Abundance, major element composition and size of components and matrix in CV, CO and Acfer 094 chondrites

Abstract

The relative abundances and chemical compositions of the macroscopic components or “inclusions” (chondrules and refractory inclusions) and fine-grained mineral matrix in chondritic meteorites provide constraints on astrophysical theories of inclusion formation and chondrite accretion. We present new techniques for analysis of low count/pixel Si, Mg, Ca, Al, Ti and Fe X-ray intensity maps of rock sections, and apply them to large areas of CO and CV chondrites, and the ungrouped Acfer 094 chondrite. For many thousands of manually segmented and type-identified inclusions, we are able to assess, pixel-by-pixel, the major element content of each inclusion. We quantify the total fraction of refractory elements accounted for by various types of inclusion and matrix. Among CO chondrites, both matrix and inclusion Mg/Si ratios approach the solar (and bulk CO) ratio with increasing petrologic grade, but Si remains enriched in inclusions relative to matrix. The oxidized CV chondrites with higher matrix/inclusion ratios exhibit more severe aqueous alteration (oxidation), and their excess matrix accounts for their higher porosity relative to reduced CV chondrites. Porosity could accommodate an original ice component of matrix as the direct cause of local alteration of oxidized CV chondrites. We confirm that major element abundances among inclusions differ greatly, across a wide range of CO and CV chondrites. These abundances in all cases add up to near-chondritic (solar) bulk abundance ratios in these chondrites, despite wide variations in matrix/inclusion ratios and inclusion sizes: chondrite components are complementary. This complementarity provides a robust meteoritic constraint for astrophysical disk models.