Aqueous alteration in carbonaceous chondrites: Mass balance constraints on matrix mineralogy

Abstract

Bulk chemical compositions of matrix material in Antarctic CM chondrites and other non-Antarctic CM and CI chondrites have been determined using microprobe defocused beam techniques. These are used, along with the results of previously published mineralogical studies, to provide mass balance constraints on the relative proportions of intergrown and intermixed phyllosilicate phases in carbonaceous chondrite matrices. Results of these calculations indicate differing amounts of PCP (a mixture of approximately 25% tochilinite and 75% cronstedtite) and serpentines (Mg-rich and Fe-rich varieties in varying proportions or intermediate compositional varieties). Additional sulfide phases are also probably necessary to account for excess Ni and S. Fe/Si ratios for matrices of individual meteorites range from 1.21 to 2.77, corresponding to PCP/(PCP + SERF) ratios of 0.16 to 0.58. Progressive aqueous alteration of matrix appears to have occurred by formation of tochilinite, then cronstedtite and Mg-rich serpentine, and finally Fe-rich serpentine and sulfides. CM matrix clearly did not behave as an isolated system during alteration. CI chondrite matrices appear to contain little if any PCP; this may be a natural consequence of the absence of chondrule-associated metal, from which PCP forms, in the unaltered precursor material. These data provide a more quantitative picture of low-temperature aqueous alteration processes in carbonaceous chondrite parent bodies than has heretofore been possible from TEM studies alone.