Carbon isotopic evolution of aqueous fluids in CM chondrites: Clues from in-situ isotope analyses within calcite grains in Yamato-791198

We report the variability in carbon and oxygen isotopic compositions, chemical compositions, and cathodoluminescence intensities within calcite grains in the Yamato-791198 CM chondrite measured by secondary ion mass spectrometry. To understand the change in carbon isotopic compositions during calcite formation, the carbon isotope-analyses were performed on a series of crystal growth bands of each calcite grain. The crystal growth of calcite grains was inferred from comprehensive analyses of oxygen isotopes, chemical compositions, and cathodoluminescence characteristics. The observed delta O-18 variations within individual grains are as large as 13 parts per thousand. The oxygen-isotope data plot on a single straight line with a slope of 0.61 +/- 0.06 (2 sigma) in an oxygen three-isotope diagram. This slope steeper than that of the terrestrial fractionation line indicates that the oxygen isotopic compositions of aqueous fluids evolved from higher delta O-18 and Delta O-17 to lower delta O-18 and Delta O-17 compositions due to the oxygen-isotope exchange between water and anhydrous silicates in the parent body. Thus, calcite crystals grew from higher Delta O-17 to lower Delta O-17 areas. The crystal growth inferred from oxygen isotopic compositions is corroborated by the morphology and cathodoluminescence characteristics of the calcite grains. The minor element concentrations of the calcite grains did not increase/decrease monotonically during calcite formation. The delta C-13 variations within individual grains are no more than 4 parts per thousand except for one grain. The intra-grain delta C-13 variations observed here are much smaller than inter-grain delta C-13 variations of similar to 80 parts per thousand previously reported. These observations indicate that the carbon isotopic compositions of dissolved carbon species did not change during calcite formation and that they were locally heterogeneous which reflects variable proportions of carbon reservoirs with different isotopic compositions. (C) 2020 Elsevier Ltd. All rights reserved.