MINERALOGY AND COMPOSITION OF MATRIX AND CHONDRULE RIMS IN CARBONACEOUS CHONDRITES

The degree of compositional variation of fine-grained minerals displayed by the members within any carbonaceous chondrite group (i.e., CI, CM, CV, CR) is a direct reflection of the range of aqueous alteration assemblages present. Matrix and fine-grained chondrule rims within any particular carbonaceous chondrite are mineralogically nearly identical to one another, but not necessarily similar in bulk elemental composition, even though they have subsequently experienced post-accretional secondary processing (aqueous alteration) under identical conditions. This may indicate preferential alteration of one component, brecciation, exchange of components between rims and matrix, and/or accretion of matrix and rims from mineralogically or compositionally distinct materials. We propose that CO chondrites experienced parent body conditions of low f(O2), low water/rock ratios, and temperatures below 50-degrees-C. CR chondrites experienced higher water/rock ratios, potentially higher temperatures (less-than-or-equal-to 150-degrees-C), and a wide range of f(O2). The alteration mineralogy of CV chondrites indicates water/rock ratios at the high end (at least) of the range for CR chondrites, Essebi, and MAC 87300. CM chondrites experienced temperatures below 50-degrees-C, low f(O2) and low water/rock ratios, except EET 83334, which probably experienced relatively higher f(O2), and B-7904 and Y-86720, which experienced postalteration temperatures in the range 500-700-degrees-C. Most CI chondrites experienced temperatures between 50 and 150-degrees-C, relatively high water/rock ratios, and variable f(O2) with a late-stage increase in the latter, due possibly to brecciation and consequent release of methane. Y-82162 witnessed post-alteration heating, possibly as high as 400-degrees-C.