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Gas pressures and redox reactions in geothermal fluids in Iceland |
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| Authors: | Andri Stefnsson Stefn Arnrsson |
| Institution: | a Institut für Mineralogie und Petrographie, ETH-Zentrum, Sonneggstrasse 5, CH-8092, Zurich, Switzerland b Science Institute, University of Iceland, Dunhagi 3, 107, Reykjavík, Iceland |
| Abstract: | The gas and redox chemistry of 100–300 °C geothermal fluids in Iceland has been studied as a function of fluid temperature and fluid composition. The partial pressures of CO2 in dilute (mCl<500 ppm) and saline (mCl>500 ppm) geothermal fluids above 200 °C are controlled by the mineral buffer clinozoisite+prehnite+calcite+quartz. Two buffers are considered to control the H2S and H2 partial pressures above 200 °C depending on fluid salinity, epidote+prehnite+pyrite+pyrrhotite for dilute fluids and pyrite+prehnite+quartz+magnetite+anhydrite+clinozoisite+quartz for saline fluids. Below 200 °C, the partial pressures of CO2, H2S and H2 also seem to be buffered but other minerals must be involved. Zeolites are expected to replace prehnite and epidote. Redox potential calculated on the assumption of equilibrium for the H+/H2 redox couple decreases in dilute geothermal fluids with increasing temperature from about ?0.5 V at 100 °C to ?0.8 V at 300 °C, whereas saline geothermal fluids at 250 °C display a redox potential of about ?0.45 V. A systematic discrepancy between redox couples of about 0.05–0.09 V is observed in the redox potential for the dilute geothermal fluids, whereas redox potentials agree within 0.02–0.04 V for saline geothermal waters. The discrepancies in the calculated redox potential for dilute geothermal fluids are thought to be due to a general lack of equilibrium between CH4, CO2 and H2 and between H2S, SO4 and H2. It is, accordingly, concluded that an overall equilibrium among redox species has not been reached for dilute geothermal fluids whereas it appears to be more closely approached for the saline geothermal fluids. The latter conclusion is based on limited database and should be treated with care. Since the various redox components are not in an overall equilibrium in geothermal fluids in Iceland these fluids cannot be characterised by a unique hydrogen fugacity, oxygen fugacity or redox potential at a given temperature and pressure. |
| Keywords: | Redox reactions Redox potential Geothermal fluids Gas pressures Chemical equilibrium Aqueous speciation |
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