses in both poikilitic (early-stage mineral assemblages) and non-poikilitic (late-stage mineral assemblages) textures. We suggest the possibility that poikilitic shergottites and olivine-phyric shergottites share a petrological link such as a common magmatic system. Our results show that though not as primitive, the parental melt compositions of the poikilitic shergottites overlapped with the range of olivine-phyric shergottites parental melt compositions suggesting similar petrogenesis and magmatic history for the two types. Five non-poikilitic melt inclusions exhibited K-enrichment (>1 wt.% K₂O), while only one poikilitic melt inclusion had K-enrichment. Each of the K-rich melt inclusions have K₂O/Na₂O ratios >1.30 wt.%, while all the K-poor melt inclusions had K₂O/Na₂O ratios <0.60 wt.% indicating that the enrichment is likely coming from an enriched source or magmatic process. We suggest that this K-enrichment is caused via assimilation or alteration in the crust and not by a metasomatized mantle source or fractionation of K-poor mineral phases. We believe the addition of K-rich crustal material is a common process during poikilitic shergottite formation, which occurs in the later stages of poikilitic shergottite melt evolution after the formation of the poikilitic textures.