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 D_U^fluid-cpx>D_Th^fluid-cpx while D_U^melt-cpx tends to be similar to D_Th^melt-cpx. The partition coefficients between fluid or melt and garnet reveal very strong light to heavy REE fractionation, D_La/D_Lu from 95 (hydrous melt) to 1600 (aqueous fluid). The LILE are highly incompatible with partition coefficients >50. The behavior of HFSE are decoupled, with D_Zr,Hf close to 1 while D_Nb,Ta>10. Garnet is characterized by D_U^{melt/fluid-garnet}<D_Th^{melt/fluid-garnet}.A comparison of our experimental partitioning results for trivalent cations as well as the results from the literature and the calculations carried out using the lattice strain model adapted to the presence of water in the bulk system indicates that H₂O in the fluid or melt phase has a prominent effect on trace element partitioning. Garnet in mantle rocks in equilibrium with an aqueous fluid is characterized by significantly higher D_o(3+) for REE in the X site of the garnet compared with the partitioning values of the optimal cation in garnet in equilibrium with hydrous melts. Our data show for the first time that the change in the nature of the mobile phase (fluid vs. melt) does affect the affinities of trace elements into the garnet crystal at conditions below the second critical endpoint of the system. The same also applies for clinopyroxene, although this is less clear. Consequently, our new data allow for refinements in predictive modeling of element transfer from the slab to the mantle wedge and of possible compositions of metasomatized mantle that sources OIB magmatism.