Oxidation of CH 4 to CO 2 and H 2O by chloritization of detrital biotite at 270 ± 5 °C in the external part of the Central Alps, Switzerland

Abstract

The upper end of the CH 4-zone in the External part of the Central Alps of Switzerland results from an oxidation reaction of methane into water and carbon dioxide at 270 ± 5 °C [Tarantola, A., Mullis, J., Vennemann, T., Dubessy, J., de Capitani, C., 2007. Oxidation of methane at the CH 4 / H 2O–(CO 2) transition zone in the external part of the Central Alps, Switzerland: Evidences from stable isotope investigations. Chemical Geology 237, 329–357.]. Petrographic investigations of the North-Helvetic Flysch revealed the presence of detrital biotite, muscovite and chlorite in the CH 4-zone. At the transition from the methane to the water zone, the sudden increase of CO 2 in the fluid is associated with the formation of chlorite and muscovite in replacement of detrital biotite. In the H 2O-zone, biotite is entirely replaced by chlorite-muscovite interlayered stacks. Mössbauer spectroscopy showed a minimum decrease of XFe 3+ from 0.18 to 0.09 within the sheet silicate fraction at the transition from the CH 4- to the H 2O–(CO 2) dominated fluid zone. Detrital biotite of the investigated host rocks incorporates at the same conditions larger amounts of Fe 3+ than chlorite in its structure and XFe 3+ (biotite) > XFe 3+ (new-chlorite) > XFe 3+ (detrital chlorite). The reaction of chloritization of detrital biotite is interpreted as a fluid controlled mineralogical isograde at 270 ± 5 °C, marking the transition from the low to the high grade anchizone, and allowing the oxidation of up to 36 moles of CH 4 per cubic meter of rock.