Fine-grained rims surrounding chondrules in the Tagish Lake carbonaceous chondrite: Verification of their formation through parent-body processes

Abstract

Listen

Translate article

A petrographic and electron microscopic study of the carbonate-poor lithology of the Tagish Lake carbonaceous chondrite reveals that most chondrules contain many pseudomorphs of opaque nodules and are surrounded by phyllosilicate-rich altered zones that were formed by replacing enstatite and opaque nodules along the chondrule peripheries. Most chondrules and other coarse-grained components are surrounded by fine-grained rims, which are commonly disaggregated and partly lacking. The altered zones and the rims are compositionally and texturally similar, although they exhibit some differences in secondary minerals. In comparison, the rims and the host matrix show more significant differences in bulk chemical composition, texture, and mineralogy. The observations suggest that the chondrules and the rims experienced aqueous alteration simultaneously, whereas the rims and the matrix experienced aqueous alteration under distinct conditions. We also found a clast that contains multiple coarse-grained components embedded in a matrix and numerous smaller matrix clasts. The coarse-grained components in the clast have no rims, and the matrices of the clasts are mineralogically identical to the rims.The results suggest that the chondrules, other coarse-grained components, and their rims (generically referred to as chondrules/rims) and the clasts originated from a common precursor region in the meteorite parent-body that was different from the location where the host meteorite was finally lithified. That is, the chondrules/rims are actually clasts produced by brecciation and later transported and incorporated into the present host matrix. The rims are, therefore, remnants of matrix material that formerly filled interspaces between the chondrules and other coarse-grained components. This model is essentially consistent with those previously proposed for the carbonate-rich lithology of Tagish Lake and the hydrated chondrules/rims in the Vigarano and Mokoia CV3 chondrites.