ISOTOPIC COMPOSITION OF CARBONATES IN THE SNC METEORITES ALLAN-HILLS-84001 AND NAKHLA

We have measured the C-13/C-12 and C-14/C-12 ratios in CO2 released by acid etching of the carbonate-bearing SNC meteorites Allan Hills 84001 and Nakhla. Most of the C released is strongly enriched in C-13. In 10 out of 12 samples, 15 parts per thousand < delta(13)C < 55 parts per thousand. Terrestrial values of carbonate delta(13)C from weathering products are generally between -10 and +10 parts per thousand. Two leachate samples especially rich in C-13, ALH 84001,27 and Nakhla 25, have elemental Si/Mg ratios much lower than those of the bulk meteorites and C-14 activities that are much lower than the values expected for terrestrial carbonates. The former observation indicates that these leachates consist primarily of carbonates and, less likely, phosphates. The latter observation implies that heavy C was introduced not by terrestrial weathering but by extraterrestrial processes. For ALH 84001,121 (sample 27) and Nakhla (BM 1913,26) delta(13)C = +41 parts per thousand and +35 parts per thousand, respectively. The measured O-18/O-16 ratios in the leaches are similar: delta(18)O similar to 15 +/- 5 parts per thousand, contrasting with 4.2 parts per thousand in the bulk silicates. We infer that the C in the carbonates retains an extraterrestrial isotopic signature, but probably not O, due to its ease of isotopic exchange (Cole and Ohmoto, 1986).