The Iceland Deep Drilling Project (IDDP): (6) Hydrothermal minerals record CO2 partial pressures in the Reykjanes Geothermal System, Iceland
Abstract
The Reykjanes Geothermal system, a target site for drilling by the IDDP, is located on the Reykjanes Peninsula in southwest Iceland, the landward extension of the Mid-Atlantic Ridge spreading center. Seawater penetrates the coastal Reykjanes geothermal system at depth, mixing with magmatic volatiles and reacting with the basaltic host rock to form secondary hydrothermal minerals. Within this system, 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 ranges 0.17 to 0.48. 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 is 0.36, which 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, we 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. 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 using both the WATCH and SOLVEQ. At reference temperatures between 275°C and 310°C, these fluids 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% 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% of the calculated pCO2 values fall within the range when calcite is not present in the drill cuttings.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFMOS13A1171F
- Keywords:
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- 1009 GEOCHEMISTRY / Geochemical modeling;
- 1034 GEOCHEMISTRY / Hydrothermal systems;
- 8416 VOLCANOLOGY / Mid-oceanic ridge processes;
- 8424 VOLCANOLOGY / Hydrothermal systems