ИСТИНА

The voluminous magmatic outputs from the old, cold, and altered Pacific slab under the Kamchatka Peninsula creates an ideal environment to utilise B and B isotopes to faithfully trace volatile release across subduction zones. Here we report new petrological and geochemical results (XRF, EMPA, SEM, ICP-MS, and TIMS) from the forearc-situated Kozelsky and Avachinsky volcanoes (Avacha Group) with respect to the back arc volcano Khangar. We will present a first geochemical and petrological typification of Kozelsky. The sampled olivine-phyric basalts from the Avacha Group record high MgO (5-15 wt%), Ni (15-250 ppm) and Cr (60-800 ppm) abundances. The whole rock 87/86Sr and 143/144Nd ratios of these volcanoes show narrow ranges (~0.70335 and ~0.51305, respectively), revealing dominantly mantle wedge (non-crustal) magma sources. Such isotopic signatures are nearly identical to those of high-MgO basaltic lava and tephra from the Tolbachik monogenetic field in the Central Kamchatka Depression (the second volcanic belt with slab depths of ~200km) (Iveson et al., 2021, 2022; Gorbtov et al., 1997]. Kozelsky magmas sample very shallow depths to the slab of less than 90km ( Syracuse and Abers, 2006). New whole rock B and B isotope data reveals serpentinite-forearc mélange influenced outputs [e.g. high B and heavy δ11B (up to >+2 to +5‰ )]; this deviates from the expected deep sourced slab melt contribution (+2‰, Li et al., 2023) which whilst has an influence, alone cannot source these magmas. This is also in contrast to AOC (altered oceanic crust) melt-dominating mantle metasomatism in the arc belts with deeper subducting slabs (i.e. Tolbachik: Iveson et al., 2021, 2022; Gorbtov et al., 1997;EB : Iveson et al., 2021). Therefore, such excess, non-sediment sourced B and enriched δ11B should be derived from dehydrated and with heavy δ11B AOC ( McCaig et al., 2018) or forearc, serpentinite, melange rocks associated with the subducting slabs, supporting the mélange model of slab material transport (Nielsen and Marschall, 2017). Moreover, Khangar, which samples a depth to the slab of ~350km, has the expected dominantly OIB signature with a much lighter δ11B signature (-5 to -10‰, EB, Iveson et al, 2022) and lower B concentrations from a volatile starved slab. We are now focussing on the analysis of δ11B and hydrous contributions in melt inclusions, to further trace the non-crustal, boron-depleted, and isotopically light arc mantle background across the Kamchatka arc. Our results will evaluate if systematic variations in B-Sr-Nd isotopes and trace element ratios can be linked to across-arc change in depth to a slab of the magma source and/or reveal details about the prevailing geodynamic regime.