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Oxygen and C isotope compositions of CO2gas 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 CO2gas during thermal decarbonation in a 3 bar He pressure environment is very small. The δ13C and δ18O values of evolved CO2gas are nearly identical to those of the carbonate, very different from the calculated equilibrium 18Ocalcite-CO2value of -4 to -5‰ at 800-900°C or from previous experimental results of decarbonation in vacuum. The kinetic Δ18Osiderite-CO2values are ~-2‰, whereas Δ13Csiderite-CO2values 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 O2were detected during the decarbonation process. The data can be explained by simultaneous oxidation and reduction by the reaction:

Abstract: A novel DSC apparatus is based on resistance thermometers comprising two legs of a modified resistance bridge (Halas-Kamiński bridge) in which one resistor can be energized from an external source. Two metal spirals installed in a regulated oven are used as thermometers and sample holders. Two scans for 20 mg of NH4NO3 demonstrate the high sensitivity of this method.