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Abstract

In the Earth’s subduction zones, some material from the down-going plate is transferred into the overlying mantle wedge and partly into subduction-related volcanic rocks. Lithium (Li) and boron (B) of subducted rocks, in particular of altered oceanic crust and siliciclastic sedimentary rocks, show a large and characteristic variation in their isotopic compositions. During progressive metamorphism and partial melting of the subducting material, Li and B are largely lost from the subducted rocks by fluid-mediated processes. The rising fluids eventually produce a geochemically and isotopically heterogeneous mantle wedge just above the subducting plate. The loss of Li and B from the down-going rocks is controlled by mineral stability in the subducting slab. Small-volume melts from this mantle wedge inherit the isotopic and chemical signature of the subducted material, i. e., of the altered oceanic crust and/or of siliciclastic sedimentary rocks. Thus, small-volume melts derived from the mantle wedge may reveal the nature of old sutures in orogenic belts.