Salt Effects on Stable Isotope Partitioning and Their Geochemical Implications for Geothermal Brines

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). INTRODUCTION It has long been recognized that dissolved salts in water can change oxygen and hydrogen isotope partitioning between water and other phases (Le., vapor, minerals) due to the hydration of ions upon the dissolution of salts in water. However, their effects have not been well determined at elevated temperatures (cf. Truesdell, 1974). We are currently conducting a series of hydrothermal experiments of the system brine-vapor or minerals to 350°C, in order to determine precisely the effects of dissolved salts abundant in brines on isotope partitioning at temperatures encountered in geothermal systems. The so-called “isotope salt effect” has important implications for the interpretation and modeling of isotopic data of brines and rocks obtained from geothermal fields. We will show how to use our new results of isotopic partitioning to help better evaluate energy resources of many geothermal fields. EXPERIMENTAL We can determine the salt effect on oxygen and hydrogen isotope partitioning between liquid water and a coexisting phase A (vapor, minerals) from, lb ln r = lblnaA-brine-lblnaA.pure water, (1) where a is an equilibrium isotope fractionation factor between a phase A and liquid water. The isotope salt effect (r), which is caused by the dissolution of salts in water, can be more rigorously defined as follows, a(HDO)/a(H,O) a(H,’80)/a(H,’60) X(HDO)/X(H,O) X(H,’80)/X(H,’60) r = or = y(HDO)/y(H,O) or ~(H,’~o)/y(H,’60), (2) where a, X, and y denote activity, mole fraction, and activity coefficient, respectively. In order to examine the effects of dissolved salts on isotope partition, a series of vapor-liquid water equilibration experiments were carried out for NaCl solutions from 50 to 350 C, CaCI, solutions from 50 to 2OO0C, and for KCI, MgCl,, Na,SO,, and MgSO, solutions from 50 to 100’C. Several mixed salt solutions mimicking natural geothermal brines (Salton Sea brine) were also studied at 50 to 1000C. In order to cover the wide range of temperature, three different liquid-vapor equilibration apparatus were employed, using both static and dynamic sampling techniques of water vapor. The overall errors in values of l b ln r are kO.l to 0.15%0 ( la) and k1.0 to 1.5%0 ( la ) for oxygen and hydrogen isotopes, respectively. The effect of dissolved NaCl on oxygen isotope