Oxygen isotope activities and concentrations in aqueous salt solutions at elevated temperatures: Consequences for isotope geochemistry

Studies of the effect of dissolved salts on the oxygen isotope activity ratio of water have been extended to 275°C. Dehydrated salts were added to water of known isotope composition and the solutions were equilibrated with CO₂ which was sampled for analysis. For comparison similar studies were made using pure water. Results on water nearly coincide with earlier calculations. Salt effects diminish with increasing temperature only for solutions of MgCl₂ and LiCl. Other salt solutions show complex behavior due to the temperature-dependent formation of ion pairs of changing character. Equilibrium fractionations (10³ ln α) between 1 molal solutions and pure water at 25, 100, and 275°C are: NaCl 0.0, −1.5, +1.0; KCl 0.0, −1.0, +2.0; LiCl −1.0, −0.6, −0.5; CaCl₂ −0.4, −1.8, +0.8; MgCl₂ −1.1, −0.7, −0.3; MgSO₄ −1.1, +0.1, −; NaF (0.8 m) 0.0, −1.5, −0.3; and NH₄Cl (0.55 m) 0.0, −1.2, −1.3. These effects are significant in the isotope study of hot saline fluids responsible for ore deposition and of fluids found in certain geothermal systems. Minor modification of published isotope geothermometers may be required.