The thermal and aqueous evolution of CM carbonaceous chondrite meteorites revealed by triple oxygen and clumped isotope compositions of their carbonates

Abstract

Carbonates (calcite, aragonite, dolomite) are a minor component (often below 4 vol. %) of CM carbonaceous chondrites and formed during the aqueous alteration of their asteroidal parent bodies in the first few million years of Solar System history. The chemical and isotopic composition of these minerals are a valuable source of information on the conditions of alteration, potentially providing information on the composition and temperature of asteroidal fluids. We report the carbon triple oxygen and carbonate clumped isotope compositions of six CM chondrites (Allan Hills 83100, Cold Bokkeveld, LaPaz Icefield 031166, Lonewolf Nunataks 94101, Murchison, Scott Glacier 06043), which span a range of degrees of aqueous alteration. To avoid issues due to the brecciated nature of these meteorites, gas aliquots produced by a single acid digestion were used to measure both the clumped isotopes and the triple oxygen isotope compositions. Where both calcite and dolomite are present, stepped acid dissolution allows us to measure their isotopic compositions separately. We found that the Δ17O values range from -1 to -2.6‰, with a 0.6‰ difference between coexisting calcite and dolomite that indicates precipitation from distinct fluids. Crystallization temperatures range from 5 to 50⁰C for calcite and 75 to 100⁰C for dolomite. CM chondrites often contain several generations of carbonates with ranges in isotopic compositions that can be determined by ion probe. Because our method relies on the bulk extraction of carbonate phases, the measured values are the modes of the distributions for each meteorite, which has to be considered carefully in their interpretation. The isotopic composition of the alteration fluids can also be calculated for each meteorite and carbonate phase. We find that their δ18OvsSMOW ranges from -6.6 to +2.3‰, with no clear relationship with temperature or the δ13C of the carbonates, indicating a variety of starting isotopic compositions for the alteration fluids. The main pattern that emerges is that chondrites with a higher degree of alteration (based on their petrology) have carbonates with lower Δ17O and higher crystallization temperatures, which is consistent with a prograde reaction in a largely closed system, and with dolomite forming after calcite in our samples.