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Dehydration of serpentinized slab mantle: Seismic evidence from southwest Japan

机译：蛇形板状地幔的脱水：来自日本西南部的地震证据

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The seismicity in the subducting Philippine Sea slab (PHS) beneath southwest Japan shows a variety of modes of occurrence. We try to explain this variety on the basis of dehydration embrittlement in the subducting oceanic crust and/or mantle. The PHS subducting along the Nankai Trough shows commonly a single narrow seismic zone shallower than 60 km, which may reflect dehydration embrittlement in the hydrated subducting oceanic crust only, implying the lack of hydrated slab mantle. The PHS beneath Kanto, however, shows a double seismic zone (Hori, 1997) in the mantle part. Here the serpentinized 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, seismic events within the slab mantle have also been detected. This indicates that the PHS mantle beneath these areas is also hydrated, which may have resulted from subduction of the serpentine stable in the Izu-Bonin back-arc area. Aqueous fluids released from the serpentinized mantle beneath Kii Peninsula may have initiated partial melting in the mantle wedge, as indicated by the presence of high 3He/4He ratios in the natural gasses and the shallow seismic swarms in this region (Wakita et al ., 1987).
机译：日本西南部俯冲菲律宾海平板（PHS）的地震活动显示出多种发生方式。我们试图根据俯冲的洋壳和/或地幔中的脱水脆性来解释这种变化。沿南开海槽的PHS俯冲通常显示出一个浅于60 km的狭窄地震带，这可能仅反映了水合俯冲洋壳中的脱水脆性，这意味着缺少水合板幔。然而，关东下方的小灵通在地幔部分显示出双重地震带（Hori，1997）。伊苏-波宁前弧的蛇形化地幔楔正在俯冲，而双重带可以用其脱水来解释。在基伊半岛和九州以下，还发现了板状地幔内部的地震事件。这表明这些区域下方的PHS地幔也被水化了，这可能是由于伊豆-波宁后弧区域蛇形稳定地带的俯冲所致。从基伊半岛下方的蛇形地幔中释放出来的含水流体可能已开始在地幔楔中部分融化，这表明天然存在高 3 He / 4 He比值天然气和该地区的浅层地震群（Wakita等，1987）。

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