URANIUM IN GRANITIC MAGMAS: PART 1. EXPERIMENTAL DETERMINATION OF URANIUM SOLUBILITY AND FLUID-MELT PARTITION COEFFICIENTS IN THE URANIUM OXIDE-HAPLOGRANITE-H2O-NA2CO3 SYSTEM AT 720-770°C, 2 KBAR

Abstract

The solubility of uranium was investigated in both carbonated aqueous fluid and granitic melt in equilibrium in the system haplogranite-uranium oxide-H2O-Na2CO3 (0.5–1 molal) at 720–770°C, 2 kbar, andƒo2 fixed by Ni-NiO, Fe3O4-Fe2O3, and Cu2O-CuO buffers. As complete solid solution exists between UO2.00 and UO2.25 (i.e., 75 mol% UO2 + 25 mol% UO3), three distinct uranium oxides: UO(2.01 ± 0.01), UO(2.1.0 ± 0.02), and UO(2.25 ± 0.02) were, respectively, obtained at equilibrium, under the three ƒo2 conditions cited above. Thus, the percentage of U (VI) in uranium oxide increased with increasing log ƒo2. The thermal decomposition of Na2CO3 to CO2 and Na2O led to the decrease of the sodium carbonate concentration from 0.5–1 molal to ~10⁻² molal in all aqueous fluids and to the dissolution of Na in the silicate melts. Crystal-free silicate glasses with four agpaitic coefficients, were obtained.