Serpentinite clasts and muds erupted from Conical Seamount, Mariana forearc, show substantial enrichment in boron (B) and 11B (δ11B up to +15‰) relative to mantle values. These elevated B isotope signatures result from chemical exchange with B-rich pore fluids that are upwelling through the seamount. If the trends of decreasing δ11B with slab depth shown by cross-arc magmatic suites in the Izu and Kurile arcs of the western Pacific are extended to shallow depths (∼25 km), they intersect the inferred δ11B of the slab-derived fluids (+13‰) at Conical Seamount. Simple mixtures of a B-rich fluid with a high δ11B and B-poor mantle with a low δ11B are insufficient to explain the combined forearc and arc data sets. The B isotope systematics of subduction-related rocks thus indicate that the fluids evolved from downgoing slabs are more enriched in 11B than the slab materials from which they originate. Progressively lower δ11B in arc lavas erupted above deep slabs reflects both the progressive depletion of 11B from the slab and progressively greater inputs of mantle-derived B. This suggests that the slab releases 11B-enriched fluids from the shallowest levels to depths greater than 200 km.