Проведено сравнение палеовариаций магнитного поля Земли, полученных по параллельным разрезам голоценового почвенно-пирокластического чехла вулкана Малый Семячик на Камчатке. Показано, что в интервале возраста 300 — 6000 лет назад наблюдается подобие изменений направления остаточной намагниченности подтверждающее первичность этой намагниченности и пригодность ее для реконструкции истории геомагнитного поля. Палеовариации, выделенные для интервала 1000 — 4000 лет назад, прослежены в одновозрастных слоях в 240 км к северу, в разрезе тефры Ключевского вулкана.
Выяснено, что вследствие связанной со спецификой формирования отложений тефры возможности выпадения из разрезов отдельных горизонтов для получения достаточно детальной картины палеовариаций необходимо изучение двух или более параллельных разрезов.
Рис. 6, библ. 3 назв.

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.

Detailed studies of volcanic tremor envelopes with frequencies ranging from 5.5⋅10-6 to 2.5⋅10-2 Hz (50 hrs - 40 sec), recorded during the Klyuchevskoi volcano eruptions of 1983 and 1984, revealed five major frequencies: 1.1⋅10-2 Hz (T1 = 1 min 34 sec), 2.5⋅10-3 Hz (T2 = 6 min 10 sec), 4.2⋅10-4 Hz (T3 = 40 min), 5.1⋅10-5 Hz (T4 = 5 hrs 30 min), 7.7⋅10-6 Hz (T5 = 36 hrs), as well as superpositions of their harmonics. In the 1993 eruption, fluctuations in the volcanic tremor envelopes have frequencies of TI = 2 hrs 48 min and TII = 6 hrs 12 min, which correspond to periodicities in the dynamics of eruptions identified by visual observations since 1932. The distribution of peak amplitudes has been found to vary in relation to eruption intensity—increasing eruption strength correlates with an increase in the amplitude of low frequency peaks, and vice versa. It is concluded that volcanic tremor allows monitoring of eruption dynamics. Possible reasons for the occurrence of periodicities are discussed, but a comprehensive model for this phenomenon has not yet been developed.

The early 21st century saw increased eruption activity of major volcanoes in the Northern Group of Kamchatka, namely, Sheveluch, Klyuchevskoy, Bezymianny, and the Tolbachik Fissure Zone. The growth of an extrusive dome on Sheveluch andesitic volcano has occurred, with the dome reaching a height of 600 m after 38 years of nearly uninterrupted eruption activity. An 8-year period of relative quiet was followed by ten summit eruptions and two lateral vent openings on the Klyuchevskoy basaltic volcano. Explosive–effusive eruptions were observed nearly every year on the Bezymianny andesitic volcano. A 36-year quiet period gave way to a new eruption in the Tolbachik regional fissure zone.