The 13C 12C and 18O 16O isotopic ratios of carbonates from carbonaceous and ordinary chondrites have been measured on CO2 released by the action of H3PO4 on whole-rock samples. Carbonates from CI, CM and CR carbonaceous chondrites exhibit a range in δ18O of ca. 15%. (+20.5%. to +35.1%. relative to SMOW). Limited data from CO2-water equilibration experiments suggest that meteoritic carbonates do not possess grossly anomalous 17O isotopic compositions; therefore, they are truly enriched in 13C, with δ13C between +23.7%. and +80.7%. relative to PDB. Large internal variations in δ13C and δ18O were found in individual meteorites and suggest that two or more isotopically distinct carbonates of different origin may be present. The abundance, δ13C and δ18O 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 δ13C ca. +50 to +60%., whereas δ13C 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 CO2 on acid treatment, which might be from carbonate dissolution, but which is not enriched in 13C, exhibiting δ13C values ca. 0 ± 10%. The exception to this is Bishunpur, with δ13C ca. -23.5%.. The difference in δ13C 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.
ASJC Scopus subject areas
- Geochemistry and Petrology