University of California, Santa Cruz, United States

We present an investigation of B-Be- (super 10) Be-epsilon Nd systematics of the incoming crust and across the wide, active Kurile arc in the NW Pacific ocean basin to address the role of recent sediment incorporation in arc magmas as a function of depth to the Wadati-Benioff Zone. The (super 10) Be flux ratio (8-14%) through the arc constrains the fate of sediments subducted at the trench and requires that a minimum ?10 m of the sediment column must reach the depths of magma generation. Enrichments in subducted sediment-derived (super 10) Be in rear-arc lavas are comparable with those in the volcanic front, despite longer subduction transit times, and require a mechanism for the prolonged release of (super 10) Be from the subducting slab. Cross-arc (super 10) Be enrichments, together with drastic reductions in B/Be, imply a protracted stability of their primary mineralogical host in subducted sediment, white mica (phengite). The persistence of phengite constrains slab surface temperatures to be < 950 degrees C to 165 km depth and limits the extent of partial melting of the slab to < 20%. The simplest interpretation of combined incoming sediment and cross-arc B/Be- (super 10) Be/9Be-epsilon Nd systematics is that the agent of element transfer changes from aqueous fluid-dominated beneath the volcanic front to melt-like beneath the deeper regions of the arc.