Journal of Volcanology and Seismology最新文献

This paper deals with the isotope composition of helium (³Не/⁴Не = R) in the groundwater of the Southern Baikal volcanic area (SBVA) and Southern Khangai volcanic area (SKhVA) during the Late Cenozoic Period. We have found differences in the behavior and value of that parameter. It was found that the differences in the concentrations of ³Не/⁴Не in the SBVA and the SKhVA resulted from mantle reservoirs that have different isotope compositions of helium. This confirms that the Late Cenozoic volcanism in the SBVA and SKhVA is controlled by mantle sources related to mantle plumes of the Central Asian hot mantle field.

We know that tectonomagmatic activity periodically increased during the Earth’s history without any visible external factors to cause these occurrences. This is obviously related to the evolution of petrological processes at depth that produce events in the outer shells of the modern Earth (the tectonosphere). However, the essence of these processes and the mechanisms that translate them to the tectonosphere remain little known. We have examined this problem for the particular case of the Late Cenozoic (Neogene to Quaternary) global activation. We know that the modern Earth is a cooling body with a solidifying liquid iron core. The process must be accompanied by a number of thermodynamic, physical, and physicochemical effects, and it is these which might cause the inner activation of our planet. We have tried to shed some light on these problems using available modern geological, petrological, geochemical, and geophysical data on the activation that is just now occurring before our eyes. We have shown that the main active element on the modern Earth must be a thin crystallization zone that is constantly rising; that zone is between the wholly solidified part of the core (the solid inner core) and its completely liquid part (the outer liquid core). It is this zone which harbors various phase transitions in a cooling melt as the melt is passing bifurcation points. The phase transitions are both of the type like a change in released solid phases that accrete to the inner core and as retrograde boiling producing drops of core fluids. It is shown that the drops are rising in a high-Fe host melt and are accumulated at the base of the mantle. Once there, they participate in the generation of mantle plumes which are the chief translators of deep impulses to the outer geospheres, and leave the core for good simultaneously with impulses. It is supposed that at one such point, fluid solubility experienced a sharp drop in the cooling high-iron liquid of the outer core. This must have led to a simultaneous intensification of retrograde boiling of this melt throughout the entire surface of the core crystallization zone, that is to say, on a global scale. It is this phenomenon which must have supplied the excess of core fluids necessary for mass generation of mantle plumes and have served as a trigger for processes involved in the Late Cenozoic global tectonomagmatic activation of the Earth.

This paper provides information on the 2022 eruptive activity of Ebeko Volcano. Phreatic explosions had been occurring in the crater lake from January 22 to June 13 due to water seepage through a plug in the upper part of the magma conduit with subsequent boiling. Vulcanian type explosions started since June 14 and dried the lake. The ash particle-size distribution changed toward smaller sizes. Petrographic, mineralogical, and geochemical studies of the tephra define this period as a phreatomagmatic eruption based on the presence of fresh juvenile material. Interaction between magma and waters of the Ebeko hydrothermal system results in its depletion in alkali and enrichment in silica. We hypothesize that the formation of amorphous water-bearing silica in the form of numerous segregations and its subsequent dehydration can favor the volcano’s explosive activity.

This study examines and compares 3D distributions of crustal and upper mantle density contrast with a set of geological and geophysical data for the heads of six plumes (Yellowstone, Emeishan, Cathaysia, Sea-of-Okhotsk, Maya–Selemdzha, and Indigirka–Kolyma plumes) down to 200 km depth. According to our data, the asthenospheric parts of the plumes have mushroom shapes, while the asthenospheric magmas are spreading out beneath the lithosphere bottom, less frequently beneath the crustal bottom. The plume heads become narrower at distances of 250–300 km from the central conduit to decrease to diameters of 200–300 km at depths of 100–120 km. In most of the cases, the plume lithospheric and crustal fragments are convex toward the ground surface. The uplifts are occasionally complicated with local depressions in the upper crust, which can be explained by subsidence of the domes of the structures above magma chambers in the subcrustal viscous layer and asthenosphere. Plumes are frequently associated with zones of lithosphere tension (rifts), resulting in linear zones of lower viscosity being mapped in the lower lithospheric and crustal cross sections of the plumes. The structural settings of the plumes under consideration here are controlled by boundaries of lithosphere plates and large segments of the second order. The identity of geometry and rheology in the plumes that were formed at different times (Triassic to Neogene), and in regions far removed from each other (Northeast Russia, Amur region, northwestern United States, South China, and Sea of Okhotsk), provide evidence of the universality of the tectonic settings that favor the penetration of mantle flows into the upper tectonic shells of the Earth. The foremost among these are tension zones in the lithosphere, especially areas where differently directed lithospheric faults intersect.

To explore an efficient approach refining seismomagnetic information through magnetic field survey technology, the author introduces the seismomagnetic monitoring system of the capital region in China. It includes field measurement techniques, data processing methods and application performance. The variation characteristics of the crustal magnetic field and seismomagnetic information in this area are systematically calculated and statistically analyzed based on the geomagnetic data products of the X, Y, and Z components. The results show that the mean amplitudes of X, Y and Z of the crustal magnetic field in the study area are 1.87, 1.90, and 1.39 nT, and the mean square deviations are 2.41, 2.48, and 1.94 nT, respectively. The epicenters of future large earthquakes will mostly lie near the “0” value line of crustal magnetic field variation with very low amplitude, which X, Y and Z elements are 18.7, 45.5 and 53.9% of the mean values in the survey area, respectively, and rise obviously after the earthquake. There is a high correlation between the location of the epicenter and the anomalous region of low spatial variation ratio of geomagnetic field secular variation.

We have studied the geological structure, material composition of the ores, and the age of the Nizhny Birkachan volcanogenic gold–silver deposit discovered recently. The ore bodies consist of veins and vein-streak zones of adularia-carbonate-quartz composition; they lie in granodiorite porphyries with U‒Pb zircon age (ID-TIMS) equal to 335 ± 2 Ma. The ores are low sulfide, low silver (Au/Ag = 1–2), with pyrite dominating the ore minerals. The Ag minerals are tennantite, Ag sulfide, native gold and silver, and hessite. From an ore vein we obtained an adularia-based ⁴⁰Ar/³⁹Ar age equal to 169 ± 4 Ma, which reflects the rejuvenation of the isotopic argon system after the emplacement of a dike of unaltered Jurassic basites that cuts through the ore body. The Nizhny Birkachan deposit has a geological structure and ore composition that are very similar to those of other Au-Ag deposits at the Kedon volcano-plutonic belt such as Kubaka and Birkachan; it was also formed in the age span 290–335 Ma.

Crustal magmatic structures possess pronounced resonant properties, which enable these structures to generate secondary seismic waves at their eigen frequencies. Strain data acquired with the help of a 75-meter laser interferometer were used to identify resonant modes and to estimate the parameters of the magmatic structures beneath the Elbrus Volcanic Center. Such resonant modes are unique features for each magmatic feature, and determine the size and physico-mechanical properties of an identified internal structure. The present study contains an analysis of a local feature that has manifested itself as a compact area in the form of weak seismic pulses that have been recorded in the Elbrus area using small-aperture seismic instruments operated by the Geophysical Survey of the Russian Academy of Sciences (GS RAS) in 2011. The results of these studies based on seismic and strain observations, as well as on the results of microseismic sounding, have shown the existence of a new peripheral shallow magma chamber 2.5‒3 km across in size as part of the Elbrus Volcanic Center.

This paper presents new results from geological-stratigraphic and isotope geochronological studies of young lavas in the northeast part of the Javakheti Highland, Lesser Caucasus, Georgia. We provide a first description of the Algeti complex-structured valley lava flow about 55 km in total length; no information on this feature is available in the geological literature. We show that young magmatism in the northeast part of the Javakheti region has been evolving in the time interval of 3.2–1.5 Ma B.P. Its earlier phases have produced the longest (up to 100 km) valley basaltic lava river of those known in the Lesser Caucasus (the Khrami flow) (3.19 ± 0.10 Ma B.P.). Subsequently (2.7–2.5 Ma B.P.), the eruptions continued to form extensive lava plateaus there (Tsalka, Gomareti, and other plateaus). In the Late Piacentian–Early Gelasian (2.7–2.0 Ma B.P.) the active vents in the northern Javakheti Range started to form the Algeti basaltoid valley flow, with this process lasting for ~1 Ma. The terminal phase of its formation (1.9–1.5 Ma B.P.) was probably related to eruptions of the volcanic cones in the area of Lake Tabatskuri. These data, along with the reconstruction of the history of young magmatism, enabled us to trace the main patterns in the generation of the present-day relief and the network of river valleys in the area of study in the Lesser Caucasus.