Shock‐induced pervasive remelting of Fe sulfides in the basaltic shergottite Northwest Africa 14672: A benchmark for shock stages S6/S7 on Mars

Abstract

AbstractNorthwest Africa (NWA) 14672, the most highly shocked Martian meteorite so far, has experienced >50% melting, compatible with peak pressure >~65 Gpa, at a transition stage 6/7. Despite these extreme shock conditions, the meteorite still preserves a population of “large” Fe sulfide blebs from the pre‐shock igneous assemblage. These primary blebs preserve characteristics of basaltic shergottites in term of modal abundance, preferential occurrence in interstitial pores along with late‐crystallized phases (ilmenite, merrillite), and Ni‐free pyrrhotite compositions. Primary sulfides underwent widespread shock‐induced remelting, as indicated by perfect spherical morphologies when embedded in fine‐grained silicate melt zones and a wealth of mineral/glass/vesicle inclusions. Extensive melting of Fe‐sulfides is consistent with the decompression path experienced by NWA 14672 after the peak shock pressure at ~70 GPa. Primary sulfides acted as preferential sites for nucleation of vesicles of all sizes which helped sulfur degassing during decompression, leading to partial resorption of Fe‐sulfide blebs and reequilibration of pyrrhotite metal/sulfur ratios (0.96–0.98) toward the low oxygen fugacity conditions indicated by Fe‐Ti oxides hosted in fine‐grained materials. The extreme shock intensity also provided suitable conditions for widespread in situ redistribution of igneous sulfur as micrometric globules concentrated in glassy portions of fine‐grained lithologies. These globules exsolved early on quenching, allowing dendritic skeletal Fe‐Ti oxide overgrowths to nucleate on sulfides.