EVALUATING FLUID-ROCK INTERACTIONS AND MINERAL EQUILIBRIA IN CO2-RICH GEOTHERMAL AREAS IN ICELAND

Volcanic and metamorphic CO₂ emissions are the primary flux of CO₂ from the solid Earth into the atmosphere over geologic time. As such, variations in volcanic CO₂ degassing have implications for balancing the global carbon budget and unraveling events in Earth’s history that have been linked to changes in atmospheric CO₂ content, such as mass extinctions and climatic warming. Basaltic geothermal areas in Iceland with an anomalously high input of magmatic CO₂ act as natural laboratories to study the flux of CO₂ from volcanic activity and the fluid-rock interactions associated with CO₂ metasomatism. In Icelandic geothermal systems >200°C, compositional zoning in hydrothermal epidotes and local equilibrium mineral assemblages of hydrothermal Ca-Al-Fe silicates reflect the variations in fluid CO₂ concentration over time. However, a corresponding mineral assemblage has not been suggested for low-temperature (≤180°C) alteration in the presence of magmatic CO₂, despite the implications for interpreting CO₂ fluxes, redox and pH conditions and associated element mobility during the later stages of a geothermal system’s lifecycle. This study investigates Icelandic geothermal areas with fluids that contain >4 mmol/kg total CO₂. The waters contain up to 80 mmol/kg dissolved inorganic carbonate (DIC), and the aqueous concentration of major cations increases with DIC and decreasing pH. Thermodynamic modeling indicates that waters approach saturation with respect to calcite and/or aragonite, kaolinite and amorphous silica, and are undersaturated with respect to plagioclase feldspar, clinozoisite and Ca-zeolites. Petrographic study of drill cuttings from CO₂-rich areas indicates that the sites have undergone at least two stages of hydrothermal alteration: initial high-temperature and late stage low-temperature alteration. CO₂-rich low-temperature fluids are not in equilibrium with correlative high-temperature hydrothermal mineral assemblages, indicating that the kinetics of mineral dissolution and secondary mineral precipitation along with fluid residence times are important controls on CO₂ alteration at low temperatures. Our results have implications for interpreting the CO₂ flux history of fossil geothermal systems and for predicting mineral products of CO₂ sequestration into basaltic rocks.