Salt Effects on Stable Isotope Partitioning and Their Geochemical Implicationsfor Geothermal Brines

Abstract

The effects of dissolved salts (NaCI, KCI, MgCl,, CaCI,, Na,SO,, MgSO,, and their mixtures) on oxygen and hydrogen isotope partitioning between brines and coexisting phases (vapor and calcite) were experimentally determined at 50-350 C and 300 C, respectively. In liquid-vapor equilibration experiments, for all of the salts studied, the hydrogen isotope fractionation factors between the salt solutions and vapor decreased appreciably (up to 20?!0) compared to pure water-vapor. Except for KCI solutions at 500 C, the oxygen isotope fractionation factors between salt solutions and vapor were higher (up to 4%0) than, or very close to, that of pure water. The observed isotope salt effects are all linear with the molalities of the solutions. Mixed salt solutions mimicking natural geothermal brines exhibit salt effects additive of those of individual salts. The isotope exchange experiments of calcite-water at 300-C and 1 kbar yielded a fractionation factor of 5.9+0.3%0 for pure water and effects of NaCl consistent with those obtained from the liquid-vapor equilibration experiments. The isotope salt effects observed in this study are too large to be ignored, and must be taken into account for isotopic studies of geothermal systems (Le., estimation of isotope ratios and temperatures of deep-seated geothermal brines).