Oxidation of methane at the CH 4/H 2O–(CO 2) transition zone in the external part of the Central Alps, Switzerland: Evidence from stable isotope investigations

Abstract

With the aim of understanding the mechanisms that control the metamorphic transition from the CH 4– to the H 2O–(CO 2)-dominated fluid zone in the Helvetic domain of the Central Alps of Switzerland, fluid inclusions in quartz, illite “crystallinity” index, vitrinite reflectance, and the stable isotope compositions of vein and whole rock minerals and fluids trapped in quartz were investigated along four cross-sections. Increasing temperature during prograde metamorphism led to the formation of dry gas by hydrocarbon cracking in the CH 4-zone. Fluid immiscibility in the H 2O–CH 4–(CO 2)–NaCl system resulted in cogenetic CH 4- and H 2O-dominated fluid inclusions. In the CH 4-zone, fluids were trapped at temperatures ≤ 270 ± 5 °C. The end of the CH 4-zone is marked by a sudden increase of CO 2 content in the gas phase of fluid inclusions. At temperatures > 270 ± 5 °C, in the H 2O-zone, the total amount of volatiles within the fluid decreased below 1 mol% with no immiscibility. This resulted in total homogenization temperatures of H 2O–(CO 2–CH 4)–NaCl inclusions below 180 °C. Hydrogen isotope compositions of methane in fluid inclusion have δD values of less than − 100‰ in the CH 4-zone, typical for an origin through cracking of higher hydrocarbons, but where the methane has not equilibrated with the pore water. δD values of fluid inclusion water are around − 40‰, in isotopic equilibrium with phyllosilicates of the whole rocks. Within the CH 4 to H 2O–(CO 2) transition zone, δD(H 2O) values in fluid inclusions decrease to − 130‰, interpreted to reflect the contribution of deuterium depleted water from methane oxidation. In the H 2O-zone, δD(H 2O) values increase again towards an average of − 30‰, which is again consistent with isotopic equilibrium with host-rock phyllosilicates. δ 13C values of methane in fluid inclusions from the CH 4-zone are around − 27‰, in isotopic equilibrium with calcite in veins and whole rocks. The δ 13C(CH 4) values decrease to less than − 35‰ at the transition to the H 2O-zone and are no longer in equilibrium with the carbonates in the whole rocks. δ 13C values of CO 2 are variable but too low to be in equilibrium with the wall rock fluids, compatible with a contribution of CO 2 from closed system oxidation of methane. Differences in isotopic composition between host-rock and Alpine fissure carbonate are generally small, suggesting that the amount of CO 2 produced by oxidation of methane was small compared to the C-budget in the rocks and local pore fluids were buffered by the wall rocks during precipitation of calcite within the fissures.