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Abstract:
We have performed in situ analyses of C and O isotopic compositions, trace element concentrations, and cathodoluminescence (CL) intensities on calcite in Murchison, a weakly altered CM chondrite. We found that the trace element (Mg, Mn, and Fe) concentrations are heterogeneous within single calcite grains. Grain to grain heterogeneity is even more pronounced. The analyzed calcite grains can be separated into two distinct types with respect to their C isotopic ratios, trace element concentrations, and CL characteristics: Calcite grains with higher delta C-13(PDB) values (similar to 7 parts per thousand) have low trace element concentrations and uniformly dark CL, while grains with lower delta C-13(PDB) values (similar to 35 parts per thousand) have higher trace element concentrations and CL zoning. In contrast to the C isotopic ratios, O isotopic ratios are similar for both types of calcites (delta(18)(OSMOW) similar to 34 parts per thousand).
The O isotopic ratios, trace element concentrations, and CL characteristics provide no evidence for C-isotope evolution in fluids from a single C reservoir by Rayleigh-type isotope fractionation (i.e., removal of C-bearing gaseous species). Also, it seems difficult to explain the O and C isotopic compositions of the two types of calcites by their formation at different temperatures from a single fluid. Instead, the delta C-13(PDB) variation suggests the presence of at least two C reservoirs with different isotopic ratios in the aqueous fluids from which the calcites precipitated. The C reservoirs with lower delta C-13 values are likely to be organic matter. The same holds for the C reservoirs with higher d delta C-13 values which might have significant contributions from the C-13-enriched grains identified in meteoritic insoluble organic matter. Thermodynamic calculations show that calcite with lower Fe concentrations formed under more reduced conditions than calcite with higher Fe concentrations. If this is the case, the C-13-rich organic grains may have been destroyed and dissolved in the fluids under more reduced conditions than other organic components. The fact that the two types of calcites were found in different domains in the same thin section suggests that microenvironments with diverse physicochemical conditions such as redox states were present at scales of 100' s mu m. (C) 2015 Elsevier Ltd. All rights reserved.
