For the first time, high-alumina mineral associations including rare minerals of the rhönite-kuratite series were found in post-caldera lava flows of the Kudryaviy and Menshiy Brat volcanoes (Kuril Islands). These lava flows are composed of olivine-bearing basalts and andesite-basalts of the calc-alkaline series. Partially crystallized olivine-hosted melt inclusions in these rocks contain high-alumina daughter phases. Residual glass with up to 25 wt% Al2O3, Al-pyroxenes and spinel with minerals of the rhönite subgroup were identified. The rhönite-kuratite series minerals are characterized by strong variations in Al, Fe2+, Fe3+, Mg, Si, as well as very low Ti, which makes it possible to name it a low-Ti Fe2+ analogue of rhönite with the formula (Ca,Mg,Na)4(Mg,Fe,Fe, Al,Ti,Mn)12O4[Si8.2Al3.8O36] – (Ca,Na,Mg)4(Fe,Mg,Al,Fe,Ti)12O4[Si6.78Al5.22O36]. It is assumed that high-alumina daughter phases form in olivine-hosted melt inclusions through water accumulation in the trapped melts during crystallization of the host olivine on the walls of melt inclusions. Water-rich melts can inhibit early crystallization of plagioclase, which promotes enrichment of the residual melt in Al2O3.

For the first time, high-alumina mineral assemblages including rare minerals of the rhönite–kuratite series were found in postcaldera lava flows of the Kudryavy and Menshiy Brat volcanoes (Kuril Islands). These lava flows consist of olivine-bearing basalts and andesite-basalts of the calc-alkaline series. Partially crystallized olivine-hosted melt inclusions in these rocks contain high-alumina daughter phases. Residual glass with up to 25 wt

Pyrolysis-free gas chromatography-mass spectrometry was used to determine the gas phase composition of inclusions in phenocrysts from basalts and rhyolites of the Men'shii Brat Volcano (Medvezh'ya caldera, Iturup Island). Among more than 300 compounds detected in the inclusions, hydrocarbons are predominant (52-92 rel.%). These hydrocarbons (C-1-C-17) are alkanes, alkenes, alcohols, polycyclic aromatic hydrocarbons, ethers and esters, aldehydes, ketones, and carboxylic acids as well as sulfonated, nitrogenated, and halogenated organic compounds. Inorganic substances, which are predominantly CO2, H2O, SO2, and N-2, are present in subordinate amounts. The organic compounds are interpreted as products of abiogenic synthesis of hydrocarbons in magmatic gases. This fundamentally new information about the composition of magmatic fluid suggests that mantle and crustal magmas can transport hydrocarbon substance.

The small volcanic islands of the central Greater Kuril Chain (GKC) produce strong explosive eruptions, two of which occurred in the recent years. The latest eruption took place at Raikoke volcano, which occurred on June 21–25 (2019) and was one of the largest on the Kuril Islands in the 21st century. This study presents mineralogical, petrographic and geochemical data on the air fall ash and pyroclastic density current deposits to elucidate the causes of high explosivity of this eruption. The magma involved in the Raikoke volcanic eruption is likely to have had a mantle origin and basaltic composition. Prior to forming a near-surface reservoir, the magma fractionated (i.e. due to crystallization of olivine and pyroxene) at depths of about 26 km, which is close to the Moho discontinuity in the middle part of GKC. The presence of amphibole among the 2019 pyroclastic minerals indicates that the magma contained significant (i.e. no less than 4 wt%) amounts of water at depth. This magma was degassed prior to intrusion into a shallower reservoir. The magma in the shallow reservoir had high crystallinity and contained viscous felsic residual melts, which prevented the development of effusive eruptions or Strombolian explosions. It is likely that magmatic fluid bubbles were still present, but their abundance was insufficient for lava fragmentation in the conduit. This led to low degree of vesiculation of the erupting melts. The results of this study demonstrate that the explosivity of the eruption is most likely related to the interaction in the volcanic edifice between meteoric waters and the magma, which was largely degassed and contained significant amounts of crystals and felsic melt (i.e. had high viscosity and low mobility).

The paper presents an original method developed by the authors for determining water content in highly polymerized rhyolite and rhyodacite glasses in melt inclusions. The method involves simultaneous determination of water content by two techniques: electron probe X-ray microanalysis (EPMA) and Raman spectroscopy, which are applied to mutual verify the results. The Raman spectroscopic technique was calibrated using a set of standard reference synthetic glasses of haplogranitic composition (SGS), containing 1.8 to 5.9 wt % water. The calibration was verified using a set of reference natural rhyolite obsidian samples (NRS) with water contents of 4.7 to 9.9 wt %. Two ratios were chosen as the calibrated parameters: (1) the area of the water and hydroxyl bands in the range of 2900–3800 cm–1 to the area of the silicate band vibration (Si–O and Al–O) in the range of 850–1200 cm–1 (Aw/As) and (2) the ratio of the intensities of the water band in the 3550–3560 cm–1 region and the 480 cm–1 band in the silicate vibration region (I3550/I480). As a result, the following calibration equations were acquired: $${{C}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}$$ = 0.8458 $$\frac{{{{A}_{w}}}}{{{{A}_{s}}}}$$ (σ) ± 0.17 wt % and $${{C}_{{{{{\text{H}}}_{{\text{2}}}}{\text{O}}}}}$$ = 11.494 $$\frac{{{{I}_{{3550}}}}}{{{{I}_{{480}}}}}$$ (σ) ± 0.21 wt %. Comparison of water concentrations in standard glasses determined by Raman spectroscopy using the Aw/As and I3550/I480 ratios and measured by X-ray microprobe analysis, showed a good consistency. Electron probe X-ray microanalysis was used to determine water content in glasses by recalculating the excess oxygen content. The method was tested on unheated glassy melt inclusions in minerals from dacitic and rhyolitic pumice from caldera eruption deposits of Okataina volcanic center, New Zealand; Kurile Lake, Kamchatka Peninsula; Vetrovoy Isthmus; and Lvinaya Past Bay of Iturup Island in the Kuriles Islands. The ability of using different Aw/As or I3550/I480 ratios makes it possible to minimize the effect of the overlaps of host-mineral bands in the low-frequency part of the Raman spectrum of the melt inclusions. We found out that the usage of the I3550/I480 ratio for melt inclusions in pyroxenes produces more reliable results than the use of the Aw/As ratio. At the same time, the Aw/As ratio is better to use for inclusions in quartz and plagioclase. Water content in the inclusions varies from 0.5 to 7.9 wt %. The proposed method can be routinely used for reliable measurements of water content in the glasses of melt inclusions no smaller than 10 μm, at water contents no lower than 0.5 wt %.

Detailed mineralogical and melt and fluid inclusion constraints on magma storage, and the evolution of melts, are presented for the large-volume caldera eruption on the Vetrovoy Isthmus on Itutrup Island (Kuril Islands, Russia). The shallow magma reservoir beneath the Vetrovoy Isthmus is composed of a mush of plagio-rhyolitic melt, phenocrysts and the products of peritectic reaction(s). The melt appears to have formed as a result of partial melting of previously erupted rocks, which probably had andesitic to basaltic compositions and were metamorphosed into amphibole-bearing assemblages. The breakdown of amphibole in the partially melted precursor rocks led to the formation of early Mg-rich clino- and orthopyroxene, along with plagioclase and Fe-Ti oxides, and the release of aqueous fluids. Variations in fluid pressure are recorded by a strong increase of An contents in plagioclase. Crystallization took place at around 850 degrees C with pressure ranging from 0.9 to 3 kbar. This study demonstrates that dacitic magmas erupted during the course of a 20 kyr voluminous eruption were the result of mixing between plagio-rhyolitic partial melts and the breakdown reaction minerals (i.e. pyroxenes, plagioclase and Fe-Ti oxides). Plagioclase and quartz were the last minerals to crystallize from these melts prior to eruption.

The paper represents results of the study of pre-eruption volatile and fluid behavior in magma chambers beneath two large calderas at the Iturup Island (Great Kuril island chain). Both magmas have similar dacitic compositions and were composed of plagiorhyolitic melts, magmatic phenocrysts and restite minerals. Minerals crystallized at 850 °C under strongly oxidized conditions (NNO+1). Magma storage of the VI eruption was shallow (~1 kbar) and high water contentsleads to degassing and release of the H2O-CO2 fluid. The magma reservoir of the LP eruption was located deeper (>1 kbar) and similar to VI water contents in magma resulted in appearance of Mg-hornblende, rather than degassing. The study revealed that behavior of water and carbon dioxide largely depends on the pressure. In both VI and LP magma reservoirs all studied volatile components (H2O, Cl, F, S) behaved like incompatible elements and were concentrated by silicate melts.

The Vetrovoy Isthmus and the Lvinaya Past' Bay on the Iturup island (Kuril island arc) are the results of large Plinian eruptions of compositionally similar dacitic magmas. This study is devoted to a comparative analysis of the storage and crystallization conditions for magma reservoirs, which were a source of large-scale explosive eruptions. The plagioclase is most informative mineral in studying of the melt evolution. The studied plagioclases possess a complex zoning patterns, which are not typical for silicic rocks in island-arc systems. It was shown that increase of Ca in the plagioclase up to unusually high An(95) is related to increase of H2O pressure in both volcanic magma chambers. The study revealed that minerals of the Vetrovoy Isthmus and Lvinaya Past' crystallized from compositionally similar melts. Despite the compositional similarity of the melts, the phenocryst assemblage of the Lvinaya Past' differs from the Vetrovoy Isthmus by the presence of the amphibole, which indicates that the pressure in the magmatic chamber exceeded 1-2 kbar at a 4-6 wt. % of H2O in the melt. The rocks of the Vetrovoy Isthmus do not contain amphibole phenocrysts, but melt and fluid inclusions assemblages in plagioclase demonstrate that the magma degassed in the course of evolution. This is an indication that the pressure did not exceed significantly 1-2 kbar.

This work is devoted to the study of one of the largest caldera eruptions of the Kurile-Kamchatka island-arc system that occurred on the island of Iturup. The object of investigation of this work are phenocrysts of quartz and plagioclase from dacite pumice of the Isthmus of the Isthmus, which is located on the island of Iturup. The purpose of this work is to determine the water content in the melts that participated in the caldera eruption of the Vetrovoy Isthmus and the patterns of their changes during the crystallization of magma. In the course of the work, the following were carried out: 1) adaptation and calibration of the Raman spectroscopy method for determining water in rhyolite melt's inclusions glasses in quartz and plagioclase from pumice stone; 2) determination of composition and estimation of water content in melt inclusions in quartz and plagioclase according to x-ray spectral analysis; 3) establishment of the regularities of the change in the water content during the evolution of the magmatic melt; 4) evaluation of fluid pressure by comparison with experimental data