A tephrostratigraphic and petrological study of the Chikurachki (1816 m)-Tatarinov-Lomonosov volcanic chain (CTL volcanic chain) and Fuss (1772 m), located at the southern part of Paramushir Island in the northern Kurile Islands, was carried out to reveal the explosive eruption history during the Holocene and the temporal change of the magma systems of these active volcanoes. Tephra successions were described at 54 sites, and more than 20 major eruptive units were identified, consisting of pumice fall, scoria fall and ash fall deposits, each of which are separated by paleosol or peat layers. The source volcano of each recognized tephra layer was confirmed by correlation with proximal deposits of each eruption center with respect to petrography and whole-rock and glass chemistry. The age of each layer was determined by radiocarbon dating and the stratigraphic relationship with the dated, widespread tephra from Kamchatka according to the thickness of paleosols bracketed between tephra layers. The Holocene activity in this region was initiated by eruptions from the Tatarinov and Lomonosov volcanoes. After the eruptions, the Fuss and Chikurachki volcanoes started their explosive activities at ca. 7.5 ka BP, soon after the deposition of widespread tephra from the Kurile Lake caldera in southern Kamchatka. Compared with Fuss located on the back-arc side, Chikurachki has frequent, repeated explosive and voluminous eruptions. Whole-rock compositions of the rocks of the CTL volcanic chain and Fuss are classified into medium-K and high-K groups, respectively. These suggest that magma systems beneath the CTL volcanic chain and Fuss differ from each other and have been independently constructed. The rocks of the Chikurachki volcano are basalt-basaltic andesite and have gradually evolved their chemical compositions; when graphed on a SiO2-oxide diagram, these form smooth trends from mafic to more felsic. This suggests that the magma system evolved mainly by fractional crystallization. In contrast, matrix glass chemistries for Fuss pumices are distinct for each eruption and show different K2O levels on a SiO2-K2O diagram. This implies that the magma system of Fuss has been frequently replaced. Both volcanoes have been active under the same subduction system. However, the Chikurachki volcano will continue eruptive activity under a stable magma system with a higher magma discharge rate, whereas Fuss may continue construction with an intermittent supply of distinct, small magma batches.

Fossil diatom assemblages in a sediment core from a small lake in Central Kamchatka (Russia) were used to reconstruct palaeoenvironmental conditions of the late Holocene. The waterbody may be a kettle lake that formed on a moraine of the Two-Yurts Lake Valley, located on the eastern slope of the Central Kamchatka Mountain Chain. At present, it is a seepage lake with no surficial outflow. Fossil diatom assemblages show an almost constant ratio between planktonic and periphytic forms throughout the record. Downcore variations in the relative abundances of diatom species enabled division of the core into four diatom assemblage zones, mainly related to changes in abundances of Aulacoseira subarctica, Stephanodiscus minutulus, and Discostella pseudostelligera and several benthic species. Associated variations in the composition and content of organic matter are consistent with the diatom stratigraphy. The oldest recovered sediments date to about 3220 BC. They lie below a sedimentation hiatus and likely include reworked deposits from nearby Two-Yurts Lake. The initial lake stage between 870 and 400 BC was characterized by acidic shallow-water conditions. Between 400 BC and AD 1400, lacustrine conditions were established, with highest contributions from planktonic diatoms. The interval between AD 1400 and 1900 might reflect summer cooling during the Little Ice Age, indicated by diatoms that prefer strong turbulence, nutrient recycling and cooler summer conditions. The timing of palaeolimnological changes generally fits the pattern of neoglacial cooling during the late Holocene on Kamchatka and in the neighbouring Sea of Okhotsk, mainly driven by the prevailing modes of regional atmospheric circulation.

A geometric technique is introduced to estimate the height of volcanic eruption columns using the generally discarded near-limb portion of geostationary imagery. Such oblique observations facilitate a height-by-angle estimation method by offering close-to-orthogonal side views of eruption columns protruding from the Earth ellipsoid. Coverage is restricted to daytime point estimates in the immediate vicinity of the vent, which nevertheless can provide complementary constraints on source conditions for the modeling of near-field plume evolution. The technique is best suited to strong eruption columns with minimal tilting in the radial direction. For weak eruptions with severely bent plumes or eruptions with expanded umbrella clouds the radial tilt/expansion has to be corrected for either visually or using ancillary wind profiles. Validation on a large set of mountain peaks indicates a typical height uncertainty of ±500 m for near-vertical eruption columns, which compares favorably with the accuracy of the common temperature method.

In a companion paper (Horváth et al., 2021), we introduced a new technique to estimate volcanic eruption column height from extremely oblique near-limb geostationary views. The current paper demonstrates and validates the technique in a number of recent eruptions, ranging from ones with weak columnar plumes to subplinian events with massive umbrella clouds and overshooting tops that penetrate the stratosphere. Due to its purely geometric nature, the new method is shown to be unaffected by the limitations of the traditional brightness temperature method, such as height underestimation in subpixel and semitransparent plumes, ambiguous solutions near the tropopause temperature inversion, or the lack of solutions in undercooled plumes. The side view height estimates were in good agreement with plume heights derived from ground-based video and satellite stereo observations, suggesting they can be a useful complement to established techniques.

Abstract We provide results of a detailed study of the first peridotite xenoliths of proven mantle origin reported from Bezymyanny volcano in the Klyuchevskoy Group, northern Kamchatka arc. The xenoliths are coarse spinel harzburgites made up mainly of Mg-rich olivine as well as subhedral orthopyroxene (opx) and Cr-rich spinel, and also contain fine-grained interstitial pyroxenes, amphibole and feldspar. The samples are unique in preserving the evidence for both initial arc mantle substrate produced by high-degree melt extraction and subsequent enrichment events. We show that the textures, modal and major oxide compositions of the Bezymyanny xenoliths are generally similar to those of spinel harzburgite xenoliths from Avacha volcano in southern Kamchatka. However, coarse opx from the Bezymyanny harzburgites has higher abundances of light and medium rare earth elements and other highly incompatible elements than coarse opx from the Avacha harzburgites. We infer that (1) the sub-arc lithospheric mantle beneath both Avacha and Bezymyanny (and possibly between these volcanoes) consists predominantly of harzburgitic melting residues, which experienced metasomatism by slab-related fluids or low-fraction, fluid-rich melts and (2) the degrees of metasomatism are higher beneath Bezymyanny. By contrast, xenolith suites from Shiveluch and Kharchinsky volcanoes 50–100 km north of the Klyuchevskoy Group include abundant cumulates and products of reaction of mantle rocks with silicate melts at high melt/rock ratios. The high melt flux through the lithospheric mantle beneath Shiveluch and Kharchinsky may be related to the asthenospheric flow around the northern edge of the sinking Pacific plate; lateral propagation of fluids in the mantle wedge south of the plate edge may contribute to metasomatism in the mantle lithosphere beneath the Klyuchevskoy Group volcanoes.

Silicate glasses in peridotite xenoliths from Avacha volcano have high SiO2 (up to 72 wt.) and highly SiO2-oversaturated characteristics; normative quartz content is up to 50 wt.. The glasses represent secondary melts solidified after interaction with mantle peridotite, i.e. crystallization of secondary orthopyroxene at the expense of olivine. We identified two kinds of silicate glasses in Avacha peridotites; one is higher in K2O and enriched in Rb, Ba, U, and Pb than the other. The glasses show basically similar chemical characteristics to the host basaltic andesite to andesite of the Avacha volcano. These chemical characteristics are inherited from slab-derived fluids/melts, which metasomatize the mantle wedge and induce partial melting. The differences of chemical features among the Avacha glasses are attributed to chemical difference of the slab-derived fluids/melts, possibly due to the difference of sediments/basalt ratio of the relevant slab. The low-degree partial melt of peridotite assisted by these fluids/melts, is primarily SiO2-oversaturated, and can conduct silicate metasomatism, evolving through interaction with surrounding mantle peridotite, i.e. formation of orthopyroxene at the expense of olivine. Highly silicic glasses, also reported from peridotite xenoliths from oceanic hotspots and continental rift zones, mostly result from assimilation of orthopyroxene by SiO2-undersaturated melts, which crystallize clinopyroxene and olivine. The glasses also show similar trace-element patterns to their host alkali basaltic magmas, as in the case of arc glasses/calc-alkali magmas. If the glasses in peridotite xenoliths are of silicate metasomatism origin, they are similar in chemistry to host magmas. Reaction between carbonatite melts and peridotites shows the same petrographical feature as that of SiO2-undersaturated silicate melts with peridotites. The glasses originated from carbonatite metasomatism, however, exhibit clearly different trace-element patterns from their host alkali basaltic magmas.

Lithospheric mantle is inferred to be more oxidized than the asthenosphere, and mantle-wedge peridotites are characterized by high oxidation state relative to abyssal and continental peridotites due to addition of slab-derived fluids or melts. We found metals (native Ni, Fe silicides, native Fe and possible native Ti) from otherwise oxidized sub-arc mantle peridotite xenoliths from Avacha volcano, Kamchatka. This is contrary to the consensus and experimental results that the metals are stable only in deeper parts of the mantle (> 250 km). The metals from Avacha are different in chemistry and petrography from those in serpentinized peridotites. The Avacha metals are characteristically out of chemical equilibrium between individual grains as well as with surrounding peridotite minerals. This indicates their independent formation from different fluids. Some of the Avacha metals form inclusion trails with fluids and pyroxenes, leading to the inference that very local metal saturation resulted from rapid supply (‘flashing’) of reducing fluids from deeper levels. The fluids, possibly rich in H2, are formed by serpentinization at the cold base of the mantle wedge just above the slab, and they reduce overlying peridotites. We propose a metal-saturated peridotite layer, underlying the main oxidized portion, within the mantle wedge beneath the volcanic front to fore-arc region.