沈み込み帯マグマの成因

抄録

This review discusses the genesis of magmas in subduction zones on the basis of experimental and geochemical data mainly obtained in the 1980s. Across-arc compositional variations in arc basalt magmas are caused by the following factors; (1) depth of magma segregation, (2) degree of partial melting, (3) density of primary magma, and (4) refreshment of polluted mantle materials by depleted materials similar to the MORB source during the process of downgoing migration. The subducted lithosphere supplies elements with larger ionic radii with H₂O to form hydrated or polluted amphibole peridotite in the mantle wedge beneath the fore-arc region. The amphibole peridotite is transported downwards on the slab by the induced convection in the mantle wedge. Magmatism on the volcanic front is controlled by the dehydration of amphibole in the dragged mantle materials. High-magnesian andesite magmas, SiO₂-rich primary magmas in subduction zones, are produced by the partial melting of hydrated amphibole peridotite at shallower levels under higher geothermal conditions.