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Abstract

Abstract-Isotopic analysis of nesquehonite recovered from the surface of the LEW 85320 H5 ordinary chondrite shows that the Ol3C and 0 18 0 values of the two generations of bicarbonate (Antarctic and Texas) are different: 013C = + 7.90/00 and +4.20/00; 0 18 0 = +17.90/00 and +12.10/00 respectively. Carbon isotopic compositions are consistent with equilibrium formation from atmospheric carbon dioxide at -2 ± 4 "C (Antarctic) and + 16 ± 4 ·C (Texas). Oxygen isotopic data imply that the water required for nesquehonite precipitation was derived from atmospheric water vapour or glacial meltwater which had locally exchanged with silicates, either in the meteorite or in underlying bedrock. Although carbonates with similar 013C values have been identified in the SNC meteorites EETA 79001 and Nakhla, petrographic and temperature constraints argue against their simply being terrestrial weathering products.