An experimental study of thorium and uranium of partitioning and diffusion in hydrous rhyolitic

Part I: Th and U partitioning between monazite and hydrous felsic melt: the effect of melt composition and temperature. We present experimental measurements of monazite/melt partition coefficients (Kmonz/melt) for thorium (Th) and uranium (U) in haplogranitic Na2O-Al 2O3-SiO2 melts (with 6-10 wt.% H2O) at 900ºC-1100ºC and 1 GPa pressure. Our results show the following trends: (1) KTh and KU in felsic melts are strongly dependent on melt Al/Na. Both partition coefficients reach maxima at Al/Na≈1.4 and decrease toward strongly peraluminous (Al/Na>1) and peralkaline ( Al/Na<1) melt compositions. KTh and KU vary by 2 orders of magnitude for Al/Na in the range of 0.4 to ~2. (2) For identical melt compositions, the effect of temperature on KU is pronounced: A 200ºC increase in temperature results in a 6-fold decrease in KU. In contrast, the same temperature change results in only 50% decrease of the less temperature-sensitive KTh. (3) One experiment with 10 wt.% H2O added does not change K, which may suggest little dependence of K upon water content from 6 to 10 wt%. (3) KTh is much larger than 1 at all natural magmatic temperature, so the removal of monazite during fractionation crystallization or its retention as a residual phase during partial melting has a significant effect on the content of Th in the coexisting melt phase. (4) U is much less compatible than Th, and could even behave incompatibly at high temperatures (e.g., at 1000ºC, KU is 0.01 for melts containing 75 wt% SiO2 and Al/Na~0.4).;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.