Paper No. 25
Presentation Time: 3:00 PM
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.