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.

The presence of primary iron sulfides that appear to be aqueously altered in CM and CR carbonaceous chondrites provides the potential to study the effects and, by extension, the conditions of aqueous alteration. In this work, we have used SEM, TEM, and EPMA techniques to characterize primary sulfides that show evidence of secondary alteration. The alteration styles consist of primary pyrrhotite altering to secondary pentlandite (CMs only), magnetite (CMs and CRs), and phyllosilicates (CMs only) in grains that initially formed by crystallization from immiscible sulfide melts in chondrules (pyrrhotite-pentlandite intergrowth [PPI] grains). Textural, microstructural, and compositional data from altered sulfides in a suite of CM and CR chondrites have been used to constrain the conditions of alteration of these grains and determine their alteration mechanisms. This work shows that the PPI grains exhibit two styles of alteration-one to form porous pyrrhotite-pentlandite (3P) grains by dissolution of precursor PPI grain pyrrhotite and subsequent secondary pentlandite precipitation (CMs only), and the other to form the altered PPI grains by pseudomorphic replacement of primary pyrrhotite by magnetite (CMs and CRs) or phyllosilicates (CMs only). The range of alteration textures and products is the result of differences in conditions of alteration due to the role of microchemical environments and/or brecciation. Our observations show that primary sulfides are sensitive indicators of aqueous alteration processes in CM and CR chondrites.

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Abstract-We have analyzed glasses in eight howardites with the aim of distinguishing their origins as impact melts or pyroclasts. Although theoretical calculations predict that pyroclastic eruptions could have taken place on Vesta, the occurrence of pyroclastic glasses in HED meteorites has not been documented. This study involved petrographic examination of textures, electron microprobe analysis of major and minor elements, and LA-ICP-MS analysis for selected trace elements. Previously documented textural and compositional differences between lunar impact-melt and pyroclastic glasses partly guided this study. This work yielded no positive identification of pyroclastic glasses. The most likely explanation is that pyroclastic glasses never formed, either because Vesta contains insufficient volatiles to have powered explosive eruptions, or because eruptive conditions produced optically dense fire-fountains that allowed melt drops to collect as lava ponds. The impact-melt glasses were grouped (low-alkali, Ca-rich, and K-rich) based on compositions. We suggest that these glasses are the result of impacts onto known HED lithologies. The low-alkali glasses are impact melts of bulk HED lithologies. We hypothesize that the Ca-rich and K-rich glasses result from oversampling of plagioclase and of mesostasis that experienced liquid immiscibility, respectively, during micrometeorite impacts into eucrite targets.