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, mixes with magmatic volatiles and reacts with the basaltic host rock to form secondary geothermal minerals in progressively higher-grade mineral alteration zones with depth where highest recorded temperatures are ~320°C. 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 most common at temperatures >250°C and <~310°C, and potentially provides a mineralogic chronometer constraining 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 2-3 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. The highest Fe(III) content in epidote that was observed to co-exist with prehnite is an Fe(III)-mol fraction of 0.36, which serves as the upper Fe(III) limit for epidotes used in this study. Because most observed prehnite crystals in the drillhole-cuttings are too small for electron micropropbe analyses (<20m), we employed a sigmoidal regression 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. Calculated values of PCO2 for the geothermal fluids using epidote compositions from crystals obtained from drillcutting at depths >600m between the reference temperatures of 275°C and 310°C range from ~1 to ~6.5 bars, when only epidote, prehnite and quartz are observed. When all members of the assemblage are present, including calcite, the calculated range in PCO2 is from ~0.5 to 6 bars. If calcite is absent from the assemblage, the computation of PCO2 using equilibria for reaction 1 provides the maximum value of PCO2 at which calcite will not be present. Actual PCO2 values of geothermal fluids from the Reykjanes system were derived from analytical data on liquid and vapor samples collected at the surface 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 measured CO2 in the Reykjanes geothermal system fluids. 74% of the calculated PCO2 values based on epidote compositions where the complete epidote-prehnite-calcite-quartz assemblage is observed fall within the range of measured present-day fluids, while 62% of the calculated PCO2 values fall within the range when calcite is not present in the assemblage. 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 method still serves as a moderately accurate predictive proxy for fluid PCO2 composition in the Reykjanes geothermal system. Ultimately, the correlation between measured and predicted fluid compositions, provides insight into future abilities to characterize spatial and temporal concentrations of CO2 in active and fossil hydrothermal and low-grade metamorphic environments in mafic lithologies based on compositional variations and paragenesis of hydrothermal minerals. In addition, this study will aid in understanding the nature of reactions that involve natural sequestration of CO2 derived from magmatic degassing, and injection of industrial CO2-rich fluids within hydrothermal environments in basaltic rocks.

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