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Oxygen and C isotope compositions of CO 2  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 CO 2  gas during thermal decarbonation in a 3 bar He pressure environment is very small. The δ 13 C and δ 18 O values of evolved CO 2  gas are nearly identical to those of the carbonate, very different from the calculated equilibrium  18 O calcite-CO2  value of -4 to -5‰ at 800-900°C or from previous experimental results of decarbonation in vacuum. The kinetic Δ 18 O siderite-CO2  values are ~-2‰, whereas Δ 13 C siderite-CO2  values increase logarithmically with time, from ~1‰ for the earliest stages of decarbonation to &gt;5‰ in the final stages. Incomplete siderite decomposition produces both magnetite (δ 18 O = 3.5‰ SMOW) and minor graphite. CO and O 2  were detected during the decarbonation process. The data can be explained by simultaneous oxidation and reduction by the reaction:

The effect of thermal decarbonation on stable isotope compositions of carbonates