Behaviour of subducted water and its role in magma genesis in the NE Japan arc: A combined geophysical and geochemical approach

Water at subduction zones is carried to mantle depths by the subducting oceanic plate and then released by dehydration. It then migrates upwards and contributes to melting of the mantle wedge to form primary arc magma. The magma thus captures and transfers water to the crust, or outgasses water to the atmosphere. Water, either in fluids or melts in both the slab and the mantle, promotes the dissolution and mobilization of elements and affects the physical properties of the sub-arc slab, mantle, and seismicity. In this paper, we present a coherent model to explain the geophysical and geochemical role of water beneath NE Japan. We first investigate the seismic structures of the downgoing slab and sub-arc mantle and examine the role of subducted water in forming these structures. We then use the Arc Basalt Simulator version 4, a petrological-geochemical model developed to describe the geochemical behaviours of water and elements in the slab, mantle, and arc basalt. Parameters governing these petrogenetic processes are also estimated by the model and compared to geophysical observations. The combined approach shows that (1) subducted sediment and igneous oceanic crust are almost fully hydrated, whereas only partial hydration occurs in the oceanic mantle; (2) this high slab water content leads to melting of the slab sediment and the uppermost basalt layer beneath the arc; (3) the released water via slab liquid promotes 3-25% melting of the mantle wedge at a depth of 50-30. km at a mantle temperature of 1250-1350. °C; (4) virtually 89% of slab water is released, 22% of the water returns to the forearc, and 38% enters the arc crust with the magma; and (5) 11% of the subducted water retained beyond a depth of 180. km is held in the slab, and 29% in nominally anhydrous minerals in the wedge mantle.