Calcium isotope systematics in the NaCl-H2O system at elevated pressure and temperature

Two sets of hydrothermal experiments were performed in order to quantify the rate of Ca isotope exchange between a Ca-bearing NaCl fluid and anhydrite and to determine the amount of fractionation between the vapor and liquid phases in the NaCl-H2O system. A flexible gold cell reactor was used for all experiments. The isotope exchange experiments employed 43Ca spikes and in-situ sampling in order to track isotope exchange as a function of time and total dissolved Ca concentration at 410°C, 500 bar. Experiment 1 lasted 282 hours and had a total dissolved Ca concentration (SCa) of 12 mmol/kg. Mass balance calculations indicate that 18.2% exchange occurred within this period. Experiment 2 contained 23 mmol/kg SCa and after 243 hrs 26.5% exchange occurred between dissolved Ca and anhydrite. The third experiment contained 45 mmol/kg SCa and 20.9% exchange occurred within 72 hours. These data indicate that increasing SCa causes an increase in exchange rate. However, these rates are slower than those calculated from similar spiked experiments below 350°C and may reflect a change in exchange mechanism or be due to the retrograde solubility of anhydrite. Two experiments were also performed at 420 and 450°C to determine the extent of Ca isotope fractionation between the vapor and liquid phases in the NaCl-H2O system. Anhydrite, of natural isotopic composition, buffered Ca concentrations in both phases. Within the pressure ranges investigated (420°C, 33-31 MPa; 450°C, 40.5-37.5 MPa), there is no observed isotope fractionation between vapor and liquid. This result implies that the same aqueous Ca species dominates both phases. All experiments presented here provide data necessary to better understand Ca isotope kinetics and fractionation in hydrothermal environments, particularly mid-ocean ridge vent systems.