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.
