| Summary: | The carbon chemistry and stable carbon isotope systematics of low-temperature geothermal waters in Iceland was studied. The waters had temperatures ranging from 3 to 97°C, pH between 6.18 and 10.15 as well as dissolved inorganic carbon concentration from 1.8 to 2853 ppm. The carbon isotopes were found to be in the range δ¹³C -1.46 to -13.96‰. The geochemistry of CO₂ and its sources and reactions in low-temperature geothermal water was approached in three ways; by carbonate mineral saturation, a component mixing model and by reaction path isotope modelling. Low-temperature geothermal waters were observed to be calcite saturated, suggesting that calcite may possibly form in low-temperature ground water systems. The source of the carbon in the water and the possible formation of calcite, along with the stable carbon isotope systematics cannot be explained simply by rock dissolution, atmospheric CO2 input and organic matter decay. Instead, progressive basalt dissolution, aqueous speciation, and calcite formation play a major role in carbon isotope systematics and the carbon concentration of the low-temperature geothermal water. For waters that contain low CO₂ concentrations (<50 ppm) and low δ13C values (-5 to -15‰) the CO₂ is thought to be derived from both atmospheric sources and primary rock dissolution. This is due to the variations of CO₂ concentrations and δ13C values generated by the concentration of CO₂ and the exact δ¹³C content of the basalt, and due to carbon isotope fractionation upon water-rock interaction. However, waters that contain high CO₂ concentrations and high δ¹³C values cannot be explained without the introduction of a highly concentrated CO₂ source with a δ13C value of less than -3‰. This source cannot be carbonate dissolution at shallow depth within the crust, as this would result in too low CO₂ concentrations. Mantle degassing through the crust is also unlikely as this would result in too low δ¹³C values. Presently, an alternative CO₂ source of unknown origin has therefore been ...
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