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

Abstract

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 arc. Trace element ratios and δ11B values of the EVF lavas show across-arc variations that are consistent with those observed in the adjacent Kurile arc: B/Nb, Pb/Nb, and δ11B are highest at the volcanic front (14 to 22, 2.0 to 2.4, and +4.9 to +5.6‰, respectively) and systematically decrease with increasing slab depth (to ∼3, ∼0.9, and −2‰, respectively) whereas Rb/Nb, Ba/Nb, K/Nb, Be/Nb, and Li/Zr do not vary significantly across the arc. The values for the SR (B/Nb = 1.7 to 2.0, Pb/Nb = 0.9 to 1.2, and δ11B = −3.7 to −1.1‰) located far behind the EVF are consistent with these across-arc trends. However, the CKD lavas exhibit elevated B/Nb (6 to 13) and δ11B values (−0.6 to +3.6‰) although the slab depths are greater than at the EVF. Despite variations in B/Nb and δ11B, 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 δ11B (+6.2 ± 0.5‰). 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 δ11B 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.