Fluorapatite solubility in H 2O and H 2O–NaCl at 700 to 900 °C and 0.7 to 2.0 GPa

Abstract

The solubility of Durango fluorapatite was measured in H 2O and H 2O–NaCl at 700–900 °C and 0.7–2.0 GPa in a piston-cylinder apparatus. Solubility was determined by weight loss using a double-capsule method. At all conditions, fluorapatite dissolves incongruently to monazite + fluid, but residual monazite crystals were below weighing detection limits of 0.6 μg. The concentration of fluorapatite dissolved in pure H 2O is low at all investigated conditions (56 ± 8 to 288 ± 8 ppm by weight), but increases with temperature ( T) and pressure ( P). The data are well described by the equation log c ap ° = − 3.56 + .00241 T + 9.17 log ρ H 2O , where c ap ° is apatite concentration in H 2O (in ppm), T is in Kelvin, and ρ H 2O is H 2O density in g cm − 3 . Fluorapatite solubility was also measured in H 2O–NaCl fluids at a range of P and T. Results indicate a strong increase in dissolved fluorapatite concentration with rising NaCl mole fraction ( X NaCl) at all P and T, and X NaCl to near halite saturation. The data were fit to the equation c ap = c ap ° + Ba NaCl 1/2 where c ap is apatite concentration in H 2O–NaCl (in ppm), a NaCl is NaCl activity calculated from an ideal mixing model for H 2O–NaCl, and B = (4.4 + 1.1 P 4)exp(0.0070 T), with P in GPa, and T in Kelvin. The results differ from previous work in that the newly determined solubilities are ≥ 10 times lower at comparable conditions, increase with P at constant T, and show a strong positive correlation with X NaCl. The discrepancy is attributed to unrecognized growth of new apatite crystals at run conditions in the earlier study. Compared to fluorite, calcite and anhydrite, fluorapatite solubility is significantly lower in H 2O and H 2O–NaCl; however, the solubility enhancement by NaCl relative to that in pure H 2O is similar to anhydrite and much greater than calcite and fluorite. The NaCl enhancement of solubility increases with T but decreases with P. The undetectable weight of monazite requires significant solubility of light rare-earth elements (LREE), likely as chloride complexes, and indicates that metasomatic interaction of brines with apatite-bearing rocks at high metamorphic grades can mobilize substantial LREE.