Published in Meteoritics, 29, p. 479.
We report the isotopic composition of carbonates present in small amounts in the SNC meteorites. The first study of such possible nonterrestrial weathering products was the pioneering measurement of Carr et al. , who observed heavy C (delta13C ~ 16-45) in CO2 produced by phosphoric acid etching of Nakhla. Weathering products known as white druse deposits have been observed in the shergottite EETA 79001 . Recently, Mittlefehldt  reported the occurrence of inclusions of an Fe-Mg carbonate in the SNC orthopyroxenite ALH 84001. The petrography of these unique features suggested an extraterrestrial origin for these carbonates. We therefore undertook a study of these carbonates to determine if they carry an isotopic record of some preterrestrial atmospheric CO2 and H2O. Previously, we have studied 14C and stable-isotope compositions of carbonates from EETA 79001, and concluded that they were either terrestrial or had exchanged with terrestrial CO2 . These samples contained 68-95% modern terrestrial 14C, and values of delta13C and delta18O were within the terrestrial range for carbonates from Antarctica. We have measured the 14C terrestrial age of EETA 79001 to be ~12,000 yr, and ALH 84001 ~13,000 yr.
Carbonates irradiated in space at this time in a small meteorite would currently have about 1% of the modern terrestrial value. Samples of CO2 evolved from acid-etching experiments of SNC carbonates showed varying values for delta13C and delta18O, and amounts of 14C. Three separate experiments were performed on samples of ALH 84001. The first two fractions of this meteorite studied show the highest values observed of both delta13C (40-53) and delta18O and the lowest values of 14C (as low as 8% modern terrestrial 14C) in the CO2 released by acid-dissolution experiments with 100% H3PO4. An experiment on a sample of Nakhla showed intermediate values of delta13C and delta18O and significant amounts of 14C. In another experiment, we collected several fractions of CO2 from a sequential etching of a sample of ALH 84001 in H3PO4.
In this series of analyses, CO2 was characterized by heavy delta13C (up to 35) and delta18O (up to 30) and lower 14C as obtained on the later etching steps. A good anticorrelation of delta13C with 14C was observed; delta18O does not show as good a correlation. These results can be interpreted as a mixing line for two discrete carbonates from ALH 84001. We can identify a recent terrestrial component, with low delta13C of about 0 and delta18O of ~20, and a modern value of 14C. By extrapolation to ~1% modern terrestrial 14C, we can identify the heavy component with a delta13C of ~45. The delta18O increases less, and may only reach a maximum of perhaps 35. There is little correlation of delta13C with delta18O, consistent with isotopic equilibration of the O with a different reservoir. We identify the heavy component as extraterrestrial on the basis of this unusual isotopic composition. We are also studying the composition of the carbonate leachates for elemental composition and other cosmogenic nuclides. This additional information will be valuable in identifying the two components discussed.
References:  Carr R. H. et al. (1985) Nature, 314, 248-250.  Gooding J. L. (1986) GCA, 50, 2215-2223; Gooding J. L. et al. (1988) GCA, 52, 909-915.  Mittlefehldt D. (1994) Meteoritics, 29, 214-221.  Jull A. J. T. et al. (1992) LPS XXIII, 641-642.