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

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 (Churikova et al., 2001, 2007; Portnyagin et al., 2007; Portnyagin & Manea,, 2008 and ref. therein) 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 (Churikova et al., 2001, 2007; Portnyagin et al., 2007). 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 (Ishikawa et al., 2001; Churikova et al., 2007) 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. However, the individual slab lithologies involved and their relative influence with depth, especially that 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 cross-arc transect and represent depths to the top of the slab of 120 to 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 (300 km Khangar) volcanoes from the same (arc front orthogonal) transect.

The olivine- and pyroxene- bearing mafic samples (preferably scoria) we collect will be used to extract melt inclusions (MI) from the mafic minerals (ol, px, hbl). The MI will be complementary to our own large 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. 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), petrological and geochemical modelling.

Kozelsky volcano is Pleistocene low-K andesitic volcano, situated in the shallowest segments of the Kamchatka arc. Although it is one of the large volcanoes proximal to the town of Petropavlovsk (population ~170 000)- it is not extensively studied and sampled. We hope to be able to sample mafic rocks from several cinder cones on the flanks of Kozelsky volcano.

Khangar caldera hosts the southernmost volcanic centres of Sredinny Ridge and represents the deepest sourced rear- arc magmatism in Kamchatka. Various domes and ignimbrites here sample Holocene eruptions as young as 500 yrs. ago. About 7000 yrs. ago it exploded in Plinian (VEI= 6) eruption that with hornblende and biotite, but there are some mafic (picritic) lavas south of the caldera and those will be our primary targets.