The behaviour of incompatible elements during hydrous melting of metasomatized peridotite at 4-6GPa and 1000°C-1200°C

Trace element behavior during hydrous melting of a metasomatized garnet-peridotite was examined at pressures of 4-6GPa and temperatures of 1000°C-1200°C, conditions appropriate for fluid penetrating the mantle wedge atop the subducting slab. Experiments were performed in a rocking multi-anvil apparatus using a diamond-trap setup. The compositions of the fluid and melt phases were measured using the cryogenic LA-ICP-MS technique. The water-saturated solidus of the K-lherzolite composition is located between 900°C and 1000°C at 4GPa and between 1000°C and 1100°C at 5 and 6GPa. The partition coefficients between fluid or melt and clinopyroxene reveal an asymmetric MREE trough with a minimum at Dy. The clinopyroxene in equilibrium with aqueous fluids is characterized by DUfluid-cpx>DThfluid-cpx while DUmelt-cpx tends to be similar to DThmelt-cpx. The partition coefficients between fluid or melt and garnet reveal very strong light to heavy REE fractionation, DLa/DLu from 95 (hydrous melt) to 1600 (aqueous fluid). The LILE are highly incompatible with partition coefficients >50. The behavior of HFSE are decoupled, with DZr,Hf close to 1 while DNb,Ta>10. Garnet is characterized by DUmelt/fluid-garnet