THE SOLUBILITY OF FLUORITE IN H2O AND H2O-NACL AT HIGH PRESSURE AND TEMPERATURE

Abstract

The solubility of fluorite in H2O and H2O-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 H2O is low at 600 °C and 0.5 GPa (0.0014 mol/kg H2O) but increases strongly with temperature (T) and pressure (P) to 0.247 mol/kg H2O at 1000 °C, 2 GPa. The data were fit with the equation log mCaF2 = - 6.587 + 2.774P1/2 + (2.266 × 10- 3 - 4.699 × 10- 4P)T, where mCaF2 is molality of CaF2, T is in Kelvin, and P is in GPa. Fluorite solubility was also measured in H2O-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 H2O at XNaCl = 0 to 1.673 mol/kg H2O at XNaCl = 0.486, where XNaCl is the mole fraction of NaCl in the fluid. The data were fit to mCaF2 = 0.01035 + 1.141XNaCl + 4.694XNaCl2. 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 H2O and H2O-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. © 2007 Elsevier B.V. All rights reserved.