Oxygen isotope equilibrium in sulfate–water systems: A revision of geothermometric applications in low-enthalpy systems

Since approximately forty years ago, the equation relating the oxygen isotope fractionation between bisulfate and water has been extensively used as a geothermometer for acidic and high-enthalpy hydrothermal systems (T > 150 °C). Its application to near-neutral or basic, low-enthalpy (T < 150 °C) systems was considered to be unreliable. Recently, empirical and theoretical geothermometric equations were formulated for the (SO42 −–H2O) oxygen isotope system, opening new perspectives on isotope geochemistry applied to geothermal research. In the present paper, these geothermometers are applied while revisiting a variety of hydrological case studies. In addition to the equilibrium fractionation, isotopic effects on geothermometric estimates due to mixing–dilution, salinity, sulfide oxidation, sulfate reduction, standardization of the analytical results and aqueous speciation are also discussed.

► Recent SO4–H2O oxygen isotope equations are tested for geothermometric application. ► Dilution, salts, water cooling and standardization effects have been examined. ► Sulfate species and CaSO4-equilibrium may control the isotope equilibrium. ► Other than isotope, chemical and thermodynamic knowledge of the systems are necessary.