Boron isotope and trace element systematics of the three volcanic zones in the Kamchatka arc

Trace element concentrations and B isotope compositions were determined for lavas from the Eastern Volcanic Front (EVF), the Central Kamchatka Depression (CKD), and the Sredinny Range (SR) in the Kamchatka are. Trace element ratios and delta B-11 values of the EVF lavas show across-arc variations that are consistent with those observed in the adjacent Kurile are: B/Nb, Pb/Nb, and delta B-11 are highest at the volcanic front (14 to 22, 2.0 to 2.4, and +4.9 to +5.6 parts per thousand, respectively) and systematically decrease with increasing slab depth (to similar to3, similar to0.9, and similar to2 parts per thousand, respectively) whereas Rb/Nb, Ba/Nb, K/Nb, Be/Nb, and Li/Zr do not vary significantly across the are. The values for the SR (B/Nb = 1.7 to 2.0, Pb/Nb = 0.9 to 1.2, and delta B-11 = -3.7 to -1.1 parts per thousand) located far behind the EVF are consistent with these across-are trends. However, the CKD lavas exhibit elevated B/Nb (6 to 13) and delta B-11 values (-0.6 to +3.6 parts per thousand) although the slab depths are greater than at the EVF, Despite variations in B/Nb and delta B-11, the values for all these three volcanic zones of Kamchatka can be explained in terms of simple mixing between a depleted mantle wedge and slab-derived aqueous fluids with a relatively homogeneous delta B-11 (+6.2 +/- 0.5 parts per thousand). This mixing trend resembles those observed in Kurile, Izu, and Mariana, suggesting that fluids derived from altered oceanic crust and/or serpentinized peridotite in the subducting slab predominate in the slab-mantle interaction beneath these western Pacific arcs. Given this mixing relationship, the high B/Nb and delta B-11 values observed in the CKD indicate particularly large fluid influx beneath this region, which may partly result from subduction of the altered crust of the Emperor seamount chain in addition to the altered Mid-Ocean Ridge Basalt (MORB) crust. The compositions of the SR lavas in this study are consistent with melting of a weakly fluid-metasomatized mantle. However, the origin of magma in this volcanic zone is complicated, judging from the variable chemical characteristics reported for the SR lavas, Copyright (C) 2001 Elsevier Science Ltd.