Mineral/melt partitioning of trace elements during hydrous peridotite partial melting

This experimental study examines the mineral/melt partitioning of incompatible trace elements among high-Ca clinopyroxene, garnet, and hydrous silicate melt at upper mantle pressure and temperature conditions. Experiments were performed at pressures of 1.2 and 1.6 GPa and temperatures of 1,185 to 1,370 degreesC. Experimentally produced silicate melts contain up to 6.3 wt% dissolved H2O, and are saturated with an upper mantle peridotite mineral assemblage of olivine + orthopyroxene + clinopyroxene + spinel or garnet. Clinopyroxene/ melt and garnet/melt partition coefficients were measured for Li, B, K, Sr, Y, Zr, Nb, and select rare earth elements by secondary ion mass spectrometry. A comparison of our experimental results for trivalent cations (REEs and Y) with the results from calculations carried out using the Wood-Blundy partitioning model indicates that H2O dissolved in the silicate melt has a discernible effect on trace element partitioning. Experiments carried out at 1.2 GPa, 1,315 degreesC and 1.6 GPa, 1,370 degreesC produced clinopyroxene containing 15.0 and 13.9 wt% CaO, respectively, coexisting with silicate melts containing similar to1-2 wt% H2O. Partition coefficients measured in these experiments are consistent with the Wood-Blundy model. However, partition coefficients determined in an experiment carried out at 1.2 GPa and 1,185 degreesC, which produced clinopyroxene containing 19.3 wt% CaO coexisting with a high-H2O (6.26 +/- 0.10 wt%) silicate melt, are significantly smaller than predicted by the Wood-Blundy model. Accounting for the depolymerized structure of the H2O-rich melt eliminates the mismatch between experimental result and model prediction. Therefore, the increased Ca2+ content of clinopyroxene at low-temperature, hydrous conditions does not enhance compatibility to the extent indicated by results from anhydrous experiments, and models used to predict mineral/melt partition coefficients during hydrous peridotite partial melting in the sub-arc mantle must take into account the effects of H2O on the structure of silicate melts.