Carbon isotope systematics and CO 2 sources in The Geysers-Clear Lake region, northern California, USA

Abstract

Carbon isotope analyses of calcite veins, organic carbon, CO 2 and CH 4 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. δ 13C 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 δ 13C values of the veins to be reset. Across The Geysers-Clear Lake region the carbon isotope composition of CO 2 gas associated with individual geothermal reservoirs shows a general increasing trend in δ 13C values from west to east. In contrast, δ 13C values of CH 4 do not exhibit any spatial trends. The results from this study indicate that regional variations in δ 13C–CO 2 values result from differences in the underlying lithologies. Regional CO 2 contains significant amounts of carbon related to degradation of organic carbon and dissolution of calcite veins and is not related to equilibrium reactions involving CH 4. CO 2 from degassing of underlying magma chambers is not recognizable in this region.