Diverse sources of volatile release across volcanic arcs: Insights from Kozelsky and Khangar volcanoes, Kamchatka

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The Kamchatka peninsula is one of the most volcanically active regions on Earth and is among the best places in the world to study the fate of subducted old (cold) oceanic crust. Uniquely the volcanoes of this arc erupt in several, rather than one, broad volcanic lineaments, possibly sampling magmatic fluid and melt sources in the forearc, beneath the main volcanic front and also from the behind-the-arc region. Recent geophysical and geochemical results show that the Kamchatka volcanoes indeed sample fluids derived from slab depths from as little as 50km to as deep as 350 km. While the majority of arc volcanoes (here and elsewhere in other arcs) result from flux melting of depleted subarc mantle wedge source, the geochemistry of the arc volcanic rocks (incl. 10Be, U- series, B & Li isotopes, B/Be, B/Nb and Sr/Y ratios, among others) also shows, sometimes significant elemental and isotopic inputs from subducted slabs. These may include subducted sediments, altered basaltic crust and/or fluids/melts resulting from the dehydration of serpentinized sections of the slabs or forearc-serpentinized (subarc) mantle wedge. Although the depleted mantle sources dominate the major and minor element budgets of the erupted products, the components released from the slab are geochemically traceable and have been shown to deliver the most important H2O, CO2, halogen and fluid mobile element (B, Cs, Li, As, Sb) contributions to arc magmas (Portnyagin et al. , 2007). However, the individual slab lithologies involved and their relative influence with depth, especially the one originating from dehydration reactions in the deeply subducted serpentinites, has not been quantified yet. This project aims to address this issue by examining the geochemistry and petrology of volcanic products erupted along a SE-NW oriented depth transect across the Eastern Volcanic Arc Front and deep into the rear arc (Sredinny Ridge) region. In previous fieldwork campaigns we have extensively sampled Avachinsky and Bakening volcanoes that are ideally situated in the middle of a cross-arc transect that represents depth to the top of the slab between 120 and 200 km, respectively. Here we will aim to conduct fieldwork in Kamchatka (2020 and/or 2021) and sample and study the volatile and fluid mobile element systematics in one additional shallow-sourced (<100 km, Kozelsky) and one very deeply sourced (>300 km Khangar) volcano from the same (arc front orthogonal) transect. Olivine- and pyroxene- bearing mafic scoria will be used to extract melt inclusions (MI). The MI will be complementary to the already available (in Durham and Leeds) Kamchatka arc MI collection. The new MI will be adding, for the first time, an exceptional high resolution look at the petrochemical variations across active volcanic arc. Moreover, we will uniquely have a chance to examine compositional changes of slab derived fluxes with increasing depth-to-slab. The proposed transect will be complementary to a cross arc transect to the north (Churikova et al., 2007), although the slab depths/geometry and the overlaying terrain boundaries there are less clear when compared with the newly proposed transect, where the Benioff zone is clearly delineated by deep regional seismicity and tomography. The project will involve training and use of various in-situ measurement techniques such as SEM, EPMA, LA-ICP-MS and SIMS (for the MI). We will provide training (mostly in-house) in textural/petrological observations, sample selection+ preparation and geochemical modelling based on bulk rock and MI datasets.References used:Churikova et al. (2007, Contrib Mineral Petrol 154, 217-239.Portnyagin et al. (2007). Earth Planet. Sci. Lett. 255, 53-69.