The transition from dilute electrolyte aqueous solution to molten salt in geologic fluids: evidence from calcite solubility measurement in Na-halide solutions at 8 kbar and 700 °C

Abstract:

Fluids are major agents of mass and heat transport in the Earth crust and in subduction zones. Fluid inclusions, metasomatic field relations and experimental evidence suggest that these fluids can contain important ligands, including halogens, sulfates, sulfides, etc. The ligands participate in the complexation of rock-forming elements during mineral dissolution to high-T and P. Although models of high- element metasomatism typically assume that H₂O dominates the fluid’s solvent properties, however, H₂O may be a relatively minor component in the high-PT brines that are increasingly recognized in the lower crust and mantle. Understanding the evolution of solubility mechanisms as fluids change from dilute aqueous solutions to salt-rich brines is hindered by the absence of experimental investigation of this transition. To address this problem, we conducted experiments on the solubility of calcite in sodium-halide solutions at 8 kbar and 700 °C using hydrothermal piston-cylinder weight-loss methods. Investigated salts were NaL , where L=F, Cl, Br, I, at concentrations ranging from 0.15 molal to 20 molal (X_NaL ~ 0.3). At these conditions, the fluid is a single supercritical fluid phase . Run durations were 4 to 20 hours. Results demonstrate systematic trends with ligand ionic size, and locate a major mechanistic transition in the vicinity of X_NaL~ 0.1 for all calcite-H₂O-NaL systems. At lower than this critical composition (X_crit), calcite solubility displays a pronounced concave shape indicating involvement of water during the dissolution process. At X_NaL> X_crit , the shape becomes convex with no apparent effect of decreased H₂O activity in the fluid. The solubility patterns suggest that the solvent properties are dominated by those of H₂O at X_NaL< X_crit, but at X_NaL> X_crit, H₂O is a solute in a solution behaving as a molten salt. Geological evidence suggests that salt concentrations may reach values similar to or greater than X_crit in a range of metamorphic and igneous systems in the middle and lower crust. Our study implies that such fluids are best modeled as molten salt solutions, even at relatively water-rich compositions. This conclusion provides a foundation for developing models of metasomatic processes involving saline fluids in the deep crust and subduction zones.