Volumetric Proportion Analyses of Carbonaceous Chondrites: Implications for Accretion and Compositional Diversity

The petrologic components of carbonaceous chondrites each contain a distinct set of physical, chemical, and isotopic characteristics. Accretion of various mixtures of these components from a finite number of reservoirs in the nebula gave rise to the parent bodies of carbonaceous chondrites. The bulk chemical composition of each parent body presumably reflects the relative proportions of these petrologic components. In this study, we investigate the volumetric abundances of the main petrologic components - CAIs, AOAs, chondrules, matrix, and opaque phases - in multiple samples of three carbonaceous chondrites of different groups: Murray (CM), Kainsaz (CO), and Al Rais (CR). Each sample is considered an appropriate representative of its respective carbonaceous chondrite group following the criteria outlined in this study. We developed a new technique for volumetric proportional analyses, entitled Isolation by Multiplication or ISLM. In this technique, the volumetric abundance of each component is determined by a sequence of image manipulation algorithms applied to a set of four elemental X-ray maps using the image processing software, ImageJ. The results were cross-referenced with previous investigations of volumetric abundances (e.g. McSween, 1977) and wet chemistry (e.g. Wiiks, 1956) to test for the validity of our results. We found strong oxygen isotopic indicators, bulk chemical congruency, and refractory mixing trends that variations in bulk chemistry are the result of variations in the proportions of petrologic components. The distinctive oxygen isotopic compositions of Murray, Kainsaz, and Al Rais have a direct correspondence with our volumetric results. This relationship strengthens the previous conclusion by Zanda et al. (2006) that the proportions of petrologic components control the bulk chemical and oxygen isotopic compositions, by providing more rigorously determined proportions of components.