Carbon isotope systematics and CO2 sources in The Geysers-Clear Lake region, northern California, USA

Carbon isotope analyses of calcite veins, organic carbon, CO2 and CH4 from 96 rock and 46 gas samples show that metamorphic calcite veins and disseminated, organically-derived carbon from Franciscan Complex and Great Valley Sequence rocks have provided a primary carbon source for geothermal fluids during past and present hydrothermal activity across The Geysers-Clear Lake region. The stable isotope compositions of calcite veins vary widely on a regional scale, but overall they document the presence of 13C-poor fluids in early subduction-related vein-precipitating events. δ13C values of calcite veins from the SB-15-D corehole within The Geysers steam field indicate that carbon-bearing fluids in the recent geothermal system have caused the original diverse δ13C values of the veins to be reset. Across The Geysers-Clear Lake region the carbon isotope composition of CO2 gas associated with individual geothermal reservoirs shows a general increasing trend in δ13C values from west to east. In contrast, δ13C values of CH4 do not exhibit any spatial trends. The results from this study indicate that regional variations in δ13C–CO2 values result from differences in the underlying lithologies. Regional CO2 contains significant amounts of carbon related to degradation of organic carbon and dissolution of calcite veins and is not related to equilibrium reactions involving CH4. CO2 from degassing of underlying magma chambers is not recognizable in this region.