Fine‐grained chondrule rims in Mighei‐like carbonaceous chondrites: Evidence for a nebular origin and modification by impacts and recurrent solar radiation heating

Abstract

The Mighei‐like carbonaceous (CM) chondrites, the most abundant carbonaceous chondrite group by number, further our understanding of processes that occurred in their formation region in the protoplanetary disk and in their parent body/bodies and provide analogs for understanding samples returned from carbonaceous asteroids. Chondrules in the CMs are commonly encircled by fine‐grained rims (FGRs) whose origins are debated. We present the abundances, sizes, and petrographic observations of FGRs in six CMs that experienced varying intensities of parent body processing, including aqueous and thermal alteration. The samples studied here, in approximate order of increasing thermal alteration experienced, are Allan Hills 83100, Murchison, Meteorite Hills 01072, Elephant Moraine 96029, Yamato‐793321, and Pecora Escarpment 91008. Based on observations of these CM chondrites, we recommend a new average apparent (2‐D) chondrule diameter of 170 μm, which is smaller than previous estimates and overlaps with that of the Ornans‐like carbonaceous (CO) chondrites. Thus, we suggest that chondrule diameters are not diagnostic for distinguishing between CM and CO chondrites. We also argue that chondrule foliation noted in ALH 83100, MET 01072, and Murchison resulted from multiple low‐intensity impacts; that FGRs in CMs formed in the protoplanetary disk and were subsequently altered by both aqueous and thermal secondary alteration processes in their parent asteroid; and that the heat experienced by some CM chondrites may have originated from solar radiation of their source body/bodies during close solar passage as evidenced by the presence of evolved desiccation cracks in FGRs that formed by recurrent wetting and desiccation cycles.