Calcite and dolomite formation in the CM parent body: Insight from in situ C and O isotope analyses

To constrain the conditions of aqueous alteration in early planetesimals, we carried out in situ C and O isotope analyses of calcite and dolomite and O isotope analyses of magnetite from the highly altered CM chondrites ALH 83100, ALH 84034, and MET 01070. Petrographic and isotopic analyses of these samples support previous findings of multiple generations of carbonate growth. We observe wide ranges in the C and O isotope compositions of carbonates of up to 8 parts per thousand and 30 parts per thousand, respectively, that span the full range of previously reported bulk carbonate values for CM chondrites. Variations in the Delta O-17 values indicate that fluid evolution varied for each chondrite. ALH 83100 dolomite-magnetite delta O-18 fractionation of 23 parts per thousand +/- 7 parts per thousand (2SD) corresponds to dolomite formation temperature of 125 degrees C +/- 60 degrees C. delta C-13 vs delta O-18 values fall into two groups, one consisting of primary calcite and the other consisting of dolomite and secondary calcite. The positive correlation between delta O-13 and delta O-18 for primary calcite is consistent with the precipitation of calcite in equilibrium with a gas mixture of CO (or CH4 ) and CO2. The isotopic composition of calcite in CM1s and CM2s overlap significantly; however, many CM1 calcite grains are more depleted in delta O-18 compared to CM2s. Altogether, the data indicate that the fluid composition during calcite formation was initially the same for both CM1s and CM2s. CM1s experienced more episodes of carbonate dissolution and reprecipitation where some fraction of the carbonate grains survive each episode resulting in a highly disequilibrium assemblage of carbonates on the thin-section scale. (C) 2019 The Author(s). Published by Elsevier Ltd.