We present here the first regional set of Sr-Nd-Pb isotopic compositions, major and trace element compositions and KAr ages for a representative suite of back-arc samples of the Sredinny Range (SR) of Kamchatka. Together with previously published analyses, this unique dataset allowed us to trace the source heterogeneity and invoke new mechanisms to explain the variably enriched geochemical signatures. Our results indicate that the Sr isotopic ratios and the LILE content of the studied rocks were mainly influenced by the subduction fluid. Neodymium isotopes, as well as the HFSE and REE distributions in the rocks, require a more complex explanation, depending on the geographical location of the studied samples and the age of their eruption. Melts representing Miocene rocks of the northern part of the SR and most of the rocks from the eastern and central flanks of the SR (from Miocene until present) were produced from a depleted MORB-like metasomatized mantle, and were then exposed to varying degrees of crustal assimilation and fractional crystallization. Higher HFSE contents and lower 143Nd/144Nd ratios in the western flank lavas, as well as high HFSE contents in the Quaternary lavas of the northern part of the SR and some of the lavas from the eastern flank of the southern part of the SR, require an alternative source for their enrichment. Delamination of the lithosphere explains the unusual, enriched signature of these rocks, whereas their variably enriched neodymium isotope ratios identify the various ages of the separation of the lithosphere from the mantle. The Nd isotopic composition of the rocks together with their HFSE content, therefore, serves as an unusual tracer for the enriched mantle domain, showing the presence of the older lithospheric blocks and indicating the timeframes for this source involvement in magma generation.

We report, here, the composition and K-Ar ages of a representative collection of volcanic rocks that erupted within three monogenetic volcanic fields in the active fault zone of the Sredinny Range of Kamchatka: Tigilsky Dol, Mount Oxi massif and Anaunsky Dol. The studied rocks display a wide range of compositions (medium-K, moderate-Mg, high-K, high-Ti and high-Mg basalts, and high-LREE picrobasalts); the high-Mg varieties are confined to faults. Five main periods of volcanic activity were investigated, 4.3-3, 2, 1.5, 1 Ma and from 0.3 to <0.05 Ma. Primitive lavas first emerged on the surface at 3.5 Ma. There was a massive outpouring of high-Mg lavas at 1.5-1 and 0.3 Ma, which could have been related to the formation of the fault zone. This is the first report of rocks in Kamchatka with a high-LREE picrobasaltic composition (1.5 Ma). The Fo content of the olivine phenocrysts reaches 93.2 mol%, which is the highest value known for Quaternary Kamchatka basalts. A very heterogeneous source, even for individual eruptions is indicated by the minor element contents in the olivine (Ni, Mn and Ca); Cr-spinel - olivine paragenesis show that all the rocks studied crystallized in the same temperature range (1111-1292 C), whereas the oxygen fugacity for the different samples varied from & UDelta;QFM +0.7 to +2.0 log. units. A melt inclusion study showed that the Mg basalts of the Mt. Oxi massif and the high-LREE picrobasalts of Tigilsky Dol had different fluid sources that were enriched and depleted in water and Cl, respectively. We argue that the fluid source for the Mt. Oxi massif was likely the remains of the Pacific slab under the Sredinny Range, whereas, for the high-LREE picrobasalts of Tigilsky Dol, it was the lithospheric lithologies. The low content of S and high content of Cu in the oxidized high-LREE basalts provide additional evidence that they originated from the re-melting of sulfur-poor lithospheric lithologies. Both the fault zone and the lithosphere reactivation in the region are likely linked to the regional stress field.

Pliocene-Quaternary volcanism in Kamchatka occurs in three volcanic chains parallel to the trench: Eastern volcanic front, Central Kamchatka Depression and the Sredinny Range (SR). Origin of Quaternary volcanism in the SR is controversial. Compositional variability of the SR volcanic rocks is explained by participation of several components in melting –depleted mantle wedge, subduction fluid and HFSE-enriched component, which invokes various geodynamic models including postsubduction setting, back-arc condition of the contemporary arc system, mantle plume, melting of the slab edge, trench roll-back, etc. Noble gas isotopes can be good tracers of the origin of magmatism because they have different features in different geochemical reservoirs such as convecting MORB-source mantle, deep-mantle and crust. To constrain the origin of the SR magmatism, we analyzed noble gases (He, Ne, Ar, Kr, and Xe) in olivines and pyroxenes separated from basalts and basaltic andesites collected from the SR.

For the first time, a series of 10 isotopic K–Ar ages was obtained for the lava centers of the Sedankinsky Dol, located in the northern part of the Sredinny Range of Kamchatka. We show that volcanism in this area started at least in the Middle Pleistocene. For the pre-Holocene time, four stages of volcanism activation were distinguished (300–270, 180–160, 100–80, and between 50 and 12 ka BP), separated by significant periods of dormancy. The beginning of volcanic activity and the first supply of basic magmas to the Dol were apparently caused by regional reasons for the increase of endogenous activity. The obtained geochronological data cast doubt on either the age or the genesis of the moraine complexes of the first and second phases of the Late Pleistocene glaciation which were previously identified for this region. Analysis of lithological-petrographic and isotope-geochronological data indicates that about 100–80 ka a large avalanche descended from the near-watershed part of the ridge. The deposits of this avalanche were previously identified as “phase I moraine”. This event could have been triggered by a high-magnitude earthquake, preceding and/or accompanying a new stage of volcanism activation, when the earliest eruptions of olivine basalts were recorded.

A first set of K–Ar isotopic ages obtained, which allowed to estimate the age of the largest volcanoes of the Anaunsky Dol (3.2, 2.2 and 1.9 Ma) and eruptive centers of the post-caldera stage of Uksichan massif (1.2 and 0.8 Ma), located in the Sredinny Range of Kamchatka. Significant time intervals separating the individual phases of volcanism suggest either repeated activation of the fault zone to which the dated eruptive centers are confined, or the presence of several fault zones here.

—We report the chemical and isotopic compositions of volcanic rocks of the Akhtang and Kostina mountain massifs in the Sredinny Range, Kamchatka. The analyzed rocks are similar in composition to the earlier studied volcanics of the eastern flank of the southern part of the Sredinny Range. Results of K–Ar isotope dating reveal three stages of volcanic activity in the two massifs. These stages are divided by long (1.4 and 2.4 Ma) periods of quiescence. In the Akhtang massif, the eruptive activity was at 4.9–4.0, 1.9–1.7, and 0.3–0.2 Ma, and in the Mt. Kostina massif, at ~8.0, 5.6–4.9, and ~3.5 Ma. Two early stages of both massifs are characterized by the eruption of island arc type rocks, and the late stage, by the eruption of rocks of hybrid geochemical type. The Mio-Pliocene (N1–N21) rocks of the Mt. Kostina massif are similar in geochemical features to the early Pliocene (N21) rocks of the Akhtang massif, and the late Pliocene (N22) lavas of the former massif are similar to the middle Quaternary (Q2) rocks of the superimposed monogenetic volcanism zone of the latter massif. For the Akhtang massif it has been first discovered that the volcanic reactivation after the long quiescence periods was accompanied by a change in the composition of rocks and in the type of eruptive activity (from the eruption of plateau-effusives rocks to the formation of stratovolcanoes and monogenetic volcanism zones). The obtained data on the age and composition of rocks as well as some morphological features of the studied massifs suggest that the plateau-effusive rocks of the Sredinny Range might be related to central-type eruptions.

The K–Ar isotopic age of the beginning of formation of Ichinsky (0.35) and Khangar (0.4) stratovolcanoes is established for the first time, as well as the age of the final stage of activity of the Lauchan Ridge (about 0.9 Ma) in the Sredinny Range of Kamchatka. Probably the formation of the studied volcanoes may be caused by the generation of the new deep NNE fault within the Sredinny Range of Kamchatka, which in turn was connected to the regional reasons of the increasing endogenous activity at the North-West Pacific.

Clusters of small-volume volcanoes that individually may be defined as monogenetic, but have interlinked and interconnected plumbing systems, are used to be categorized as monogenetic volcanic fields (MVF). We argue that such volcanic clusters should be distinguished as separate type of volcanism, intermediate between monogenetic and polygenetic. The magma plumbing system structure of the MVF (its complexity and polymagmatic character) is the key argument for the potential separation of them in a classification. To avoid confusion caused by genetic meaning of the used words we suggest using a term “areal volcanism” or “areal volcanic fields” (AVF instead of MVF) as defining this special type of volcanic activity. Here we provide a review of the main characteristic features of one of the largest Holocene AVF, which is active now – the Tolbachik field of cinder cones in the southern part of Klyuchevskaya volcano group (Kamchatka), known in the literature as Tolbachinsky Dol. This paper is focused on the research of magma plumbing system. We consider structural, morphological, geological, geochemical and petrological data on the erupted basalts and their genesis. Specially planned seismic experiments made in 2010–2015 (seismic tomography and microseismic sounding) allowed modeling of the principal elements of the magma plumbing system of Tolbachik AVF. Analysis of the investigations made in this area shows that Tolbachik AVF has a complex, dynamic, variable magmatic feeding system, which can be visualized as a superposition of subvertical and sublateral magma conduits. The contrast composition of the erupted rocks is caused by their different, although genetically connected, magma sources and mixing processes. One of the long-lived eruptive centers of Tolbachik AVF is Plosky Tolbachik stratovolcano, which lost its independent activity and was captured by Tolbachik AVF in Holocene. The AVF formed rejuvenated volcanism using the feeding system of the stratovolcano like an “old anthill”. The magma plumbing system characteristics of Tolbachinsky Dol strongly support the idea of separation of AVF from monogenetic volcanism type in the classification.

The Kamchatka volcanic arc (Russia) is one of best-studied, but most complex tectonic margins on Earth, with an extensive geologic history extending back to the Late Cretaceous. Unlike many other subduction zones, primitive basalts with Mg# > 65 are abundant in Kamchatka, thereby allowing characterization of the mantle source through compositional analyses of near-liquidus minerals in the rocks. In this paper, we present a comprehensive dataset on the composition of Cr-spinel inclusions in olivine for all main Late Quaternary volcanic zones in Kamchatka, comprising 1604 analyses of spinel inclusions and their host-olivine in 104 samples from 30 volcanic complexes (single volcanoes and volcanic fields). The studied rocks are basalts, basaltic andesites and high-Mg andesites, which cover the whole compositional range of the primitive Late Quaternary volcanic rocks in Kamchatka. The spinel composition shows large variability. Spinel inclusions with the lowest Cr# and Fe3+/Fe2+ ratios were found in basalts from Sredinny Range and Northern Kamchatka, whereas the most Cr-rich and oxidized spinel inclusions occur in basalts and high-Mg andesites from the Central Kamchatka Depression. Intermediate Cr-spinel compositions characterize the Eastern Volcanic Belt of Kamchatka. The compositions of olivine-spinel pairs were used to quantify the oxidation state of parental Kamchatka magmas and the degree of partial mantle melting. The redox conditions recorded in spinel compositions range from Delta QFM = +0.7 to +3.7. Delta QFM for spinel from the Sredinny Range and Northern Kamchatka correlates with a number of whole-rock proxies for the involvement of slab-derived components (e.g., La/Nb and Ba/La), which suggests a coupling between mantle oxidation state and slab-derived fluid/melt metasomatism. These correlations were not observed in frontal Kamchatka volcanoes with the highest estimated Delta QFM, which possibly indicates buffering of the mantle oxidation state by sulfur. The estimated degrees of partial mantle melting range from 8 to >20% for Kamchatka volcanoes. Spinel from the Central Kamchatka Depression has the highest Cr# and could crystallize from magmas generated from the most depleted sources. In contrast to the Eastern Volcanic Belt, spinel Cr# and the inferred degrees of melting in the Central Kamchatka Depression do not correlate with spinel TiO2 content. The apparent decoupling between the proxies of mantle depletion in the CKD spinel is interpreted to reflect refertilization of the CKD mantle by oxidized Ti-rich slab- or mantle lithosphere-derived melts near the northern edge of the subducting Pacific Plate. This study demonstrates that the composition of Cr-spinel in volcanic rocks in combination with bulk-rock compositions can be a powerful tool to map regional variations of the mantle source depletion, oxidation state, and involvement of various slab-derived components in island-arc magmatism. (C) 2018 Elsevier B.V. All rights reserved.

Based on the geochemical characteristics of the Miocene-Quaternary volcanic rocks of the Sredinny Range of Kamchatka, we divide it into northern and southern provinces; the latter comprises the "eastern", "western", and "central" flanks. We present new data on the composition of Neogene-Quaternary volcanic rocks in the southern part of the Sredinny Range of Kamchatka: Khangar and Icha volcanic massifs and Mt. Yurtinaya on the "western" flank, Bystrinsky and Kozyrevsky Ridges on the "eastern" flank, and Anaunsky Dol and Ulcsichan massif located in between. We show systematic differences in the composition of rocks from the "western" and "eastern" flanks. During the Neogene, a typical island-arc volcanism took place within the "eastern" flank. Quaternary volcanic rocks of this area have both island-arc and within-plate geochemical features. We propose to call rocks of this type hybrid rocks. Within the "western" flank, hybrid volcanism has been manifested since the Neogene, while typical island-arc rocks arc not found. Magma generation processes on the "western" flank of the Sredinny Ridge arc influenced by an enriched mantle source; the effect of fluid is less pronounced here as compared to the rocks of the "eastern" flank, where it is clearly traced. (C) 2018, V.S. Sobolev IGM, Siberian Branch of the RAS. Published by Elsevier B.V. All right reserved.

For the first time, the age of the beginning of the volcanic activity within Sredinny metamorphic Massif is determined (7–6 Ma). We suppose that this event was caused by the collision of Kamchatka with the Kronotsk arc that started about 7 Ma from accretion of Shipunsky peninsula. We demonstrate that at least two types of rocks were erupted within Sredinny Range of Kamchatka in late Miocene times: typical islandarc rocks were produced in the central and northern parts of the Range, and hybrid type rocks—in its southernmost part.

Here we present the results from monitoring of the composition of rocks produced during the 2012–2013 fissure eruption at Tolbachik volcano (FTE). Major and trace element concentrations in 75 samples are reported. Products of this eruption are represented by high alumina basaltic trachyandesites with higher alkalis and titanium contents than in all previously studied rocks of the Tolbachik monogenetic volcanic field. Rocks erupted during the first three days (27–30 November) from the northern (also called Menyailov) group of vents are the most silica- and alkali-rich (SiO2 concentrations up to 55.35wt.% and K2O up to 2.67wt.%). From December onwards, when the eruptive activity switched from the Menyailov vents to the southern (Naboko) group of vents, silica content dropped by 2wt.%, concentrations of MgO, FeO, TiO2 and Mg# increased, and K2O and Na2O concentrations and K2O/MgO ratio decreased. For the rest of the eruption the compositions of rocks remained constant and homogeneous; no systematic compositional differences between lava, bombs and scoria samples are evident. Trace element distributions in the rocks of the Menyailov and Naboko vent lavas are relatively uniform; Menyailov lavas have slightly higher Th, Nb, Hf, Y, and HREE concentrations than the Naboko vent lavas at more or less constant element ratios. We explain the initial change in geochemistry by tapping of a slightly cooler and fractionated (~3% Mt and 8% Cpx) upper part of the magma storage zone before the main storage area began to feed the eruption. Thermodynamic constraints show that apparent liquidus temperatures varied from 1142°C to 1151°C, and thermodynamic modeling shows that variations in compositions are consistent with a high degree of low pressure (100–300MPa), nominally anhydrous fractionation of a parent melt compositionally similar to the 1975 Northern Breakthrough high-Mg basalt. Geochemistry, petrological observations and modeling are in agreement with the newly erupted material being derived from remnant high-Al magma from the 1975–76 Southern Breakthrough eruption with only slight amounts of cooling (less than 1°C per year) during the intervening 36years.

We present a reconstruction of the chronological sequence of events that took place during the first days of the 2012–2013 Tolbachik fissure eruption using petrological data and remote sensing methods. We were forced to use this approach because bad weather conditions did not allow direct observations during the first two days of the eruption. We interpreted infrared images from the scanning radiometer VIIRS Suomi NPP and correlated the output with the results of the geochemical study, including comparison of the ash, deposited at the period from 27 to 29 November, with the samples of lava and bombs erupted from the Menyailov and Naboko vents. We argue that the compositional change observed in the eruption products (the decrease of SiO2 concentration and K2O/MgO ratio, increase of MgO concentration and Mg#) started approximately 24h after the eruption began. At this time the center of activity moved to the southern part of the fissure, where the Naboko group of vents was formed; therefore, this timeframe also characterizes the timing of the Naboko vent opening. The Naboko group of vents remained active until the end of eruption in September 2013.

We estimated the subsidence rate of the lava flow formed during the 2012–2013 eruption of Tolbachik volcano from satellite radar interferometry with using Sentinel-1 satellite images between 2017 and 2019. The maximum subsidence values were 285 mm/yr in 2017, 249 mm/yr in 2018, and 261 mm/yr in 2019. The subsidence rate increases with the flow thickness. This trend is observed for most of locations except a small area in the vicinity of the active vent where the subsidence is anomalously high. We show that the main observed trend can be well explained by a thermal compaction with a 1D mathematical model that takes into account the latent heat of crystallization, temperature dependences of physical parameters (heat capacity, thermal conductivity, density), temperature dependence of crystal concentration in the melt volume, and the percentage of uncrystallized material (glass or melt), porosity and lava layer formation rate. The much faster subsidence rates observed close to the vent can be explained by contraction of buried lava tubes that is not accounted in the thermal model.

We present a broad overview of the 2012–13 flank fissure eruption of Plosky Tolbachik Volcano in the central Kamchatka Peninsula. The eruption lasted more than nine months and produced approximately 0.55 km3 DRE (volume recalculated to a density of 2.8 g/cm3) of basaltic trachyandesite magma. The 2012–13 eruption of Tolbachik is one of the most voluminous historical eruptions of mafic magma at subduction related volcanoes globally, and it is the second largest at Kamchatka. The eruption was preceded by five months of elevated seismicity and ground inflation, both of which peaked a day before the eruption commenced on 27 November 2012. The batch of high-Al magma ascended from depths of 5–10 km; its apical part contained 54–55 wt.% SiO2, and the main body 52–53 wt.% SiO2. The eruption started by the opening of a 6 km-long radial fissure on the southwestern slope of the volcano that fed multi-vent phreatomagmatic and magmatic explosive activity, as well as intensive effusion of lava with an initial discharge of > 440 m3/s. After 10 days the eruption continued only at the lower part of the fissure, where explosive and effusive activity of Hawaiian–Strombolian type occurred from a lava pond in the crater of the main growing scoria cone. The discharge rate for the nine month long, effusion-dominated eruption gradually declined from 140 to 18 m3/s and formed a compound lava field with a total area of ~ 36 km2; the effusive activity evolved from high-discharge channel-fed 'a'a lavas to dominantly low-discharge tube-fed pahoehoe lavas. On 23 August, the effusion of lava ceased and the intra-crater lava pond drained. Weak Strombolian-type explosions continued for several more days on the crater bottom until the end of the eruption around 5 September 2013. Based on a broad array of new data collected during this eruption, we develop a model for the magma storage and transport system of Plosky Tolbachik that links the storage zones of the two main genetically related magma types of the volcano (high-Al and high-Mg basalts) with the clusters of local seismicity. The model explains why precursory seismicity and dynamics of the 2012–13 eruption was drastically different from those of the previous eruption of the volcano in 1975–76.