Constraints on the anhydrous precursor mineralogy of fine‐grained materials in CM carbonaceous chondrites

Abstract

Abstract— Mass balance calculations were performed to constrain the precursor mineralogy of fine‐grained, aqueously altered materials in CM carbonaceous chondrites. The bulk composition of unaltered fine‐grained CM materials was estimated and then used to calculate phase proportions for several different initial assemblages. All starting assemblages contain relic, unaltered Fe‐poor phases observed in CM chondrites, plus iron sulfides. The original sources of Fe are uncertain, because most primary Fe‐rich phases were aqueously altered. Four endmember assemblages are considered by adding Fe metal, Fa50, Fa100, or FeO‐rich amorphous materials to the Fe‐poor phases. These represent the Fe‐bearing phases in CM and/or other chondritic classes. Results indicate that the precursor CM assemblage may have contained a maximum of either ∼10 vol% Fe metal, 57 vol% Fa50, ∼28 vol% Fa100, or 37.0 vol% FeO‐rich amorphous materials. Additional calculations were performed in which Fe metal was added to the various FeO‐bearing assemblages. These reveal a strong positive correlation between the forsterite/(forsterite + enstatite) ratio and the amount of FeO‐bearing phases that coexist with metal. If forsterite was more abundant than low‐Ca pyroxene in the accreted CM materials, then these materials must have also contained significant amounts of FeO‐rich phases (e.g., at least 36 vol% Fa50, 10 vol% Fa100, or 17 vol% FeO‐bearing glasses). Calculated mineral proportions suggest that intact calcium‐aluminum‐rich inclusions (CAIs) represent only about one‐half of the original CAI budget, which is consistent with previous hypotheses that the initial CAI contents of CM and CO chondrites were similar. Some similarities exist between the primary CM assemblages calculated here and the mineralogies of other chondrite classes, but the initial CM materials do not appear to be represented in our meteorite inventory.