Hydrothermal minerals record CO2 partial pressures in the Reykjanes geothermal system, Iceland

The Reykjanes Peninsula in southwest Iceland is the landward extension of the Mid-Atlantic Ridge spreading center. At present seawater penetrates the coastal Reykjanes geothermal system at depth, where the highest recorded temperature is ∼320°C. It mixes with magmatic volatiles and reacts with the basaltic host rock to form secondary hydrothermal minerals in progressively higher-grade mineral alteration zones with increasing depth. Within the epidote-chlorite and portions of the epidote-actinolite zones of alteration, epidote-prehnite-calcite-quartz-fluid constitutes a quadra-variant assemblage that, under conditions of specified temperature, pressure, and activity of H2O allows prediction of geothermal fluid PCO2 as a function of the composition of the solid solution minerals epidote or prehnite. This assemblage is typically found at temperatures >250°C and ≲310°C, and potentially provides a mineralogical recorder that constrains fluid CO2 concentrations based on compositional zoning in hydrothermal epidote. Analysis of epidote crystals separated from drillhole-cuttings from three geothermal wells (RN-9, RN-10, RN-17) display complex chemical zoning, generally with Fe(III)-rich cores and Al-rich rims. The Fe(III)-mol fraction of epidote at depths between 0.5 to 1 km ranges from 0.21 to 0.38, between 1 to 2 km depth the range is 0.17 to 0.48 and between 2 to 3 km it is 0.17 to 0.30. The Fe(III)-mol fraction of prehnite ranges from 0.11 to 0.59 in the upper portions of drillhole RN-17, where the highest Fe(III) content in epidote, 0.36, serves as the upper Fe(III) limit for epidotes coexisting with prehnite in this study. Because most observed prehnite crystals in the drillhole-cuttings are too small for electron microprobe analyses (<20μm), we employed a sigmoidal correlation of available compositional data from active geothermal systems to calculate the Fe(III)-Al composition of prehnite using measured compositions of epidote in the Reykjanes system. In drill cuttings that contain epidote, prehnite, quartz and calcite, using measured epidote compositions between the reference temperatures of 275°C and 310°C, calculated values of PCO2 for the geothermal fluids range from ∼0.6 to ∼6.2 bars. When only epidote, prehnite and quartz are observed in the drill cuttings, the calculated range of PCO2 is from ∼1.3 to ∼6.8 bars, which provides the maximum value of PCO2 at which calcite will not be present. The present day PCO2 values of geothermal fluids from the Reykjanes system were derived from analytical data on liquid and vapor samples collected at the surface from wet-steam well discharges using both the WATCH and SOLVEQ speciation programs. The geothermal fluids at reference temperature between 275°C and 310°C have PCO2 concentrations ranging from 1.3 bars to 4.0 bars. The calculated PCO2 values based on epidote compositions are in close agreement with present-day fluid PCO2 in the Reykjanes geothermal system. 72 percent of the calculated PCO2 values based on epidote compositions where the assemblage of epidote, prehnite, quartz and calcite are observed in drill cuttings are within the range of measured present-day fluids, while 58 percent of the calculated PCO2 values fall within the range when calcite is not present in the drill cuttings. Therefore, our method for calculating fluid PCO2 is proven quite reliable when all four index minerals are present. Additionally, if only epidote, prehnite and quartz are observed, our model calculations still serve as a moderately accurate predictive proxy for maximum fluid PCO2 composition in the Reykjanes geothermal system. Ultimately, these correlations between the measured and calculated PCO2 fluid compositions will in the future provide a method, based on compositional variation and paragenesis of hydrothermal minerals in mafic lithologies, by which to characterize spatial and temporal concentrations of CO2 in both active and fossil hydrothermal systems and in low-grade metamorphic environments. The generally observed zoning pattern in epidotes in the Reykjanes geothermal system from Fe(III)-rich cores to Al-rich rims suggests that if the zoning formed while equilibrium was maintained among the epidote-prehnite-calcite-quartz assemblage under near isothermal conditions, there must have been an increase in PCO2 with time. This geochemical signature may then be employed to make large scale inferences concerning the evolution of the Reykjanes geothermal system. Analysis of geothermal fluids collected at the wellhead suggest that the four index minerals that comprise the assemblage are in equilibrium with the fluids, thus enabling the application of the PCO2 predictive method discussed in this study to modern epidote samples. In addition to aiding in understanding the history of reactions that involved natural sequestration of CO2 derived from magmatic degassing, this study may also provide useful insights into reactions that could result from the injection of industrial CO2-rich fluids into hydrothermal environments in basaltic rocks.