The carbon and oxygen isotopic composition of meteoritic carbonates

Abstract

The 13C 12C and 18O 16O isotopic ratios of carbonates from carbonaceous and ordinary chondrites have been measured on CO 2 released by the action of H 3PO 4 on whole-rock samples. Carbonates from CI, CM and CR carbonaceous chondrites exhibit a range in δ 18 O of ca. 15%. (+20.5%. to +35.1%. relative to SMOW). Limited data from CO 2-water equilibration experiments suggest that meteoritic carbonates do not possess grossly anomalous 17O isotopic compositions; therefore, they are truly enriched in 13C, with δ 13 C between +23.7%. and +80.7%. relative to PDB. Large internal variations in δ 13 C and δ 18 O were found in individual meteorites and suggest that two or more isotopically distinct carbonates of different origin may be present. The abundance, δ 13 C and δ 18 O of carbonate in CM2 chondrites may be related to the extent of aqueous alteration of the meteorites. Carbonates in CI and CR chondrites have a median δ 13 C ca. +50 to +60%., whereas δ 13 C of CM meteorites lie in the range +40 to +50%., although exceptions exist in both sets of samples. CV3 and CO3 carbonaceous chondrites and unequilibrated ordinary chondrites release small amounts of CO 2 on acid treatment, which might be from carbonate dissolution, but which is not enriched in 13C, exhibiting δ 13 C values ca. 0 ± 10%. The exception to this is Bishunpur, with δ 13 C ca. −23.5%.. The difference in δ 13 C of the CI, CM and CR vs. CV, CO and ordinary chondrite carbonates may be a result of the progressive enrichment in 13C of percolating fluids, brought about by increasing solubilization of “exotic” 13C-enriched grains.