Dehydration of serpentinized slab mantle: Seismic evidence from southwest Japan

The seismicity in the subducting Philippine Sea slab (PHS) beneath southwes t Japan shows a variety of modes of occurrence. We try to explain this vari ety on the basis of dehydration embrittlement in the subducting oceanic cru st and/or mantle. The PHS subducting along the Nankai Trough shows commonly a single narrow seismic zone shallower than 60 km, which may reflect dehyd ration embrittlement in the h drated subducting oceanic crust only, implyin g the lack of hydrated slab mantle. The PHS beneath Kanto, however, shows a double seismic zone (Hori, 1997) in the mantle part. Here the serpentinize d mantle wedge of the Izu-Bonin fore-arc is subducting, and the double zone can be explained by its dehydration. Beneath Kii Peninsula and Kyushu, sei smic events within the slab mantle have also been detected. This indicates that the PHS mantle beneath these areas is also hydrated, which may have re sulted from subduction of the serpentine stable in the Izu-Bonin back-arc a rea. Aqueous fluids released from the serpentinized mantle beneath Kii Peni nsula may have initiated partial melting in the mantle wedge, as indicated by the presence of high He-3/He-4 ratios in the natural gasses and the shal low seismic swarms in this region (Wakita et al., 1987).