Geocheminar fall-2019

Carbonaceous chondrites are useful analogues for many water-rich solar system bodies, as they show evidence of water-rock interaction early in the solar system's history. To investigate the chemistry and timescales of these aqueous environments, it is possible to measure the stable isotopic compositions and relative ages of carbonate minerals which precipitated directly from the water. Though diverse carbonates (including calcite, dolomite, and magnesite) are present in carbonaceous chondrites, most previous work has focused on calcite due to analytical difficulties associated with analyzing carbonates with substantial Fe contents. We show results of in-situ measurements by SIMS of carbon and oxygen stable isotopes in calcite and dolomite in the highly altered CM chondrite ALH 84034. We find that calcite and dolomite formed under substantially different alteration regimes, and that matrix-matched standards are essential for interpretation of the oxygen isotopic results. We also give an update on the development of dolomite standards for relative age dating using the Mn-Cr system, which will be applied to the same carbonates to place the aqueous environments of these bodies into the timeline of solar system formation.