Primary iron sulfides in CM and CR carbonaceous chondrites: Insights into nebular processes

Abstract We have carried out a systematic study involving SEM, EPMA, and TEM analyses to determine the textures and compositions of sulfides and sulfide–metal assemblages in a suite of minimally to weakly altered CM and CR carbonaceous chondrites. We have attempted to constrain the distribution and origin of primary sulfides that formed in the solar nebula, rather than by secondary asteroidal alteration processes. Our study focused primarily on sulfide assemblages associated with chondrules, but also examined some occurrences of sulfides within the matrices of these meteorites. Although sulfides are a minor phase in carbonaceous chondrites, we have determined that primary sulfide grains are actually a major proportion of the sulfide grains in weakly altered CM chondrites and have survived aqueous alteration relatively unscathed. In minimally altered CR chondrites, we have determined that essentially all of the sulfides are of primary origin, confirming the observations of Schrader et al. ( ). The pyrrhotite–pentlandite intergrowth (PPI) grains formed from crystallization of monosulfide solid solution (mss) melts, while sulfide‐rimmed metal (SRM) grains formed from sulfidization of Fe,Ni metal. Micron‐sized metal inclusions in some PPI grains may have formed by co‐crystallization of metal and sulfide from a sulfide melt that experienced S volatilization during the chondrule formation event, or alternatively, may be a remnant of sulfidization of Fe,Ni metal that also occurred during chondrule formation. Sulfur fugacity for SRM grains ranged from −18 to −10 (log units) largely in agreement with predicted solar nebular values. Our observations show that understanding the formation mechanisms of primary sulfide grains provides clues to solar nebular conditions, such as the sulfur fugacity during chondrule formation.