An experimental study of Th and U of partitioning and diffusion in hydrous rhyolitic melt

Xing, Lingbo
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Watson, E. Bruce
Katz, Miriam E.
Roecker, Steven W.
Spear, Frank S.
Tomozawa, Minoru
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Part II: Th and U diffusion in granite melt: Experimental determination in diffusion couples. In order to improve our understanding of the transport mechanisms of Th and U in melt, 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. The experiments were done using the diffusion couple method and the concentration profile that developed in each couple were modeled to retrieve the self diffusion coefficients of Th and U. 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.
May 2013
School of Science
Dept. of Earth and Environmental Sciences
Rensselaer Polytechnic Institute, Troy, NY
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