Slab contributions to the mantle source of backarc magmas: A neodymium and hafnium perspective from arcs in the northwest Pacific

Arc magmas result from the partial melting of an asthenospheric mantle source that has been metasomatized by the addition of one or more slab-derived fluxes (e.g., Elliot et al., 1997 and Hawksworth et al., 1993). However, the nature of the slab flux, its sources, and the degree to which the flux modifies the mantle source of arc volcanism remains a matter of considerable debate. New major and trace element concentration and Sr, Nd, Hf, and high-precision Pb isotope data for basalts erupted in the Mariana, Izu, and Kurile arcs are used to develop an internally consistent model of arc magma genesis.;Backarc basalts erupted from all three arcs have lower Hf isotope ratios and large negative Hf concentration anomalies, relative to basalts erupted at the volcanic front. Although the relative importance of sediment to altered oceanic crust varies slightly between arcs, at the volcanic front and in the backarc the slab flux is derived mostly from altered oceanic crust; sediment is not only a volumetrically minor component of the slab, but also of the slab-derived flux. The nature of the slab flux, however, differs between the volcanic front and backarc. Trace element concentrations and Pb-Nd-Hf isotope ratios are consistent with an aqueous fluid slab flux at the volcanic front and a hydrous partial melt of the slab in the backarc. Negative Hf concentration anomalies require slab melts to be in equilibrium with trace quantities of zircon (<0.001%). Hf isotopes and concentration anomalies place constraints on the temperature of the slab at depths beneath the backarc, and correlate with the speed and age of the subducting slab. Inferred depths of slab melting are shallowest for younger slabs that are subducting faster.