Sulfide mineralogy and redox conditions in some shergottites

Abstract

Magmatic sulfide mineralogy has been studied in 2 olivine-phyric shergottites (DaG 476 and SaU 005) and 4 basaltic shergottites (Zagami, Shergotty, Los Angeles, and NWA 480). Modal abundances of magmatic sulfides, as estimated by image analysis on thin section, are high (0.16 to 0.53 area percent) and correlate positively with abundances of Fe-Ti oxides. Sulfides are mesostasis minerals, being mostly interstitial grains or locally enclosed in post-cumulus melt inclusions (e.g., SaU 005) in olivine. Sulfides in shergottites are composed of major pyrrhotite containing pentlandite exsolutions associated with minor amounts of Cu sulfides (chalcopyrite and/or cubanite). Hot desert finds (e.g., DaG 476) show abundant fracture-filling iron (oxy)hydroxides of probable terrestrial origin. Unaltered sulfides show metal-rich hexagonal pyrrhotite compositions with metal/sulfur (M/ S) ratio ranging between 0.936 +/- 0.005 and 0.962 +/- 0.01. This compositional range corresponds to the two-phase structural domain 2C + nC of the Fe-S system; however, the high-temperature disordered hexagonal 1C pyrrhotite structure would be in better agreement with magnetic properties of shergottites. Ni contents in pyrrhotite increase from Los Angeles (<0.05 at%) to Shergotty, Zagami, and NWA 480 (0.2-0.5 at%), and DaG 476 and SaU 005 (up to 3 at%). The higher Ni values of pyrrhotite in olivine-phyric shergottites correlate with the abundance of pentlandite exsolutions, both reflecting more primitive Ni-rich sulfide liquids where abundant olivine crystallized. This result and the strong correlation between sulfide abundances and Fe-Ti oxides argue for a primary magmatic origin of these sulfides. Although they reproduce the trend of magmatic oxygen fugacity conditions determined from Fe-Ti oxide pairs, observed pyrrhotite compositions are systematically more metaldeficient compared to those calculated from the Fe-S-O system. This suggests post-magmatic oxidation during cooling on Mars, followed by terrestrial weathering for hot desert finds.