Th and U in hydrous rhyolite melts

We measured diffusion coefficients of thorium (Th) and uranium (U) in hydrous (up to 6 wt.% H₂O) melts of Lake County Obsidian at 1 GPa and temperatures ranging from 900°C to 1200°C. Diffusion couples were assembled from pre-synthesized capsule halves with high (~500 ppm) and low (~100 ppm) concentrations of U and Th. Experiments were run for 4 to 54 hours, depending on temperature, and the resulting concentration profiles were characterized by LA-ICP/MS in the case of U and both EPMA and LA-ICP/MS in the case of Th. Th and U have almost identical diffusivities, ranging from 10^-9 to 10^-7 cm²/s over the temperature range examined. We observed Arrhenius behavior for both Th and U, and constrain activation energies to E ~140 and 116 kJ/mole, respectively. Measured diffusivities are insensitive to dissolved H₂O contents at > 6 wt%. Modest down-temperature extrapolation to conditions relevant to the Earth's crust (700 - 850°C) give Th and U diffusivities of ~10^-10 cm²/s. Our results are comparable with known values for the major structural constituents of accessory minerals that concentrate U and Th (e.g., zircon, monazite, apatite, xenotime), so the diffusive supply of U and Th to growing crystals is adequate to preclude significant disequilibrium uptake during growth. The results complement and extend previous results on Th and U diffusion in hydrous molten granite.