Enstatite chondrites: condensation and metamorphism under extremely reducing conditions and contributions to the Earth

Abstract

AbstractEnstatite chondrites are a small clan of meteorites, only ~ 1% out of all meteorite collection. However, they are the most reduced meteorites and have almost identical isotopic compositions to those of the Earth, suggestive of significant contributions to the latter and other terrestrial planets. Enstatite chondrites contain a unique mineral inventory of sulfides of typical lithophile elements, Si-bearing metal, silicide and phosphide, which record the nebular processes and the thermal metamorphism in asteroidal bodies under extremely reducing environments. EH group is mainly characteristic of the higher Si content of metallic Fe–Ni and the higher MnS contents of sulfides than EL group, indicative of a more reducing condition than the latter. However, the fugacity pH2S could be the same in both EH and EL regions, because it was buffered by kamacite and troilite. The majority of sulfides condensed from the nebula, partially enclosing schreibersite micron-spherules formed probably by early melting. Another part of troilite, sphalerite and djerfisherite, intergrown with perryite, were produced via sulfidation of metallic Fe–Ni. Minor exotic components were also found in enstatite chondrites, including Ca-, Al-rich inclusions and FeO-rich silicate clasts. The Ca-, Al-rich inclusions are identical to those in carbonaceous chondrites except for the alteration under reducing environments, and the FeO-rich silicate clasts show reduction reactions, both suggestive of migration of dust in the protoplanetary disk. The highly reducing conditions (as C/O ratios) might be established via repeating evaporation and condensation of water ice and organic matter across the snow line along the protoplanetary disk, but need to find evidence. Another issue is the preservation of submicron-to-micron-sized presolar grains during high-temperature condensation of the major constituent minerals. After accretion, the parent bodies of EH and EL chondrites probably experienced distinct thermal histories, indicated by their distinct petrologic-type distributions and different correlations with the closure temperatures determined by the FeS contents of sulfides in contact with troilite.The composition of (Mg, Mn, Fe)S, a key indicator for condensation and metamorphism of enstatite chondrites.