The composition of volcanic rocks erupted in complex plate tectonic settings can provide information on the nature of the underlying mantle. We show here that the geochemistry of alkali basalts from Japan and eastern Asia varies systematically with distance from the Japanese island-arc. Samples from northeastern Japan, relatively close to the Japan Trench, are enriched in K, Sr, Ba and Rb and depleted in Ta, Nb and Ti as compared with samples from southwestern Japan. Both sets show an island-arc influence on their composition, but alkali basalts from still further west (Korea and northeastern China) have chemistries which are indistinguishable from ocean island basalts. We suggest that the northeastern island-arc type of alkali basalts were derived from a 'normal' upper mantle source altered by fluids or melts released from the underlying subducted Pacific plate. The extent of this island-arc-related alteration decreases with distance from the trench.
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