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

Deciphering variable mantle sources and hydrous inputs to arc magmas in Kamchatka

The chemistry of primitive arc rocks provides a window into compositional variability in the mantle wedge, as well as slab-derived inputs to subduction-related magmatism. However, in the long-term cycling of elements between Earth's internal and external reservoirs, a key unknown is the importance of retaining mobile elements within the subduction system, through subduction-related metasomatism of the mantle. To address these questions, we have analysed olivine-hosted melt inclusions and corresponding bulk rocks from the Kamchatka arc. Suites of melt inclusions record evidence for entrapment along melt mixing arrays during assembly of diverse parental magma compositions. Systematic variations in parental magma B/Zr, Nb/Zr, Ce/B, and B are also apparent among the different eruptive centres studied. These element ratios constrain the nature of subduction-related metasomatism and provide evidence for ambient mantle heterogeneity and variable degrees of mantle melting. High Nb/Zr and low B/Zr in back-arc rocks indicate smaller degree melts, lower slab-derived inputs, but relatively enriched mantle compositions. Similarly, small monogenetic eruptive centres located away from the main stratocones also tend to erupt magmas with relatively lower slab contribution and overall smaller melting degrees. Conversely, arc-front compositions reflect greater slab contributions and larger degree melts of a more depleted ambient mantle. Across-arc variations in B (ranging from ca. ‰ in the rear-arc and Sredinny Ridge to ‰ in the Central Kamchatka Depression) are generally consistent with variable addition of an isotopically heavy slab-derived component to a depleted MORB mantle composition. However, individual volcanic centres (e.g. Bakening volcano) show correlations between melt inclusion B and other geochemical indicators (e.g. Cl/K2O, Ce/B) that require mixing between isotopically distinct melt batches that have undergone different extents of crustal evolution and degassing processes. Our results show that while melt inclusion volatile inventories are largely overprinted during shallower melt storage and aggregation, incompatible trace element ratios and B isotope compositions more faithfully trace initial mantle compositions and subduction inputs. Furthermore, we suggest that the signals of compositional heterogeneity generated in the sub-arc mantle by protracted metasomatism during earlier phases of subduction can be preserved during later magma assembly and storage in the crust