Trace element partitioning between aqueous fluids, silicate melts and minerals

A series of experiments was conducted to examine the capacity of H2O fluids to concentrate and transport incompatible elements (including Rb, Sr, Ba, Y, Zr, Hf, Nb, Ta, La, Lu, Th and U) in the peridotitic mantle and subducted ocean crust. An electron microprobe and proton microprobe were used to measure concentrations of major and trace elements in solutes from aqueous fluids equilibrated (at 900-1100°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 trondhjemitic and basanitic melts have granitic compositions. Total solute concentrations in fluids equilibrated with trondhjemitic melts are between 34 and 48 wt.%. The fluid equilibrated with basanitic melt contains about 30 wt.% of solutes. Fluids in equilibrium with trondhjemitic melts are moderately enriched in Rb and K. Other trace elements partition either neutrally or are moderately depleted in the fluid. 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 melt is moderately enriched in Rb and K, but depleted in Sr, Ba, Y, Ti, Zr, Hf, Nb, Ta, La, Lu, Th and U. Except for alkalis, minor and trace element concentrations are greater in amphibole and clinopyroxene than in co-existing fluid. The results from this study show that H2O fluids in equilibrium with mantle peridotites will not be efficient metasomatic agents. They also indicate that, at 2.0 GPa and 900°C, H2O fluids and hydrous melts in equilibrium with eclogitic mineral assemblages will have broadly similar compositions. The data, when combined with recent partitioning results between eclogitic minerals and trondhjemitic melts (Jenner et al., 1993; Rapp & Shimizu, 1995), suggest that H2O fluids derived from subducted ocean crust will be a viable source of incompatible elements for the mantle beneath volcanic arcs. Moreover, the distinctive incompatible element concentrations found in volcanic arc magmas (including Nb and Ta depletions) are more likely to be the result of crystal-chemical controls than due to the properties of H2O fluids.