Oxygen and C isotope compositions of CO2 gas released by thermal decomposition of siderite, calcite, and dolomite were measured using a new “real-time” continuous-flow technique to determine whether fractionation associated with simple thermal decarbonation could explain the large isotopic variations and mineralogy such as those found in the ALH84001 meteorite.
Oxygen and C isotope fractionation between calcite or dolomite and evolved CO2 gas during thermal decarbonation in a 3 bar He pressure environment is very small. The δ13C and δ18O values of evolved CO2 gas are nearly identical to those of the carbonate, very different from the calculated equilibrium Δ18O calcite-CO2 value of −4 to −5‰ at 800–900 °C or from previous experimental results of decarbonation in vacuum. The kinetic Δ18Osiderite-CO2 values are ~–2‰, whereas Δ13Csiderite-CO2 values increase logarithmically with time, from ~1‰ for the earliest stages of decarbonation to >5‰ in the final stages. Incomplete siderite decomposition produces both magnetite (δ18O = 3.5‰ SMOW) and minor graphite. CO and O2 were detected during the decarbonation process. The data can be explained by simultaneous oxidation and reduction by the reaction:
where x and y are between 0 and 1. Siderite decomposition in the presence of H2 gas produces wüstite and Fe metal in place of oxidized Fe minerals.
The experiments in this study are not a perfect analog for possible decarbonation conditions that might have occurred to the carbonates in ALH84001. Nevertheless, the large δ13C and δ18O variations observed in ALH84001 (>10‰ for O) are significantly larger than those expected by thermal decarbonation, suggesting instead a low-temperature mechanism for their formation.