Experimental Determination of Trace Element Partition Co-efficients in High-MgO arc Basalts at 1.15 GPa, 1235° C.

Island arc basalts are chemical hybrids of melts and fluids derived from subducted slab material and mantle wedge peridotite. In terms of their trace elements, island arc basalts are characterised by enrichment in LILEs relative to REEs and, in particular, relative to the HFSEs. Depletion of HFSEs has been attributed to a variety of subduction related processes most of which infer retention of HFSEs in a titanate phase (e.g. rutile) in the subducting slab during dehydration andor partial melting. However the relative contribution of such phases is unknown because the ability of the silicate mantle wedge peridotite itself to fractionate HFSEs is currently unquantified. Peridotite partial melting in the mantle wedge is a significantly different process to that responsible for melt generation beneath mid-ocean ridges in that melting takes place in the presence of a slab-derived H2O-rich fluid. Addition of water and other components to peridotite both lowers the melting temperature and alters the melting reactions. Trace element partitioning between mineral and melt is primarily a function of phase composition, pressure and temperature. Since pressure and temperature affect different valence cations to differing extents (Wood & Blundy, 1997; Blundy & Green, 2000) it is possible that the unusual low-T melting in the wedge is characterised by quite different HFSEREE fractionations than the relatively high-T melting responsible for MORB generation. Accurate determination of partition co-efficients (Ds) at pressures and temperatures relevant to melt production are thus imperative for characterising processes involved in generating primary arc magmas from the wedge, and in particular establishing the baseline effect of peridotite in generating HFSE fractionations. In this study we determine Ds for REEs, LILEs and HFSEs between cpx, opx and melt at 1235^oC, 1.15 GPa, and melt water contents of{ ~}1.5 wt%. These PT conditions correspond with the point of multiple phase saturation of high MgO calc-alkaline basalts from St Vincent, Lesser Antilles arc, determined by Pichavant et al. (in review). Hydrous glass was synthesised with 1.5wt% H2O, doped with trace elements at the ppm level, and seeded with spl, opx, cpx and ol of appropriate equilibrium compositions. These results represent the first trace element partitioning data directly applicable to hydrous batch melting within the hot core of the mantle wedge.