The Immobility of Uranium (U) in Metamorphic Fluids Explained by the Predominance of Aqueous U(IV)

The solubility of uranium (U) in hydrothermal fluid is thought to be controlled by oxidation. In general, uranium is mainly transported as U(VI) in oxidized fluid, but precipitated as U(IV) in reduced fluid. However, many geological observations indicate that metamorphic fluids, which are buffered by metamorphic rocks with oxidized protoliths such as oxidized pelite or altered marine basalt, are not enriched in U. To explore the reason of the low solubility of U in metamorphic fluids, we simulated the hydrous speciation and solubility of U in fluids that are in equilibrium with rocks. The simulations were conducted at pressure-temperature (P-T) conditions of greenschist and amphibolite facies metamorphism. The results show that U is mainly dissolved as U(IV), instead of U(VI), in metamorphic fluids. The solubility of U remains at a low level of similar to 10(-12) molal, and is not significantly influenced by metamorphic temperature, pressure, and fluid salinity. This result is consistent with geological observations and, thus, can explain the low-U nature of natural metamorphic fluids. The simulation also shows high solubility of U(VI) (1.3 x 10(-7) molal) in oxidized pelite-buffered fluids at low temperature (<250 degrees C), consistent with the geological fact that U can be mobilized by low-temperature geofluids.