Solubility of CePO4 and YPO4 in H2O, H2O-NaCl, H2O-NaF and H2O-Albite Fluids at 800°C and 1 GPa: Implications for REE Transport During Subduction-Zone Metasomatism

Monazite (CePO4) and xenotime (YPO4) are important accessory minerals in subducted metasediments because they host significant REE and are useful for geochronology. It is therefore essential to understand their behavior during metasomatic processes that attend subduction. In order to set constraints on the solubility of CePO4 and YPO4 at high pressure and temperature, we carried out a preliminary series of weight-loss experiments on synthetic single crystals of each phosphate in a piston- cylinder apparatus at 800°C and 1 GPa. The CePO4 and YPO4 crystals were grown from a Pb-free flux at a temperature of 1280°C for 15 hr, which was then cooled down to 870°C with a cooling rate of 3°C per hour. For each experiment, a single crystal was placed in a Pt envelope, which was added to a 3.5 mm OD Pt capsule with ultrapure H2O, H2O-NaCl, H2O-NaF or H2O-albite. Results indicate that the concentration (m, mol/kg H2O) of CePO4 (mz) and YPO4 (xt) dissolved in pure H2O is very low (mz, 0.0006 m; xt, 0.0002 m) but mz solubility increases strongly with increasing NaCl to 0.0059 m at 30 mol% NaCl. Solubility of xt only increases moderately to 0.0033 m at the same NaCl concentration, thus indicating a different solubility behavior of the two phosphates. In 10 wt% albite the solubility of both phosphates increases slightly, similar to that in a 10 mol% NaCl solution (mz, 0.0014 m; xt, 0.0011 m). Preliminary data on the solubility in a 20 mol% NaF solution indicate an enormous increase in solubility of both phosphates: mt, 0.70 m; xt, 0.75 m. These correspond to enhancements by factors of 1100 and 3000, respectively, supporting previous indications that REE mobility is strongly promoted by complexing with F in the aqueous phase. Because F is readily partitioned into subduction-zone fluids, even small amounts of this element in metasediments will yield at least local REE metasomatism during devolatilization reactions associated with subduction-zone metamorphism.