ABSTRACT

⁸⁷Sr/⁸⁶Sr ratios of ocean-floor sediments from the NW Pacific result from mixing between island-arc detritus (0.7037) and open-ocean detritus derived from the upper continental crust (0.7175). Sediments lying within and immediately in front of the Kurile-Kamchatka trench change from a c. 500 m sequence rich in open-ocean detritus in front of the south-central Kuriles to a c. 1000 m sequence, the Meiji sediment tongue, rich in island-arc detritus in front of the northern section of the arc. Overall, the sediments of this northern section have lower Th/U but higher K/Cs and Rb/Cs ratios, and these features are mirrored by all basalts from the northern section of the arc, presumably through sediment contamination of the subarc mantle sources. In detail, however, a simple mechanical addition of any type or mixture of known sediments to the sub-arc source mantle fails to reproduce the overall geochemistry of the arc basalts and selective transfer of mobile elements (Cs, Rb, K, Ba, Pb, Sr) and a low-⁸⁷Sr/⁸⁶Sr component (altered MORB, sub-arc mantle) seem required. Although ⁸⁷Sr/⁸⁶Sr ratios of the bulk northern pre-arc sediments (0.7073) are lower than in the south-central section (0.7104), the flux of Sr into the subduction zone is thought to be greater in the north and could explain the higher ⁸⁷Sr/⁸⁶Sr ratios of basalts from the northern section of the arc.