Petrogenesis of basaltic shergottite NWA 8656

Most basaltic shergottites are too Mg-rich to represent parent melt compositions because they contain some cumulus pyroxenes. However, basaltic shergottite Northwest Africa (NWA) 8656 with subophitic texture can be used as the parent melt composition in petrogenetic studies because it contains no or rare cumulus pyroxenes. Its pyroxene cores (Mg# 66-68, the most magnesian) are in equilibrium with the bulk rock composition based on major (Fe-Mg) and trace elements (REE—rare earth elements). The patchy zoning of pyroxenes has been interpreted as reflecting a two-stage crystallization history: 1) crystallization of Mg-rich pyroxene cores at depth (50 km, the base of Martian crust), 2) crystallization of Fe-rich pyroxene rims at the shallow depth near the Martian surface with a fast cooling history. The crystallization of Fe-rich pyroxenes and the existence of different symplectites indicate that NWA 8656 underwent eruption. The oxygen fugacity of NWA 8656 (QFM –0.9±0.5) suggests an oxidized condition at the late-stage crystallization process, and the CI-normalized REE patterns of different minerals show enrichment in LREE, compared to that of depleted shergottites. Both of these observations suggest a relatively ITE (incompatible trace elements)-enriched signature of NWA 8656, similar to those of other enriched shergottites. The REE compositions of augite core and rim and plagioclase can be successfully reproduced by progressive crystallization without exogenous components, which indicates a closed magmatic system for NWA 8656. Consequently, we conclude that the ITE-enriched signature of NWA 8656 is inherited from an enriched mantle source rather than caused by crustal assimilation. Moreover, partial melting of depleted Martian mantle could not directly yield magmas that have geochemical characteristics similar to enriched shergottite parent magmas, so the enriched and depleted shergottites are derived from distinct mantle sources, and the mantle source of enriched shergottites would be expected to contain ilmenite.