2010 Volume 119 Issue 6 Pages 1054-1062
Primary arc magmas are thought to originate through interactions among subducting slab, slab-derived fluid (slab-fluid) or slab-derived melt, and mantle wedge. In most subduction zones, slab-fluid is thought to play a major role in lowering the solidus temperature of the mantle wedge, inducing partial melting to produce arc magmas. The slab-fluid is composed of an aqueous fluid derived from materials associated with the subducting slab, such as sediment and altered oceanic crust. The chemical composition of slab-fluid depends on the mobility of elements during dehydration, and affects the chemical and isotopic compositions of the fluid-added mantle wedge and arc volcanic rocks. Accordingly, geochemical analyses of arc volcanic rocks provide valuable information on slab-fluid, fluid-added mantle, and melting conditions. Examples from the Japan arcs support the validity of this approach in deciphering the composition and amount of slab-fluid, including both across-arc and along-arc variations: the amount of slab-fluid is largest in the Central Japan arc due to overlapping subduction of the two plates, while it is significantly less in the other arcs. In the Northeast Japan arc and the Izu-Bonin arcs the amount of slab-fluid decreases from the front to the rear arc. Accurate identification of slab-fluid also resolves a large-scale along-arc variation in the isotopic composition of mantle wedge, involving the Indian-type and Pacific-type MORB mantles, which contributes to an understanding of the large-scale mantles flow.