Calcite Solubility in H2O-KCl-NaCl-LiCl Solutions at 700 °C and 8 kbar: Experimental Determination and Modeling

Abstract:

The calcium carbonate (CaCO₃) minerals calcite and aragonite are the primary reservoir for oxidized carbon in the continental crust and upper mantle, and are the dominant carriers of carbon into the Earth’s interior. Their solubility in subduction zone fluids strongly influences the amount of carbon retained in downgoing lithosphere. Previous work suggests modest solubility in pure H₂O, but high solubility in NaCl-H₂O solutions; however, natural solutions may contain additional cations such as KCl. We evaluated the role of salt on calcite solubility in salt-H₂O solutions involving NaCl, KCl, LiCl and CsCl. All experiments were conducted in a piston-cylinder apparatus at 700 °C and 8 kbar. These conditions were used because pressure is not expected to have a large effect on the nature of calcite dissolution in saline fluids. Results show that increasing the concentration of any salt exponentially increases calcite solubility for all salt-H₂O and mixed salt-H₂O solutions. At a given salt mole fraction in one-salt solutions, calcite solubility increases with decreasing salt cation size: CsCl<KCl<NaCl<LiCl. Experiments in mixed salt solutions revealed that calcite solubility varied systematically with salt components and their concentrations. The data were used to derive a simple empirical model for calcite solubility in any H₂O-KCl-NaCl-LiCl solution. The model reproduces the data in two-salt and three-salt solutions with an average deviation of 6% and 1% respectively. This study shows that aqueous fluids with high concentrations of halide salts have the potential to promote substantial carbon mass transfer. With the growing evidence for highly saline brines in a range of high pressure geologic environments, these fluids may play an important role in the deep carbon cycle.