CARBON AND HYDROGEN ISOTOPE FRACTIONATION DURING OXIDATION OF METHANE BY METAL OXIDES AT TEMPERATURES FROM 400O TO 530OC

Abstract

The oxidation of methane by metal oxides (Fe2O3, MnO2 and CuO) in closed systems is controlled by the oxides and temperature. Abiogenic oxidation of methane can readily occur, even at temperatures below 200°C in the Earth's upper crust. The existence of residual CH4 relatively enriched in 13C during oxidation can be explained by a kinetic isotope effect. The deuterium concentration of residual methane in the MnO2 and CuO oxidation systems is controlled by a kinetic isotope effect, whereas for the Fe2O3 system, an isotope exchange between CH4 and H2O was observed. For the MnO2 system, the temperature dependence of hydrogen isotope fractionation is similar to that for CuO, and was obtained as: 103(αH − 1) = 134 × 106T2−127 The relative change in the isotope concentrations in CH4 during abiogenic oxidation is ΔHΔC ≈ 21 at 200°C. This change is greater than that of other methane oxidation processes, indicating that carbon and hydrogen isotope compositions of methane may be useful in identifying abiogenic oxidation from other oxidation processes.