Classification, metamorphic history, and pre-metamorphic composition of chondrules

Sears et al. (1992) and DeHart et al. (1992) have proposed that the existing scheme for chondrule classification, which is based on modal mineralogy, texture, and mineral composition, should be replaced by a new scheme that is based primarily on the compositions of olivine phenocrysts and mesostases, but also on their cathodoluminescence. We have used published and unpublished compositional data for chondrules in ordinary and carbonaceous chondrites to compare the two classification schemes. We find that the new scheme does not preserve the identity of the existing chondrule groups, even in the least metamorphosed chondrites. Because of difficulties in assigning chondrules to the proposed groups and the lack of olivine grains or mesostasis areas of sufficient size, 30% of chondrules in some type 3 ordinary chondrites cannot be classified in the new scheme using compositional data. For type 4-6 chondrites, the existing scheme provides more information about the history of individual chondrules than the new scheme. We conclude that textural and modal criteria should be retained for the classification of chondrules. © 1994.