Carbonaceous chondrites show evidence for fluid alteration in planetesimals within a few million years of the Solar Systenm's formation. To understand the formation conditions of carbonates in CM chondrites, we measured the C and O isotopic composition of Ca-carbinates and the O isotopic composition of magnetite. These minerals form via direct precipitation from the fluid and/or water-rock interactions in the parent asteroid. We observe a wide range in the C and O isotope composition of carbonates. 13 C vs 18 O fall into two groups, one consisting of primary calcite and the other consisting of dolomite and secondary calcite. Our results are consistent with a CO2-CH4 gas mixture as the source of C for carbonate formation. These gases were likely released during melting of ice in the parent body. Dolomite and magnetite oxygen isotope fractination are consistent with formation temperatures between 125 +/- 60 C (2SD). We discuss whether our data support current carbonaceous parent body models.
Speaker: Myriam Telus, UC Santa Cruz
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