Detailed tephrochronology and composition of major Holocene eruptions from Avachinsky, Kozelsky, and Koryaksky volcanoes in Kamchatka

摘要

Avachinsky, Kozelsky, and Koryaksky volcanoes form one of the most volcanically active clusters in the Kamchatka volcanic arc and are located in close proximity of the cities of Petropavlovsk-Kamchatsky and Elizovo - the most populated area in Kamchatka. In this paper, we report a compilation of new and revised previously published data on the eruptive history of these volcanoes during the past 13.5 kyrs. We identify 217 explosive eruptions of these volcanoes, determine their ages using 207 radiocarbon dates and Bayesian statistical modeling, and characterize their tephra geochemically using major and trace element compositions of bulk samples (40 samples) and volcanic glass (75 samples).Avachinsky has been the most active during the Holocene time and had 150 explosive eruptions; Koryaksky produced-60 eruptions; and Kozelsky had only two eruptions in the early Holocene. Our new data confirm the onset of the Avachinsky postglacial activity at 11.3 cal ka BP and previously distinguished two major stages of Avachinsky Holocene eruptive history: stage I (8-3.8 cal ka BP) and stage II (3.8 cal ka BP - present). During stage I, eruptions were relatively rare, but they included at least six large pumice eruptions with tephra volumes exceeding 0.5 km(3). Stage I tephras had low-K andesitic bulk compositions and low-K rhyolitic matrix glasses. The andesites likely sampled volatile-rich crystal mush from a long-lived magma chamber under Avachinsky volcano. The stage II started at similar to 3.8 cal ka BP with a powerful eruption and was related to the construction of the Young Cone inside the Avachinsky somma. The subsequent late Holocene eruptions were frequent, but most of them did not exceed the volume of 0.3 km(3). The stage II tephras are mostly cindery basaltic andesites containing well-crystallized groundmasses of andesitic composition. These tephras originate from smaller, perhaps more shallow magmatic reservoirs, and their matrix glasses are likely products of in-situ crystallization of relatively mafic magmas on their ascent to the surface. Koryaksky volcano was mostly active in the early Holocene when Avachinsky was quiet. Koryaksky tephras had a relatively constant bulk medium-K andesitic composition during the Holocene.Thanks to characteristic compositions, high frequency, and well-constrained ages, tephras of Avachinsky and Koryaksky volcanoes can be used for high resolution dating of local sediments. Some eruptions of Avachinsky volcano reached volcanic explosivity index (VEI) 5 and produced widely dispersed tephras. These eruptions could have had global environmental effects, and their tephras can be used for the correlation of disparate sedimentary archives.Some Avachinsky and Koryaksky eruptions were closely spaced in time. However, their tephras are easily distinguished by respective low-K and medium-K compositions and by different trace element patterns, which imply compositionally different sources in the mantle wedge. We interpret these differences to reflect the increasing slab surface temperature and transition of slab component from a relatively low-temperature fluid-like phase under Avachinsky to more high-temperature and solute-rich supercritical fluid or melt under Koryaksky. The transition appears to be very sharp in Kamchatka, causing a large compositional shift in magmas just behind the volcanic front. (C) 2020 Elsevier B.V. All rights reserved.