Fluorapatite solubility in H2O and H2O–NaCl at 700 to 900 °C and 0.7 to 2.0 GPa

The solubility of Durango fluorapatite was measured in H2O and H2O–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 H2O 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 cap° = − 3.56 + .00241T + 9.17 logρH2O, where cap° is apatite concentration in H2O (in ppm), T is in Kelvin, and ρH2O is H2O density in g cm− 3. Fluorapatite solubility was also measured in H2O–NaCl fluids at a range of P and T. Results indicate a strong increase in dissolved fluorapatite concentration with rising NaCl mole fraction (XNaCl) at all P and T, and XNaCl to near halite saturation. The data were fit to the equation cap = cap° + BaNaCl1/2 where cap is apatite concentration in H2O–NaCl (in ppm), aNaCl is NaCl activity calculated from an ideal mixing model for H2O–NaCl, and B = (4.4 + 1.1P4)exp(0.0070T), 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 XNaCl. 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 H2O and H2O–NaCl; however, the solubility enhancement by NaCl relative to that in pure H2O 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.