Modal abundances of coarse-grained (>5 µm) components within CI-chondrites and their individual clasts – mixing of various lithologies on the CI parent body(ies)OPEN ACCESS 

Julian Alfing, Markus Patzek, Addi Bischoff

Available online 20 August 2019



“For the bulk rocks of CI chondrites, various values are given for the modal abundance of matrix (95-100 vol%) and the accompanying mineral constituents. Here, we have determined the modal abundance of phases >5 µm in the CI chondrites Orgueil, Ivuna, Alais, and Tonk. Considering this cut-off grain-size to distinguish between matrix and coarse-grained constituents, then, we find the modal abundance of the minor phases magnetite, pyrrhotite, carbonate, olivine, and pyroxene to be 6 vol% in total, and these phases are embedded within the fine-grained, phyllosilicate-rich matrix, making up 94 vol%. The values vary slightly from meteorite to meteorite. Considering all four chondrites, the most abundant phase is – by far – magnetite (4.3 vol%) followed by pyrrhotite (˜1.1 vol%). All four CI chondrites are complex breccias, and their degree of brecciation decreases in the sequence: Orgueil > Ivuna > Alais ˜ Tonk. Because these meteorites contain clasts with highly variable modal abundances, we therefore also studied individual clasts with high abundances of specific coarse-grained phases. In this respect, in Orgueil we found a fragment with a 21.5 vol% of magnetite as well as a clast having 31.8 vol% phosphate. In Ivuna, we detected an individual clast with a 21.5 vol% of carbonates. Thus, since the CI composition is used as a geochemical standard for comparison, one also should keep in mind that sufficiently large sample masses are required to reveal a homogeneous CI composition. Small aliquots with one dominating lithology may significantly deviate from the suggested standard CI composition.”