The solubility of fluorite in H 2O and H 2O–NaCl at high pressure and temperature

Abstract

The solubility of fluorite in H 2O and H 2O–NaCl has been measured at 600–1000 °C, 0.5–2.0 GPa in a piston–cylinder apparatus. Fluorite dissolves congruently at all conditions investigated. The data indicate that the concentration of fluorite dissolved in H 2O is low at 600 °C and 0.5 GPa (0.0014 mol/kg H 2O) but increases strongly with temperature ( T) and pressure ( P) to 0.247 mol/kg H 2O at 1000 °C, 2 GPa. The data were fit with the equation log m CaF 2 = − 6.587 + 2.774 P 1/2 + (2.266 × 10 − 3 − 4.699 × 10 − 4 P) T, where m CaF 2 is molality of CaF 2, T is in Kelvin, and P is in GPa. Fluorite solubility was also measured in H 2O–NaCl fluids at 800 °C and 1.0 GPa. Results indicate a strong increase in dissolved fluorite concentration with increasing NaCl content, from 0.028 mol/kg H 2O at X NaCl = 0 to 1.673 mol/kg H 2O at X NaCl = 0.486, where X NaCl is the mole fraction of NaCl in the fluid. The data were fit to m CaF 2 = 0.01035 + 1.141 X NaCl + 4.694 X NaCl 2. The results indicate that F mobility will be further enhanced by saline brines in high P–T igneous and metamorphic environments. High F solubility in H 2O and H 2O–NaCl in the deep crust and upper mantle can promote mobilization of high field-strength elements, rare earth elements, U, and Th through transport as F-bearing complexes.