Trace element partitioning between aqueous fluids, silicate melts and minerals

EPMA and proton microprobe techniques were used to measure concentrations of major and trace elements in solutes from aqueous fluids equilibrated experimentally (at 900-1100 degrees C, 2.0 GPa) with water-saturated melts of trondhjemitic and basanitic composition; the compositions of co-existing mineral phases (including staurolite, amphibole and clinopyroxene) were also determined. Solutes in equilibrium with both melts have granitic composition. Total solute concentrations in fluids equilibrated with trondhjemitic melts are 34-48 wt.% and are moderately enriched in Rb and K; Cl-bearing fluids are enriched in alkalis relative to non-alkaline trace elements, whereas F has little effect on element partitioning between fluids and melts. Fluid in equilibrium with basanitic melts is moderately enriched in K and Rb, but depleted in Sr, Ba, Y, Ti, Zr, Hf, Nb, Ta, La, Lu, Th and U. These results show that H (sub 2) O fluids in equilibrium with mantle peridotites will not be efficient metasomatic agents and that, at 2.0 GPa and 900 degrees C, H (sub 2) O fluids and hydrous melts in equilibrium with eclogitic mineral assemblages will have broadly similar compositions. It is suggested that H (sub 2) O fluids derived from subducted ocean crust will be a viable source of incompatible elements for the mantle beneath volcanic arcs, though the distinctive incompatible element concentrations found in volcanic arc magmas are more likely to be the result of crystal-chemical controls that due to the properties of H (sub 2) O fluids.