In situ synchrotron-radiation XRF study of REE phosphate dissolution in aqueous fluids to 800 °C

The temporal evolution of the concentrations of La, Ce, Pr, Nd, Sm, Gd, Y, Pb, Th, and U in the fluid during dissolution of synthetic LaPO4 monazite, natural monazite, and xenotime in aqueous HCl ± NaCl solutions was studied in situ at temperatures from 25 to 800 °C and pressures to about 2 GPa using hydrothermal diamond-anvil cells and synchrotron-radiation XRF analyses. Constant (within error) La concentrations in the fluid were attained in less than 50 min at temperatures ≥ 300 °C and in less than 90 min at 200 °C. At 100 °C, the time required to reach a constant La concentration varied with the HCl molality from less than 3 h at 5.1 m HCl to ≥ 11 h at 1 m HCl. The experiments indicate a significantly lower mobility of Y, Gd, and Th compared to that of the LREE, particularly La, over a large range of crustal P–T conditions if controlled by interaction of monazite with acidic and chloridic fluids. The corresponding change in the REE pattern is in close agreement with literature reports on natural monazites, which underwent greisenization or sericitization. The Th/Pb ratio in the fluid increased with temperature and HCl concentration, but was far below the Th/Pb ratio in the monazite even at 600 °C and 2 GPa. The experimental results imply that monazites leached in acidic and chloridic fluids are characterized by a strong enrichment of Th relative to the LREE and Pb.