Solubility of CePO4 monazite and YPO4 xenotime in H2O and H2O–NaCl at 800 °C and 1 GPa: Implications for REE and Y transport during high-grade metamorphism

Monazite and xenotime are important accessory minerals in metasediments because they host REE and are useful for geochronology and geothermometry. It is therefore essential to understand their behavior during metasomatic processes that attend metamorphism and subduction. In order to constrain the solubility of monazite and YPO4 xenotime at high pressure and temperature, we carried out weight-loss experiments on synthetic single crystals of CePO4 (monazite) and YPO4 (xenotime) using hydrothermal piston-cylinder methods at 800 °C and 1 GPa, in H2O and H2O–NaCl fluids. Results indicate that CePO4 and YPO4 dissolved congruently, and that their solubilities in pure H2O are very low: 0.04 ± 0.04 and 0.25 millimolal, respectively. The solubility of CePO4 rises to an increasing extent with added NaCl, to 7.94 ± 0.07 millimolal at 50 mol% NaCl. In contrast, the solubility of YPO4 rises to a decreasing degree with increasing NaCl, to 4.36 ± 0.08 millimolal at 50 mol% NaCl. Best fit equations for the solubilities of the two phases aremCePO4 = 3.56 · 10− 5 + 5.82 · 10−3XNaCl + 1.97 · 10−2X2NaClandmYPO4 = 2.48 · 10−4 + 1.26 · 10−2XNaCl − 8.89 · 10−3X2NaCl.The solubility of YPO4 is greater than that of CePO4 at NaCl mole fractions (XNaCl) of 0.00–0.27. The solubility enhancement behavior implies that Ce dissolves as anhydrous chloride complexes, whereas Y forms mixed Cl–OH solutes. The results provide a simple mechanism for redistributing REE and Y in deep-crustal and upper mantle environments. The H2O/Ce ratio inferred for subduction-zone melts and silicate-rich fluids can also be produced by a CePO4-saturated fluid with XNaCl = 0.1. In addition, neutral-pH H2O–NaCl fluids can transport substantial REE and Y, obviating the need to invoke highly acid solutions in environments where they are unlikely.

Research Highlights
► Solubility experiments at 800 °C and 1 GPa indicate that CePO4 and YPO4 dissolve congruently, and solubilities in pure H2O are very low.
► Solubility of CePO4 increases with added NaCl, to 7.94 ± 0.07 millimolal at 50 mol% NaCl.
► In contrast, solubility of YPO4 decreases with increasing NaCl, to 4.36 ± 0.08 millimolal at 50 mol% NaCl.
► The solubility enhancement behavior implies that Ce dissolves as anhydrous chloride complexes, whereas Y forms mixed Cl–OH solutes.
► The results provide a simple mechanism for redistributing REE and Y in deep-crustal and upper mantle environments.