Redistribution of chondrules in a carbonaceous chondrite parent body: A model

Kazushige Tomeoka, Ichiro Ohnishi

Geochimica et Cosmochimica Acta
In Press, available online 16 May 2015
doi:10.1016/j.gca.2015.05.012

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Carbonaceous chondrites mainly consist of chondrules and inclusions embedded in a fine-grained matrix. This texture is widely believed to have formed primarily by direct accretion of solar nebular materials, although it may have been modified to various extents by subsequent parent-body processes.

Recently, we studied all chondrules and inclusions larger than 400 μm in diameter and their rims (referred to as chondrules/rims) in the Mokoia CV3 carbonaceous chondrite using a scanning electron microscope, and found that the chondrules/rims experienced various degrees of aqueous alteration and that some also exhibit evidence of thermal metamorphism. The mineralogical and petrographic characteristics of the chondrules/rims suggest that the alteration and metamorphism occurred within the meteorite parent body. In contrast, however, the surrounding matrix does not show evidence of such alteration and metamorphism. These findings indicate that the alteration and metamorphism of the chondrules/rims did not occur in situ. Based on these results, we proposed a model that the chondrules/rims are actually clasts transported from regions in the parent body different from the location where the host meteorite was finally lithified.

If it can be assumed that the chondrules and inclusions studied are representative of all chondrules and inclusions in Mokoia, the results and interpretation pose a fundamental challenge regarding the formation of the whole Mokoia lithology; that is, it cannot be explained by either direct accretion of the solar nebula or conventional parent-body brecciation. We propose a model for the development of the Mokoia lithology through formation of chondrules/rims and fine matrix grains by fragmentation in different regions in the parent body, followed by transportation, mixing, and accumulation in a fluid state, and finally lithification of those objects. These processes may have been repeated, cyclically, within the parent body.