Major and trace element and Sr–Nd–Pb isotopic variations in mafic volcanic rocks hve been studied in a 220 km transect across the Kamchatka arc from the Eastern Volcanic Front, over the Central Kamchatka Depression to the Sredinny Ridge in the back-arc. Thirteen volcanoes and lava fields, from 110 to 400 km above the subducted slab, were sampled. This allows us to characterize spatial variations and the relative amount and composition of the slab fluid involved in magma genesis. Typical Kamchatka arc basalts, normalized for fractionation to 6% MgO, display a strong increase in large ion lithophile, light rare earth and high field strength elements from the arc front to the back-arc. Ba/Zr and Ce/Pb ratios, however, are nearly constant across the arc, which suggests a similar fluid input for Ba and Pb. La/Yb and Nb/Zr increase from the arc front to the back-arc. Rocks from the Central Kamchatka Depression range in 87Sr/86Sr from 0·70334 to 0·70366, but have almost constant Nd isotopic compositions (143Nd/144Nd 0·51307–0·51312). This correlates with the highest U/Th ratios in these rocks. Pb-isotopic ratios are mid-ocean ridge basalt (MORB)-like but decrease slightly from the volcanic front to the back-arc. The initial mantle source ranged from N-MORB-like in the volcanic front and Central Kamchatka Depression to more enriched in the back-arc. This enriched component is similar to an ocean-island basalt (OIB) source. Variations in (CaO)6·0–(Na2O)6·0 show that degree of melting decreases from the arc front to the Central Kamchatka Depression and remains constant from there to the Sredinny Ridge. Calculated fluid compositions have a similar trace element pattern across the arc, although minor differences are implied. A model is presented that quantifies the various mantle components (variably depleted N-MORB-mantle and enriched OIB-mantle) and the fluid compositions added to this mantle wedge. The amount of fluid added ranges from 0·7 to 2·1%. The degree of melting changes from ∼20% at the arc front to <10% below the back-arc region. The rocks from volcanoes of the northern part of the Central Kamchatka Depression—to the north of the transect considered in this study—are significantly different in their trace element compositions compared with the other rocks of the transect and their source appears to have been enriched by a component derived from melting of the edge of the ruptured slab.

The geochemical variations of magmas across and along supra-subduction zones (SSZ) have been commonly attributed to profound changes in the phase and chemical compositions of the mantle source and subduction-derived melt and fluid fluxes, as well as the physical parameters (e.g. depth, temperature, oxygen fugacity etc) of slab dehydration, mineral breakdown and melting. Here we test the variability of the Late Quaternary primitive magmas in the southern and northern parts of the meridionally oriented Eastern Volcanic Belt (EVB) of Kamchatka, with a slab depth varying from 60 to 160 km. Eight high-Mg (Mg# > 60 mol%) basalts were characterized for major, trace and platinum-group element (PGE) abundances, as well as the compositions of olivine phenocrysts and olivine-hosted spinel inclusions. The basalts in our study are geochemically typical of SSZ magmas and contain similar liquidus assemblages of forsteritic olivine (Mg# 78–92 mol%), low-Ti Cr-spinel and clinopyroxene. Although the absolute abundances of major and trace elements, and their ratios, in the basalts fluctuate to some extent, the observed variability cannot be correlated with any of considered parameters in the geometry of the Kamchatka SSZ and conditions of melting. This unexpected result led to the evaluation of the platinum-group element (PGE) systematics against the lithophile and chalcophile trace element geochemistry and the compositions of phenocrysts. Total whole-rock PGE content varies from 2.3 to 11.7 ppb, whereas the normalized PGE concentration patterns are typical for supra-subduction zones magmas and broadly similar in all studied samples. They are enriched in Rh, Pd and Pt relative to mid-ocean ridge basalts (MORB) and have nearly identical concentrations of Ir-group PGE. The only parameter that correlates well with PGE contents is the average Mg# of olivine phenocrysts from 84 to 90.3 mol%. This is interpreted to result from minor cryptic fractionation of sulfide melt, together with primitive olivine, in low-to-mid crustal conditions. Negative Ru anomalies on chondrite-normalized diagrams correspond to the Fe2+/Fe3+ ratios in spinel (a proxy for magma redox conditions), which reflects a replacement of monosulfide solid solution by laurite in the mantle wedge during oxidation.

This paper presents the results of studying the spatial distribution and structural setting of magnesian basalts and andesites in the Northern group of Kamchatkan volcanoes and in the junction zone of the Kuril-Kamchatka and Aleutian island arcs. The morphology and geologic structure of unique Kamchatkan magnesian basalt stratovolcanoes are described: Kharchinsky, Zarechnyi, and the Kharchinsky regional zone of cinder cones. The reported evidence includes the ages and eruptive histories, and productivities of the volcanoes and the volumes and weights of their edifices. The magnesian basalts were erupted 40-50 thousand years ago, for the first time during the Holocene.

Most of the Kharchinskii and Zarechnyi products, as well as those of the Kharchinskii cinder cones, are magnesian rocks. Mineralogical data suggest that both the basaltic and the andesitic magma were rich in water (≥3-4 and >6-7 wt., respectively) and crystallized at high oxygen fugacity (2.0-2.5 orders of magnitude higher than the NNO buffer). These features, coupled with the geochemical characteristics of these basalts and andesites, indicate that they are similar to the rocks of Shiveluch, a volcano also located on the northern flank of the Northern volcanic group, but differ from the rocks of the other volcanoes of this group which are located further south. The Kharchinskii, Zarechnyi, and Shiveluch magnesian basalts differ from the rocks of the Klyuchevskoi volcano and Tolbachik lava field by their higher K, Ba, Sr and lower Ca, Sc, Yb contents at higher La/Yb, Ni/Sc, and La/Ta ratios, while their initial magmas were more hydrous and more oxidized.

Kamchatka is a volcanic region of great interest.
About 100.000 km2 are covered by recent and quaternary lavas and tuffs; there are 20 volcanoes, active and solfatoric, 109 extinct volcanoes, 68 groups of thermal springs, 17 large and many small geysers. The Klyuchevskoy volcano is the most active one with a most varied volcanic activity. Besides, it is situated at the juncture of the Kourilsk-Kamchatka and Aleutian volcanic and tectonic arcs.
This fact explains for the organization of a volcanological station in the vicinity of the Klyuchevskoy volcano.
Since the station has been in existence, i. e, sinece 1935 the following eruptions have been recorded: the explosive eruption of the Klyuchevskoy volcano in 1935, and with lava flows from April 1937 till March 1939, and with formation of a number of parasitic craters; the eruption of Avacha from March till December 1938 accompanied by the issuing of an incandescent agglomerate stream rather the kind of the avalanche of the La Soufriere, the Merapi and partly the Mont Pele; the eruption of the Ploskij Tolbachik from September 1939 till June 1941, at the end of which a parasitic cone was formed from which flows issued; a small explosion of the Zhupanov volcano in the beginning of 1940; an eruption of the Karim volcano during 1940 and in the beginning of 1943.
Owing to the almost simultaneous ejection of lava at different heigtsts and at different distances from the central conduite of the Kluchevskoy volcano S. I. Naboko could drawn a picture of the gravitative differentiation in the magmatic reservoir.
There are given temperatures of the flowing bazaltic lava taken by V. P. Popkov and I. Z. Ivanov during their remarkable drift on the crust of the lava stream as well as the composition of gases collected by them from the hot lava.
A number of other volcanological observations is also given as well as short description of the volcanological station.
The Kamchatka Station was organized by F. I. Loevinson-Lessing. Since 1940 the station has been run by A. N. Zavaritsky, who is now organizing a Laboratory of Volcanology in Moscow.

Most of the Kharchinskii and Zarechnyi products, as well as those of the Kharchinskii cinder cones, are magnesian rocks. Mineralogical data suggest that both the basaltic and the andesitic magma were rich in water (≥3-4 and >6-7 wt.%, respectively) and crystallized at high oxygen fugacity (2.0-2.5 orders of magnitude higher than the NNO buffer). These features, coupled with the geochemical characteristics of these basalts and andesites, indicate that they are similar to the rocks of Shiveluch, a volcano also located on the northern flank of the Northern volcanic group, but differ from the rocks of the other volcanoes of this group which are located further south. The Kharchinskii, Zarechnyi, and Shiveluch magnesian basalts differ from the rocks of the Klyuchevskoi volcano and Tolbachik lava field by their higher K, Ba, Sr and lower Ca, Sc, Yb contents at higher La/Yb, Ni/Sc, and La/Ta ratios, while their initial magmas were more hydrous and more oxidized.

This paper presents the results of studying the spatial distribution and structural setting of magnesian basalts and andesites in the Northern group of Kamchatkan volcanoes and in the junction zone of the Kuril-Kamchatka and Aleutian island arcs. The morphology and geologic structure of unique Kamchatkan magnesian basalt stratovolcanoes are described: Kharchinsky, Zarechnyi, and the Kharchinsky regional zone of cinder cones. The reported evidence includes the ages and eruptive histories, and productivities of the volcanoes and the volumes and weights of their edifices. The magnesian basalts were erupted 40-50 thousand years ago, for the first time during the Holocene.