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

Mass balance calculations were performed to constrain the precursor mineral ogy of fine-grained, aqueously altered materials in CM carbonaceous chondri tes. The bulk composition of unaltered fine-grained CM materials was estima ted and then used to calculate phase proportions for several different init ial assemblages. All starling 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 alt ered. Four endmember assemblages are considered by adding Fe metal, Fa(50), Fa(100), or FeO-rich amorphous materials to the Fe-poor phases. These repr esent the Fe-bearing phases in CM and/or other chondritic classes. Results indicate that the precursor CM assemblage may have contained a maximum of e ither similar to 10 vol% Fe metal, 57 vol% Faso, similar to 28 vol% Fa(100) , or 37.0 vol% FeO-rich amorphous materials. Additional calculations were p erformed in which Fe metal was added to the various FeO-bearing assemblages . These reveal a strong positive correlation between the forsterite/(forste rite + enstatite) ratio and the amount of FeO-bearing phases that coexist w ith metal. If forsterite was more abundant than low-Ca pyroxene in the accr eted CM materials, then these materials must have also contained significan t amounts of FeO-rich phases (e.g., at least 36 vol% Fa(50), 10 vol% Fa(100 ), or 17 vol% FeO-bearing glasses). Calculated mineral proportions suggest that intact calcium-aluminum-rich inclusions (CAIs) represent only about on e-half of the original CAI budget, which is consistent with previous hypoth eses that the initial CAI contents of CM and CO chondrites were similar. So me similarities exist between the primary CM assemblages calculated here an d the mineralogies of other chondrite classes, but the initial CM materials do not appear to be represented in our meteorite inventory.