Unequilibrated EH chondrites contain silica-bearing chondrules with abundant niningerite [(Mg,Fe,Mn)S] and troilite (FeS), distinguishing them from the silica-bearing chondrules in ordinary and carbonaceous chondrites. The conventional explanation for the origin of niningerite and oldhamite (CaS) is that they are condensates from C-rich nebular gas. However, models of condensation from a solar gas with an elevated C/O ratio predict mineralogy that is inconsistent with petrographic observations. We report petrographic and chemical evidence from 45 silica-bearing chondrules from the EH3 chondrites Sahara 97072 and Alan Hills 84170 for formation of niningerite and oldhamite by sulfidation of ferromagnesian silicates. The results indicate extensive thermal processing of the chondrules including melting before, during, and after sulfidation. Bulk compositions of chondrules exhibiting varied degrees of sulfidation suggest that depletion of Mg and enrichment of Fe, Mn, and Na accompany the reactions.Sulfidation of FeO-bearing silicates, which formed at oxidizing conditions (at least IW-3), can occur with exposure to a H-poor, C- and S-rich gaseous reservoir 6-8 fO2 log units below the IW buffer at temperatures high enough for partial melting of silicates. Physicochemical analysis of mineral reactions inferred from sulfidized chondrules suggests a melted metal-sulfide assemblage (in a H-poor environment) is capable of generating sufficient S vapor to drive sulfidation. The reaction of silicates with the S gas will result in progressive extraction of Fe, Ca, and Mg into sulfides, with the stoichiometric amounts of silica either reacting with olivine to form enstatite or, when olivine is exhausted, precipitating as free silica. The sulfidation environment drastically increases Mg volatility, resulting in evaporative loss until saturation in the ambient gas is reached. The newly formed Mg-rich niningerite will tend to reach equilibrium composition by losing Mg and gaining Fe plus Mn from the ambient gas, consistent with the observed chemical fractionation in the sulfidized chondrules. Therefore, sulfidation of ferromagnesian silicates can also explain the low bulk Mg/Si ratios of the enstatite chondrites if the sulfidizing gas was lost before accretion of the enstatite chondrite parent body.
ASJC Scopus subject areas
- Geochemistry and Petrology