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)

1,2Julian Alfing,1Markus Patzek,1Addi Bischoff
Geochemistry (Chemie der Erde) Link top Article [https://doi.org/10.1016/j.chemer.2019.08.004]
1Institut für Planetologie, Westfälische Wilhelms-Universität Münster, Wilhelm-Klemm Str. 10, D-48149, Münster, Germany
2Institut für Mineralogie, Westfälische Wilhelms-Universität Münster, Corrensstr. 24, D-48149, Münster, Germany
Copyright Elsevier

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.

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