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Abstract: The upper end of the CH4-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 CH4 / H2O–(CO2) 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 CH4-zone. At the transition from the methane to the water zone, the sudden increase of CO2 in the fluid is associated with the formation of chlorite and muscovite in replacement of detrital biotite. In the H2O-zone, biotite is entirely replaced by chlorite-muscovite interlayered stacks. Mössbauer spectroscopy showed a minimum decrease of XFe3+ from 0.18 to 0.09 within the sheet silicate fraction at the transition from the CH4- to the H2O–(CO2) dominated fluid zone. Detrital biotite of the investigated host rocks incorporates at the same conditions larger amounts of Fe3+ than chlorite in its structure and XFe3+ (biotite) >XFe3+ (new-chlorite) >XFe3+ (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 CH4 per cubic meter of rock. [Copyright &y& Elsevier]