“Geothermal mineral equilibria”. Reply to a comment by M. A. Grant

The dominant process limiting CO₂-contents of fluids in high temperature (>240°), liquid-dominated systems consists of the conversion of primary plagioclase by CO₂ to calcite and clay according to log P_CO2 = 15.26 ? 7850/(t + 273.2), temperature t in °C, with a likely error in log P_CO2 due to variations in the activities of the anorthite and kaolinite components of the mineral phases involved of around ± 1 log unit. Secondary processes such as adiabatic expansion, conductive cooling and mixing with cooler groundwaters are largely responsible for subsequent variations in P_CO2 at lower temperatures (>240°).