Partitioning of protactinium, uranium, thorium and other trace elements between columbite and hydrous silicate melt

U-series disequilibria are a unique powerful tool to constrain the time-scales and processes of magmatism in mid-ocean ridge, intra-plate, and convergent margin settings. 235U-231Pa is one of the important parent-daughter pairs (231Pa half life = 33 kyr) because protactinium is normally much more incompatible than U during magmatism and thus the ubiquitously observed 231Pa excess in young igneous rocks most likely reflects melting processes. However, because of the extreme incompatibility of protactinium in most silicate minerals (mineral/meltDPa < 10-5), it is quite challenging to experimentally investigate partition coefficient of Pa. Furthermore, it is unclear whether Pa is always +5 or possibly more reduced at natural magmatic conditions, in particular in a reduced mantle. This significantly limits our understanding of the chemical behavior of Pa and applications of U-series disequilibrium data to the study of magmatism. Columbite (Mn(Nb,Ta)2O6) is a mineral with Nb5+ and Ta5+ as major elements, and occurs in per-aluminous granites and pegmatites. Because protactinium is expected to be +5 charged at crustal oxygen fugacities, and Pa5+ has an ionic radius close to Nb5+ and Ta5+, Nb and Ta have been used as proxy elements to constrain partitioning of Pa between minerals and melt using lattice strain modeling. Nb and Ta are strongly compatible in columbite (columbite/meltDNb-Ta ~ 10), and lattice strain modeling based on physical characteristics of the Nb-Ta site where Pa should be incorporated predicts that columbite/meltDPa5+ should be around 0.2, high enough to be experimentally determined at the permissible Pa doping level of 10 ppm (bulk). Experiments were run in a piston cylinder apparatus at 0.5 GPa and 1115 oC using Pt double capsules with NNO or FMQ as external fO2 buffers. The starting material is a hydrous per-aluminous granitic composition, doped with Pa solution in the Paul Scherrer Institute and also contains other trace elements including U, Th, REE, Zr, Hf, W, Mo, Ti, V, Ge, P, and Sn. Experiments employing an oscillating rather than constant temperature have produced columbite crystals up to 30 μm and large areas of crystal-free water-saturated silicate melts. Columbite/silicate melt partition coefficients of non-radioactive trace elements are presently measured by LA-ICP-MS, and Pa will be added as a next stage. The result will give the oxidation state of Pa at oxygen fugacities of the mantle and crust, and have important implications for the partitioning of Pa between silicate minerals and melt and on our understanding of 231Pa-235U disequilibrium in natural magmatic systems.