Boron isotope systematics of slab fluids as inferred from a serpentine seamount, Mariana forearc

Serpentinite: clasts and muds erupted from Conical Seamount, Mariana forear c, show substantial enrichment in boron (B) and B-11 (delta B-11 up to +15 parts per thousand) relative to mantle values. These elevated B isotope sig natures result from chemical exchange with B-rich pore fluids that are upwe lling through the seamount. If the trends of decreasing delta B-11 with sla b depth shown by cross-are magmatic suites in the Izu and Kurile arcs of th e western Pacific are extended to shallow depths (similar to 25 km), they i ntersect the inferred delta B-11 of the slab-derived fluids (+13 parts per thousand) at Conical Seamount. Simple mixtures of a B-rich fluid with a hig h delta B-11 and B-poor mantle with a low delta B-11 are insufficient to ex plain the combined forearc and are data sets. The B isotope systematics of subduction-related rocks thus indicate that the fluids evolved from downgoi ng slabs are more enriched in B-11 than the slab materials from which they originate. Progressively lower delta B-11 in are lavas erupted above deep s labs reflects both the progressive depletion of B-11 from the slab and prog ressively greater inputs of mantle-derived B. This suggests that the slab r eleases B-11-enriched fluids from the shallowest levels to depths greater t han 200 km. (C) 2001 Elsevier Science B.V. All rights reserved.