¹Makoto Kimura, ^2,3Michael K. Weisberg, ⁴Richard C. Greenwood, ^1,5Akira Yamaguchi
Geochemistry (Chemie der Erde) 85, 126343 Link to Article [https://doi.org/10.1016/j.chemer.2025.126343]
¹National Institute of Polar Research, 10-3 Midoricho, Tachikawa, Tokyo, 190-8513, Japan
²Kingsborough College and Graduate Center of the City University of New York, USA
³American Museum of Natural History, New York, USA
⁴Planetary and Space Sciences, The Open University, Milton Keynes, MK7 6AA, United Kingdom
⁵Department of Polar Science, the Graduate University for Advanced Studies, SOKENDAI, 10-3 Midoricho, Tachikawa, Tokyo, 190-8518, Japan
Copyright Elsevier

In this paper, we examine the diverse features of CM (Mighei-type) and related carbonaceous chondrites, including their petrologic classification, secondary heating, brecciation, and we explore anomalous CM-like chondrites. CM chondrites experienced varying degrees of aqueous alteration, resulting in a range of petrologic subtypes from 3.0 to 2.0. The most abundant subtypes are 2.3–2.0, which may reflect melting of significant amounts of ice, resulting in the formation of the heavily altered CM chondrites in the inner regions of the parent body. Additionally, some CM and related chondrites have undergone secondary heating after aqueous alteration. The source of heat for these chondrites is still uncertain, but impacts are the most likely the cause due to the evidence for a short duration of heating. CM chondrites are mainly genomict breccias and contain clasts of various petrologic grade and degree of heating, though some CMs contain xenolithic clasts. Highly recrystallized clasts are particularly important, as they might have formed in the interior, hotter regions of the CM parent body. Subsequently, these clasts may have been mixed with other typical CM lithologies due to impact events. CM chondrite fragments are commonly found in other meteorites, such as HED meteorites and ordinary chondrites. This indicates a widespread distribution of CM chondrite fragments in the main asteroid belt, with incorporation into other meteorites taking place significantly later than chondrule formation. There have been numerous descriptions of anomalous CM or related chondrites. We tentatively classify these anomalous CMs into four categories: highly 16O-rich, medium 16O-rich, an unusual mineral group, and others. However, the processes involved in the formation of these anomalous chondrites and their relationships to the more typical CMs remain unclear, as detailed documentation of most of the anomalous CMs is currently lacking. CM chondrites primarily consist of chondrules, refractory inclusions, opaque minerals, and a matrix material, similar to other C chondrites. The petrographic and bulk chemical features of CMs are most similar to CO chondrites. However, CM and CO chondrites did not originate from a single parent body. CM chondrites provide valuable information about the conditions and processes that operated in the outer region of the early solar system.