No 4 (2023)

The paper provides information about the eruptive activity of the Ebeko volcano in 2022. From January 22 to June 13, phreatic explosions occurred in the crater lake caused by water seeping through the plug formed in the upper part of the magma channel and its boiling. On June 14, Vulcanian explosions began, destroying the lake. The granulometric composition of the ashes has changed in the direction of reducing the particle size. Petrographic and mineralogical-geochemical studies of tephra allow us to define this period as a phreatomagmatic eruption by the presence of fresh juvenile material. It is established that the interaction of magma with the waters of the hydrothermal system of the Ebeko volcano leads to its depletion with alkali metals and enrichment with silica. It is suggested that the formation of amorphous water-containing silica in the form of numerous separations and its subsequent dehydration may contribute to the explosive activity of the volcano.

The paper considers isotopic composition of helium (³Не/⁴Не = R) in groundwater of South-Baikal (SBVA) and South-Khangai volcanic areas (SKhVA) during the Late Cenozoic period. The differences in the behavior and magnitude of the parameters were established. It is found that the mentioned differences in ³Не/⁴Не concentrations within South-Baikal and South-Khangai volcanic areas correspond to mantle reservoirs with different helium isotope compositions. This confirms that the Late Cenozoic volcanism of South-Baikal and South-Khangai volcanic area is controlled by mantle sources related to mantle plumes of the Central-Asian hot mantle field.

By means of the gravity models reflecting a rheological states of geological environments 3D distribution of density contrast in the heads of six plumes (Yellowstone, Emeishan, Indigiro-Kolyma, Sea of Okhotsk, Indigiro-Kolyma, and Maya-Selemdzha) up to the depth of 200 km are studied and compared with all geological-geophysical data. According to the obtained data, astenospheric parts of plumes have mushroom-like shape, and astenosperic magmas spread under the lithosphere bottom and more rare ‒ under the crust bottom. At the distance of 250‒300 km from central trunks of plumes they are narrowed to diameter of 200–300 km at a depth of 100‒120 km. In heads of the majority of plumes astenospheric magmas merge with the subcrustal viscous layer and approach the Earth’s surface to 40‒50 km. In the majority of the considered plumes their lithospheric and crustal fragments are curved towards the Earth’s surface, In the upper crust layers upwards are sometimes complicated by local downwards (Yellowstone and May-Selemdzha plumes) that is explained by sagging of the dome roofs over the magmatic chambers into subcrustal viscous layer and in asthenosphere. Plumes are often accompanied by zones of the lithosphere stretching (rifts) therefore in the lower lithospheric and crustal sections of plumes linear zones of the lowered viscosity are mapped. The structural position of considered plumes is controlled by borders of lithospheric plates and large segments of the second rank. Identical geometry and rheology of plumes created at different times (Triassic‒Neogene) in the regions which are far removed from each other (the North East Russia, North West of the USA, Southern China, Sea of Okhotsk) demonstrate universality of the tectonic situations promoting penetration of mantle streams into upper layers of the Earth. The main of them are the lithosphere stretching zones, in particular ‒ sites of crossing of multidirectional fractures of a lithosphere and crust.