HYDROTHERMAL MINERALS RECORD CO_2 PARTIAL PRESSURES IN THE REYKJANES GEOTHERMAL SYSTEM, ICELAND

摘要

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 H_2O allows prediction of geothermal fluid P_(co_2) 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 CO_2 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 P_(CO_2) 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 P_(CO_2) is from ～1.3 to ～6.8 bars, which provides the maximum value of P_(CO_2) at which calcite will not be present. The present day P_(CO_2)_2 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 P_(CO_2) concentrations ranging from 1.3 bars to 4.0 bars. The calculated P_(CO_2) values based on epidote compositions are in close agreement with present-day fluid P_(CO_2) in the Reykjanes geothermal system. 72 percent of the calculated P_(CO_2) 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 calcinated P_(CO_2) values fall within the range when calcite is not present in the drill cuttings. Therefore, our method for calculating fluid P_(CO_2) 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 P_(CO_2) composition in the Reykjanes geothermal system. Ultimately, these correlations between the measured and calculated P_(CO_2) fluid compositions will in the future provide a method, based on compositional variation and paragenesis of hydrothermal minerals in mafic lithologies, by which to character-ize spatial and temporal concentrations of CO_2 in both active and fos