ABSTRACT

Bulk carbon isotopic compositions of 26 Antarctic carbonaceous chondrites (33 specimens) are reported and discussed with relevance to the processes involved in their alteration and the existence of solvent-extractable organic matter. The δ¹³C values (relative to PDB) vary from -16.6 to +0.9‰. The average value (-6.2‰) is higher than that for non-Antarctic carbonaceous chondrites by -6‰. The difference may be explained by contamination of terrestrial organic carbon on non-Antarctic chondrites and/or different populations of parent bodies. Most Antarctic carbonaceous chondrites have an apparent trend (“a major sequence”) that the δ¹³C value becomes higher with increasing carbon content. The trend is likely to beexplained by a mixing model of two components; isotopically heavy and labile component (solvent-extractable organic matter and carbonate), and isotopically light and inert acid-insoluble component (kerogen-like matter). In addition, an unique group (altered specimens) is characterized by relatively high content and isotopically light carbon. For seven CM chondrites, δ¹³C values of residues after the hot H₂O and HCl/HF treatment are also reported. Despite considerable isotopic variations of bulk carbon, the δ¹³C values of acid-insoluble residues may be classified into two groups; isotopically light (-15 to -13‰) and heavy (-9 to -7‰) ones. Most kerogen-like matters belong to the light group, which are isotopically similar to non-Antarctic kerogen-like matter. The altered specimens have isotopically light bulk carbon (∼-15‰), which is similar to those of their kerogen-like matter; they probably lost heavy components such as solvent-extractable organic matters (e.g., amino acid and carboxylic acid) and carbonates during aqueous alteration and thermal metamorphism on parent bodies. This is in accord with mineralogical and oxygen isotope studies. The presence of solvent-extractable organic matter is one of the important factors controlling bulk carbon isotopes in carbonaceous chondrites.