The variable carbon isotopic composition of type 3 ordinary chondrites

Abstract

A survey of bulk carbon isotopic compositions has been carried out for a suite of H, L, and LL ordinary chondrites, from the most unequilibrated type 3 to the metamorphosed type 6. The isotopic composition of the higher petrologic types 4–6 is lighter (in the range −25 to −30°‰) and less variable than specimens of type 3 (−11 to −28°‰). There is no simple relationship between bulk carbon and its isotopic composition; several carbon phases of different δ13C may be present in the meteorites in variable proportions. To characterize the variability of type 3 chondrites, a selection has been treated with orthophosphoric acid to test for the presence of terrestrial weathering products of high δ13C. Some contamination and weathering products seem to be present, but such an interpretation is less than straightforward. The heavier isotopic composition and variability of type 3 meteorites may also be due, in part, to the presence of a component of graphite‐magnetite matrix in the H and L group samples.