At subduction zones, crustal material is recycled back into the mantle. A certain proportion, however, is returned to the overriding plate via magmatism. The magmas show a characteristic range of compositions that have been explained by three-component mixing in their source regions: hydrous fluids derived from subducted altered oceanic crust and components derived from the thin sedimentary veneer are added to the depleted peridotite in the mantle beneath the volcanoes. However, currently no uniformly accepted model exists for the physical mechanism that mixes the three components and transports them from the slab to the magma source.Here we present an integrated physico-chemical model of subduction zones that emerges from a review of the combined findings of petrology, modelling, geophysics, and geochemistry: Intensely mixed metamorphic rock formations, so-called mélanges, form along the slab-mantle interface and comprise the characteristic trace-element patterns of subduction-zone magmatic rocks. We consider mélange formation the physical mixing process that is responsible for the geochemical three-component pattern of the magmas. Blobs of low-density mélange material, so-called diapirs, rise buoyantly from the surface of the subducting slab and provide a means of transport for well-mixed materials into the mantle beneath the volcanoes, where they produce melt. Our model provides a consistent framework for the interpretation of geophysical, petrological and geochemical data of subduction zones.Geochemical studies of volcanic rocks erupted at convergent plate margins have identified element and isotopic ratios that seem to require material present in their source that is absent in volcanic rocks from other settings, such as mid-ocean ridge basalts [1]. This subduction zone geochemical fingerprint has led to the establishment of three-component mixing models to explain the composition of magmatic rocks produced at continental and island arcs [2, 3]. Their trace-element characteristics are believed to have originated from two distinct sources added to the depleted mantle source: hydrous fluids derived from subducted altered ocean crust and a component derived from the thin sedimentary veneer of the slab. However, the details of the physical process that allows the mixing of these components within the * Corresponding author. Tel: +1-508-289-2776. Fax: +1-508-457- E-mail: hmarschall@whoi.eduEmail address: hmarschall@whoi.edu (Horst R. Marschall)."subduction factory" are unknown. Current models propose that a hydrous component, enriched in incompatible trace elements, is released from the subducting slab and migrates into the overlying mantle wedge. The wedge is at much higher temperatures than the slab due to the 'corner flow' of the asthenospheric mantle, driven by viscous coupling of the mantle to the subducting slab [4]. Mantle-wedge peridotite partial melting is thought to be promoted by the influx of slab-derived fluids [5]. Yet, no uniformly accepted model exists for the actual physical mechanism tha...

13We present whole rock Li and Mg isotope analyses of 33 ultramafic xenoliths from 14 the terrestrial mantle, which we compare with analyses of 30 (mostly chondritic) 15 meteorites. The accuracy of our new Mg isotope ratio measurement protocol is 16 substantiated by a combination of standard addition experiments, the absence of mass 17 independent effects in terrestrial samples and our obtaining identical values for rock 18 standards using 2 different separation chemistries and 3 different mass-spectrometric 19 introduction systems. Carbonaceous, ordinary and enstatite chondrites have 20

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