No 4 (2025)

Mutnovsky volcano in the last four thousand years is characterized by predominantly magmatic activity in the northern sector, where the Mutnovsky geothermal field was formed. Magmatic activity is detected by the Frac-Digger method using seismic data of KF FIC EGS RAS, the most powerful sequence of shallow dykes of NE strike is manifested at the surface by blowing geothermal wells. Dyke emplacement in March 2024 is synchronized with a hydrothermal explosion with the formation of a crater with a volume of up to 0.36 million m³ at the site of well 022 (Crater 022+). Unaltered debris from the explosion indicates lithoclastic character in the head gas-saturated part of the dyke. The base of the explosion crater is a hydrothermally altered (opal, zeolites) plane of 160×75 m². The mechanism of the hydrothermal explosion is considered as a result of hydraulic fracturing of the two-phase geothermal reservoir by a shallow dyke, with subsequent pressure increase in the shallow hydrothermal reservoir near the well with a closed wellhead valve.

The isotopic K–Ar age of volcanoes in interfluve of the Kozyrevka and Malaya Romanovka rivers in the Sredinny Range of Kamchatka has been established for the first time. Six stages of activation have been identified: the termination of the Miocene (5.8–5.5 Ma), the Early Pliocene (4.9–4.5 Ma), the Middle Eopleistocene (1.5–1.4 Ma), the Late Eopleistocene (1.1–1.0 Ma), the Middle Pleistocene (0.4–0.3 Ma). No more recent volcanic activity has been recorded in the area. It has been established that the beginning of Quaternary volcanic activity was preceded by a period of rest lasting about 3 million years. It has been established for the first time that the formation of a large fault of the upper strike occurred at the northern foot of the Romanovka volcano no earlier than 0.4–0.3 million years ago. The analysis of K–Ar isotopic dates allowed us to suggest that the activation stages of Quaternary volcanic activity at ~1.5 and ~0.3 Ma had a regional character. The regional activity is less clearly traced for the stages of ~1.0 and ~0.6 Ma. Over the last six Ma, rocks ranging from basalts to dacites have erupted in the interfluve. It was found that extensive covers and large lava structures are composed of rocks of medium and acidic composition, while basalts are confined to small monogenic centers. In general, the studied volcanics are chemically similar to the Late Miocene-Quaternary rocks of the southern part of the Sredinny Range. The results of our work fundamentally change the previously existing estimates of the age of volcanoes in the studied area of the Sredinny Range. It is suggested that there were no significant glaciations in the interfluve of Kozyrevka and Malaya Romanovka at least in the Quaternary.

The mineralogical and petrochemical compositions of gabbro from the Esmeralda underwater volcano, located in the southern part of the Mariana Island Arc, dredged during the 4^th and 5^th cruises of the research vessel “Vulcanologist”, were studied. It has been established that gabbros belonging to the tholeiitic series are highly fractionated rocks, in contrast to the dredged gabbroids in the basins of the southern part of the Mariana Island Arc system. It is shown that gabbro is characterized by elevated iron content. The elevated iron content of rocks is confirmed by high iron content in pyroxenes and plagioclases, which allows us to classify the bulk of dredged gabbro as an association of Island Arc ferruginous tholeiites. Low concentrations of highly charged and heavy rare earth elements relative to the composition of MORB, as well as their interelement ratios, suggest the formation of primary melts from a depleted mantle source. The enrichment of gabbro with large-ion lithophile elements indicates the participation in magmogenesis of not only high-temperature sedimentary melt, but also the significant role of a low-temperature fluid component.