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Vol. 126 (2017) No. 2 特集号:沈み込む海洋プレート科学の最前線-アウターライズ海洋掘削に向けて- p. 195-206




 Geoscientists, who previously had limited direct knowledge of the petrological/geochemical mantle below oceanic regions, were largely restricted to areas near mid-ocean ridges, back-arc spreading centers, and hotspots. Petit-spot lavas and xenoliths provide direct information on the asthenosphere and the lithosphere of subducting plates because the magma that erupts from petit-spot volcanoes originates from the asthenosphere and ascends along the concavely flexed zone prior to the outer-rise along the trench. Such volcanoes have been reported at subduction zones worldwide (e.g., the Japan, Chile, Java, and Tonga trenches). The isotopic composition of petit-spot lavas indicates a heterogeneous asthenosphere, and geobarometric analyses of xenoliths show a higher geothermal gradient in the lithosphere than that predicted previously by the GDH1 model, meaning that conventional theory about the subducting lithosphere needs to be revised in the light of recently obtained petit-spot data. Melt fractionation is thought to occur in the middle lithosphere, given that bulk compositions show fractionation trends in the absence of phenocrysts, in spite of raising lherzolitic xenoliths from ∼45 km depth. The most important indicators of petit-spot input to the lithosphere are high levels of carbon dioxide (CO2) in petit-spot magma, which might explain the low seismic velocity and high electrical conductivity of the oceanic asthenosphere just below the subducting oceanic plate. Because carbon-rich melt ascends through the lithosphere to the seafloor as a petit-spot, it is likely to metasomatize the lithosphere just prior to its subduction.

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