Trace Element Partitioning and Accessory Phase Saturation during H2O-Saturated Melting of Basalt with Implications for Subduction Zone Chemical Fluxes

Experimental phase equilibrium and trace element partitioning data are reported for H2O-saturated mid-ocean ridge basalt at 25 GPa, 750^9008C and oxygen fugacities at the nickel^nickel oxide buffer. Garnet, omphacite and rutile are present at all temperatures. Amphibole and epidote disappear as residual phases above 8008C; allanite appears above 7508C.The Na^Al-rich silicate glass present in all run products is likely to have quenched from a supercritical liquid. Trace element analyses of glasses demonstrate the important control exerted by residual minerals on liquid chemistry. In addition to garnet, which controls heavy rare earth elements (HREE) and Sc, and rutile, which controlsTi, Nb and Ta, allanite buffers the light REE (LREE; La^Sm) contents of liquids to relatively low levels and preferentially holds back Th relative to U. In agreement with previous experimental and metamorphic studies we propose that resi-dual allanite plays a key role in selectively retaining trace elements in