Mafic enclaves preserve a record of deep differentiation of primitive magmas in arc settings. We analyze the petrology and geochemistry of mafic enclaves from Shiveluch volcano in the Kamchatka peninsula to determine the differentiation histories of primitive magmas and to estimate their pressures, temperatures, and water contents. Amphibole inclusions in high forsterite olivine suggest that the primitive melt was superhydrous (i.e. >8 wt% H2O) and was fractionating amphibole and olivine early on its liquid line of descent. We find that the hydrous primitive melt had liquidus temperatures of 1062±48°C and crystallized high Mg# amphibole at depths of 23.6-28.8 km and water contents of 10-14 wt% H2O. The major and trace element whole rock chemistry of enclaves and of published analyses of andesites suggest that they are related through fractionation of amphibole-bearing assemblages. Quantitative models fractionating olivine, clinopyroxene, and amphibole reproduce geochemical trends defined by enclaves and andesites in variation diagrams. These models estimate 0.2%-12.2% amphibole fractionated from the melt to reproduce the full range of enclave compositions, which overlaps with estimates of the amount of amphibole fractionated from parental melts based on whole rock dysprosium contents. This contribution extends the published model of shallow processes at Shiveluch to greater depths. It provides evidence that primitive magmas feeding arc volcanoes may be more hydrous than estimated from other methods, and that amphibole is an important early fractionating phase on the liquid line of descent of superhydrous, primitive mantle-derived melts.

Выделено пять стадий эволюции четвертичного Кекукнайского вулканического массива (западный фланг Срединного хребта Камчатки): 1) докальдерная трахибазальтовая-андезибазальтовая, 2) экструзивная трахиандезит-трахидацитовая, 3) ранняя трахибазальтовая, 4) средняя гавайит-муджиеритовая (с единичными проявлениями андезибазальтов) и 5) поздняя трахибазальт-гавайит-муджиеритовая (с единичными проявлениями андезитов) - ареального вулканизма. По петрологическим данным среди пород массива выделены островодужный и внутриплитный геохимические типы. Ведущую роль в пет-рогенезисе играла динамика флюидной фазы при подчиненной роли процессов фракционной кристаллизации и гибридизма. Последовательное насыщение пород флюидной фазой в ходе эволюции расплавов было прервано в период кальдерообразования, когда осуществилась экстракция большей части флюидомобильных элементов и кремнезема. Геологические и петрологические материалы свидетельствуют о том, что формирование массива произошло в обстановке задугового вулканического бассейна в условиях начавшегося рифтогенеза, при активном участии компонентов мантийного плюма.

This study presents new geochemical data for Mutnovsky Volcano, located on the volcanic front of the southern portion of the Kamchatka arc. Field relationships show that Mutnovsky Volcano is comprised of four distinct stratocones, which have grown over that past 80 ka. The youngest center, Mutnovsky IV, has produced basalts and basaltic andesites only. The three older centers (Mutnovsky I, II, III) are dominated by basalt and basaltic andesite (60–80 by volume), but each has also produced small volumes of andesite and dacite. Across centers of all ages, Mutnovsky lavas define a tholeiitic igneous series, from 48–70 SiO2. Basalts and basaltic andesites have relatively low K2O and Na2O, and high FeO* and Al2O3 compared to volcanic rocks throughout Kamchatka. The mafic lavas are also depleted in the light rare earth elements (REEs), with chondrite-normalized La/Sm < 1.0. Andesites have generally higher REE abundances and are more enriched in light REEs, some showing negative Eu anomalies. All samples are depleted in field strength elements (HFSEs) relative to similarly incompatible REEs (e.g., low La/Ta, Nd/Hf compared to MORB), similar to island arc volcanic rocks worldwide. Radiogenic isotope ratios (Sr, Nd, Pb, Hf) are similar for samples from all four eruptive centers, and indicate that all samples were produced by melting of a similar source mixture. No clear age-progressive changes are evident in the compositions of Mutnovsky lavas. Mass balance and assimilation-fractional crystallization (AFC) modeling of major and rare earth elements (REEs) indicate that basaltic andesites were produced by FC of plagioclase, clinopyroxene and olivine from a parental basalt, combined with assimilation of a melt composition similar to dacite lavas present at Mutnovsky. This modeling also indicates that andesites were produced by FC of plagioclase from basaltic andesite, combined with assimilation of dacite. Dacites erupted from Mutnovsky I and II have low abundances of REEs, and do not appear to be related to mafic magmas by FC or AFC processes. These dacites are modeled as the products of dehydration partial melting at mid-crustal levels of a garnet-free, amphibole-bearing basaltic rock, which itself formed in the mid-crust by emplacement of magma that originated from the same source as all Mutnovsky magmas. Lead isotope data indicate that subducted sediment is likely present in the source beneath Mutnovsky and most Kamchatka volcanoes, but uniformly radiogenic Hf and Nd in mafic samples (εNd = 8.7–9.3, εHf = 15.4–15.9), and significant variation in trace element ratios at nearly constant εNd and εHf, indicate that sediment plays a minor roll in controlling subduction trace element patterns in Mutnovsky lavas. Mafic lavas with Ba/Th > 450 require an aqueous fluid source component from subducting oceanic crust, but mixing patterns in isotope versus trace element ratio plots for Hf and the REEs (εNd and εHf vs. ratios with Ce, Nd and Hf) demonstrate that a source component with radiogenic Nd and Hf, and fractionated (arc-type) trace element ratios must be present in the source of Mutnovsky lavas. This source component, which is interpreted to be a partial melt of subducted basalt in the eclogite facies (eclogite melt source component), appears to be present in the source of all Kamchatka volcanoes. Cross-arc geochemical patterns at Mutnovsky and in other arc systems (Isu-Bonin, Tonga-Kermadec) suggest that the aqueous fluid component diminishes and the eclogite melt component is increased from volcanoes at the arc front compared to those in rear-arc positions.

Significant variations in temperature, water chemistry and volume of the lake in the crater of Maly Semyachik volcano, Kamchatka, in 1965–2020 can be divided in three main stages. The first stage, until about the mid-1980s, was characterized by intense volcanic-hydrothermal activity, high (above 40 °C) temperature, high salinity (up to 40 g/l) and an increase in the level (and volume) of the lake. During the second stage, which lasted 25–30 years, since the mid-1980s until the mid-2000s, the lake cooled down, until freezing in winter, the mineralization decreased (up to 1.6 g/l), and the volume of the lake remained almost constant. After the mid-2000s until present, there is a period of activation of the Maly Semyachik volcano, which is expressed in an increase in temperature, mineralization and a sharp increase in the volume of the crater lake. The water balance of the lake and the input and composition of the thermal acid chloride solution are estimated based on the analysis of changes in chemical and physical parameters over time, using a box model.