Reformulation and performance evaluation of the sulfate-water oxygen isotope geothermometer

Recent utilization of the dissolved sulfate-water oxygen isotope exchange geothermometer (δ¹⁸O SO₄-H₂O) to estimate geothermal fluid temperatures at depth appears infrequent, even with general literature support regarding its usefulness. Here, the authors provide support for continued use of this method through revision of the sulfate geothermometer equation and through analysis of publically available chemical and correlative wellbore temperature data. The utility of the δ¹⁸O SO ₄-H₂O exchange reaction for calculating equilibration temperatures is based on a slow reaction exchange rate in pH neutral conditions: water-rock equilibration to 90% can take 500 years at 100°C and 2 years at 300°C. These high-temperature reaction rates, considered to be the slowest in the suite of established geothermometers, validate the use of the SO ₄-H₂O geothermometer to help in estimating the temperatures of deep reservoir waters that have experienced long residence times. The reliability of this geothermometer can be affected by multiple processes that include geothermal and meteoric water mixing, δ¹⁸O fractionation of H₂O during boiling, formation of SO₄ by oxidation of H₂S, influence by biologic activity, dissolution and precipitation of anhydrite, and an elevated exchange rate in acidic conditions. However, with the appropriate caveats applied and regard given to dissolved ion chemistry the δ¹⁸O SO ₄-H₂O geothermometer can be a valuable tool. Several iterations of the geothermometer have been published, based on regressions of two experimental datasets - Lloyd (1968) and Mizutani and Rafter (1968)- and on a combination of the two datasets (Seal et al., 2000). McKenzie and Truesdell (1977) pointed out that the two datasets should be corrected for a revised CO₂-H₂O fractionation factor (used to determine the oxygen isotope value of water). The Seal et al. (2000) equation for the geothermometer did not take the revised fractionation factor into account. It was not until 1985 that the IAEA proposed that the new fractionation factor should be applied by laboratories (Hoefs, 2009). Therefore, for the purposes of this study, the Seal et al. (2000) equation was used to calculate δ¹⁸O SO ₄-H₂O geothermometer temperatures for data collected prior to 1985. For data collected after 1985 (and where the updated fractionation value is assumed to have been employed), a revised geothermometer equation combining the corrected Lloyd (1968) and Mizutani and Rafter (1969) datasets is as follows: (Equation Presented) Herein, this method, previous δ¹⁸O SO₄-H₄O methods, and methods commonly employed for silica and cation geothermometer calculations are evaluated and compared. Results of this evaluation on a select dataset indicate a high level of statistical accuracy for the δ¹⁸O SO ₄-H₂O geothermometer when compared to the results of other geothermometers, with comparably little data spread.