Source, composition and distribution of the fluid in the Kurile mantle wedge: Constraints from across-arc variations of B/Nb and B isotopes

B and Nb concentrations and B isotope composition were determined for lavas covering the entire length of the Kurile are. B/Nb and beta(11)B of these lavas show clear across-are Variations in which both are highest at the trench side and continuously decrease as the slab depth increases. B/Nb in the volcanic front lavas (8-25) is significantly higher than those of MORB and OIB, indicating that the mantle wedge is extensively metasomatized by B-rich fluid derived from the subducting slab. The frontal delta(11)B values are also out of the range of MORB and OIB, and systematic difference exists between Northern Kurile (NK: + 5.4 +/- 0.5 parts per thousand) and Central and Southern Kurile (CSK: + 4.3 +/- 0.3 parts per thousand). However, these two parameters approach or coincide with mantle values at the most back-are side. The delta(11)B and Sr-87/Sr-86 linearly correlate with Nb/B, suggesting control by simple mixing of two isotopically homogeneous components; that is, slab-derived fluid and mantle wedge. The estimated delta(11)B and Sr-87/Sr-86 of the fluid are +7.0 +/- 0.6 parts per thousand and 0.70326 +/- 0.00005 for NK, and + 6.3 +/- 0.3 parts per thousand and > 0.7033 for CSK, respectively. These fluid compositions are consistent with a source constituted by > 95% altered oceanic crust and < 5% oceanic sediment, with the slight difference between NK and CSK resulting from the different lithologies of subducting sediments. The mantle wedge beneath the Kurile are has B, Sr and Nd isotope compositions identical with MORB but shows significantly higher B/Nb. The above trends bear a close resemblance to those observed in the Izu are, strongly suggesting their universality in the are system. The across-are decline of B/Nb observed both in Kurile and Izu implies that the fluid flux from the slab continuously decreases as the slab depth increases. However, this relates only to a final stage of slab dehydration since most of the volatile components are lost at the fore-are region. In contrast to B, K is uniformly enriched across the are. This requires control by a K-bearing mineral phase either in the slab (phengite) or the mantle wedge (phlogopite), although such a mineral is responsible for only a small part of the whole fluid transport. (C) 1997 Elsevier Science B.V.