Volatile and fluid-mobile element systematics of mantle xenoliths from selected Kamchatka arc volcanoes

Contrary to the studies of arc volcanic rocks, mantle xenoliths offer a direct means to study mantle processes above subduction zones. The Kamchatka arc comprises a unique group of volcanoes that erupt veined (metasomatized) mantle xenoliths. The wide spatial distribution of mantle xenolith-bearing volcanoes spanning from the volcanic front (Avachinsky and Shiveluch, this study) to the rear-arc (Bakening, this study) makes Kamchatka the perfect place to investigate metasomatic subarc processes. A combined study of major and trace element compositions with B contents and δ11B of metasomatic minerals in Avachinsky and Shiveluch mantle xenoliths is used to constrain the composition and source of fluids and melts responsible for melt-rock reactions occurring in the subarc and rear-arc mantle. A close inspection of Bakening mantle xenoliths revealed that they do not contain any hydrous metasomatic minerals suitable for B and δ11B analyses. Thus, the study of metasomatic reactions in the rear-arc mantle at Bakening is limited here to major and trace element mineral compositions. Multiple pulses of compositionally diverse fluids and melts derived from progressively deeper portions of the subducting slab percolate through the subarc mantle. The low B contents and negative δ11B of vein minerals in Avachinsky and Shiveluch xenoliths indicate that they are products of fluids and melts released from subducted and already dehydrated altered oceanic crust and, to a lesser extent, from serpentinite. Avachinsky and Shiveluch mantle xenoliths, however, were later overprinted by evolved melts in the upper crust prior to the eruption. Vein amphibole major and trace element compositions indicate their equilibration with evolved melts similar to amphibole- and plagioclase-hosted melt inclusions in Shiveluch volcanic rocks (Humphreys et al., 2008). Contrastingly, Bakening xenoliths lack any evidence of extensive fluid-fluxing of the rear-arc mantle, which was instead fluxed by pyroxenite melt mixed with a small amount of carbonatite component.