Indicators of aqueous alteration in CM carbonaceous chondrites: Microtextures of a layered mineral containing Fe, S, O and Ni

A petrographic and transmission electron microscopy study of the Mighei, Murchison. and Murray CM carbonaceous chondrites shows that much of the CM matrix material was probably produced by aqueous alteration of olivine, pyroxene, sulfide, and metal. The amount of CM matrix appears to be proportional to the degree of alteration, as suggested by McSween (1979), and microtextures of PCP ("poorly characterized phase") provide evidence of the progressive alteration. PCP is divided into two major types; one occurs in chondrules and aggregates and consists largely of an Fe-Ni-S-O phase (Type-I), and the other occurs in matrix and consists of the Fe-Ni-S-O phase and cronstedtite in various proportions (Type-II). Microtextures of PCP suggest that it resulted from a three-stage alteration process. 1. (1) Type-I was produced by alteration of kamacite in chondrules and aggregates, presumably early, in the parent body regolith. 2. (2) As the alteration advanced, olivine and pyroxene were converted to serpentine. Type-I PCP separated from chondrules and aggregates (into the matrix) during regolith gardening. Simultaneously, the Fe-Ni-S-O phase reacted with Si (released by alteration of olivine and pyroxene), producing well-formed platy cronstedtite and coherent intergrowths of the Fe-Ni-S-O phase and cronstedtite. The Fe-Ni-S-O phase also recrystallized into platy and pod-like crystals. Fe. S. Ni, Cr. and P were leached out of Type-I PCP and were deposited as small grains of Fe-Ni Sulfides, magnetite, chromite. and a mineral (unidentified) containing Fe. Ni, Cr, and P. As a result, PCP came to consist primarily of the Fe-Ni-S-O phase and cronstedtite, i.e., Type-II PCP. 3. (3) During continued alteration, the well formed crystals of the Fe-Ni-S-O phase, cronstedtite, and their intergrowths in Type-II PCP were replaced by poorly formed fibers. In comparison to other CM chondrites, Mighei. Murchison, and Murray are relatively unaltered. Their matrices retain abundant amounts of the Fe-Ni-S-O phase and cronstedtite, commonly as distinctive PCP grains, which account for a large proportion of the Fe in these meteorite matrices. In more altered CM chondrites, much of the Fe-Ni-S-O phase was probably consumed to produce cronstedtite, magnetite, and sulfides. With further alteration, cronstedtite itself reacted with the serpentine to form ferroan serpentine. Thus the CM matrix was increasingly enriched in Mg with alteration, and PCP was increasingly degraded and intimately mixed with the magnesian phyllosilicates.