Home – Home – Jornals – Russian Geology and Geophysics 2025 number 1

The article presents new data on multiphase inclusions in two diamonds sampled from placers in the northeastern Siberian craton. Diamond HLS-4 is a round variety V crystal containing a multiphase mineral inclusion with more or less strongly oxidized iron carbides and moissanite in one part and calcite and iron oxides in another part. Another sample, HI-180, is a partly dissolved yellow cuboid that hosts a multiphase inclusion and numerous submicrometer inclusions delineating the crystal zoning. Sample HI-180 is deformed, with cracks and cavities up to 200 µm in size exposed in a polished section. The submicrometer inclusions plot in the field of microinclusions in fibrous diamonds from the world database, mostly near the silicic corner. They must have a hydrous composition, judging by the water-carbonate ratio of H₂O/(H₂O + CO₂) = 0.80-0.82 estimated from FTIR data. The multiphase inclusion consists of quartz, Fe-armalcolite, anatase, and diamond grains in an amorphous matrix, as well as moissanite and calcite detected by SEM-EDS, FTIR and Raman spectroscopy. One diamond grain in the inclusion, in turn, encloses moissanite. The coexistence of calcite and moissanite in multiphase inclusions is evidence that cracks were fully healed up under extreme redox variations, possibly, during diamond growth in a subduction setting.

We present the results of Nd and Pb isotope studies of late Neoproterozoic-early Paleozoic granitoids of the Kaa-Khem batholith, which formed in different geodynamic settings within the early Caledonian structures of Eastern Tuva (Altai-Sayan sialic area). Based on the obtained isotope data, we assess the role of different source materials in the formation of melts for these granitoids and compare these rocks with coeval granitoids from the Lake Zone of Mongolia. The plagiogranitoid associations of the island arc stage of the Kaa-Khem batholith formation (572-562 Ma) are characterized by mantle Nd isotope values. In contrast, the Pb isotope characteristics of these intrusive associations correspond to an upper-crust source formed predominantly by terrigenous sediments. The granitoid associations of the accretion-collision stage of the Kaa-Khem batholith formation (512-450 Ma) have a less radiogenic primary Pb isotope composition as compared with the island arc granitoids. Evidently, during the magma formation at this stage, the relative role of terrigenous material decreased, while the role of depleted mantle material increased. At the same time, the decrease in ɛ_Nd values in the granitoids as compared with the depleted mantle indicates that, along with the upper-mantle component, the role of the lower-crust component increased. These Nd and Pb isotope characteristics of the accretionary granitoids of the Kaa-Khem batholith indicate a significant contribution of the lower-crust source to their formation. This source is the crystalline basement of the Tuva-Mongolian microcontinent. The Pb isotope characteristics of the accretion-collision granitoids of the Kaa-Khem batholith are shifted to the upper-crust compositions relative to those of the Lake Zone granitoids. The reason is that the latter formed predominantly through the transformation of early Caledonian island arc complexes, which were generated with a higher portion of mantle component.

The study area is located in the Eastern Sayan hydrogeological folded area. The objects of the study were groundwaters, surface waters and ground ice in the Sentsa River basin on the Oka Plateau. Cold and thermal groundwaters are associated with Proterozoic and Paleozoic metamorphic and igneous rocks. Their discharge as springs occurs in the river valleys along fault zones. Ground ice was studied in mineral frost mounds (lithalsas) composed of clays, clayey silts and silts of lacustrine-alluvial and fluvioglacial genesis. It has been established that thermal and cold groundwaters have HCO₃ Ca-Na compositions, river and lake waters, as a rule, have HCO₃ Ca compositions, and ground ice melts - HCO₃, SO₄-HCO₃ and NH₄-HCO₃ Ca. Thermal waters are largely enriched in Li, Be, B, Si, Mn, Ga, Ge, Se, As, Br, Rb, Sr, Cs, Ba and all REE relative to river and rain waters, and have the highest values of trace elements enrichment factor (EF_REE). The latter shows that atmospheric precipitation participates in the formation of the composition of groundwaters (cold and thermal) and surface waters. The specificity of the geochemistry of ground ice is determined by the composition of precipitation, the injection of ice-forming groundwater from taliks, the interaction in the water-rock system, and the presence of organic matter in unconsolidated sediments. The participation of river water and groundwater in the formation of the frost mound ice core is also evidenced by similar values of stable isotopes (δ¹⁸O, δD) in surface water, groundwater and ground ice. The ³He/⁴He ratio points to a possible influx of mantle helium into thermal waters, and δ¹³С points to the magmatic and thermometamorphic mechanisms of carbon dioxide accumulation in thermal waters. The ⁸⁷Sr/⁸⁶Sr ratio in travertines indicates a significant contribution of intrusive rocks to the formation of fluid composition.

Some intervals of the Jurassic-Cretaceous strata of the Anabar-Lena sedimentary basin have a certain oil and gas production potential, which can be realized in the synchronous offshore horizons of the adjacent territories of the Arctic shelf. Among the most representative objects in this regard are the outcrops of Upper Jurassic and Lower Cretaceous formations of the Nordvik Peninsula. The main data on the composition and structure of this section were obtained mainly at the beginning of the second half of last century. The results of complex mineralogical, petrographic, and lithochemical studies of the Urdyuk-Khaya and Paksa formations of Cape Urdyuk-Khaya of the Nordvik Peninsula presented in here enabled us to identify 10 boundaries for changing of sedimentation regimes of the paleobasin. It was found that the Urdyuk-Khaya Formation was formed mainly in the conditions of the shelf transition zone (moderate deep water) with low rates of terrigenous material intake, some depletion of oxygen in bottom waters, and a trend towards an increase in the depths of the basin. The basal stratum of the Paksa Formation was formed in offshore conditions with periodically occurring dioxic conditions in bottom waters and extremely low rates of terrigenous material intake. The overlying part of the formation was formed in various parts of offshore transition conditions with a gradual decrease in the depths of the basin, an increase in the oxygen content in the bottom layer of water, and the rate of terrigenous material intake. The main provenance area was igneous rocks of mafic, possibly intermediate composition. There was some influence of felsic igneous rocks, or ancient sedimentary rocks rich in quartz. The parent strata were subjected to moderate and severe chemical weathering, in a warm humid climate. The revealed features of the studied strata are similar to the characteristics of the same-age sediments of the lower reaches of the Anabar River, which determines their high correlation potential and allows us to judge the evolution of the western part of the Anabar-Lena basin.

Lithological studies of sedimentary rocks are often challenged by manifestations of the superimposed processes which are related neither to the stages of lithogenesis, nor to its zonality/direction. This can be exemplified by skarns in the carbonate-terrigenous sections of the unified Yukta and Nakokhoz-Kalargon formations of Devonian age which were studied in the area of Lake Baselak and the Ambardakh River basin during the expedition works. Since the latter contain potentially oil and gas-bearing strata in the area of the Yenisei-Khatanga regional trough, special attention was paid to the dark-colored lithotypes during the research. It was revealed that among the studied samples, the dark color of some of them was of mineral nature, rather than due to the presence of OM. In these samples, the mineral composition was studied in detail, with determination of the association of newly formed minerals, which allowed us to propose hypothesized mechanism and stages of their formation. The dark color of sedimentary rocks has been shown to be not a sufficient criterion for searches for oil and gas accumulation. New results of the field studies of pre-Triassic sedimentary complexes in the northern part of Kureyka syneclise are also presented.

The formation mechanisms of sedimentary basins are considered as a response of deep processes in the mantle, therefore they carry important information about the geodynamics and thermal regime of the lithosphere. For different sectors of the northern margin of the Siberian Platform, the dynamics of sedimentation and subsidence was reconstructed. The analysis of subsidence curves shows that during the late Paleozoic the sedimentary infill formed in the foreland basin environment. In the Late Permian-Early Triassic time, in the central and western sectors, the subsidence was accelerating due to the development of a thick trap complex; after the Permian and Triassic boundary the subsidence slowed down. During the period of trap magmatism, an anomalously high subsidence rate up to 4.8 km/ Myr in the central and up to 0.5-1.1 km/ Myr in the eastern and western parts was reconstructed. The high rate and short duration of accumulation of volcanogenic sediments can be explained by an episode of short-term extension under the influence of a plume, followed by a long period of thermal subsidence. Numerical modelling of the temperature regime near mafic intrusive bodies was carried out, which showed that when determining the paleoheat flow, the influence of trap intrusions can be traced up to 400-500 m from the contacts. Estimates of the paleoheat flow for the Permian-Triassic stage of tectonic evolution of the eastern sector were obtained. It was calibrated using the PetroMod software package, based on laboratory measurements of modern values of vitrinite reflectance for rock samples from wells, modern temperature and heat flow in the sedimentary cover. It was determined that trap magmatism occurred at temperatures increased to 100 mW/m², while the mantle component of the heat flow reached 38-72 mW/m²; it is several times higher as compared to modern one. The obtained paleoheat flow estimates for the Late Permian-Early Triassic stage appear to correspond to anomalously high values of modern continental rifts.

In 2019, employees of the Research and Production Enterprise Luch jointly with specialists of the Seismological Branch of the Geophysical Survey of the Russian Academy of Sciences (GS RAS) carried out river seismic exploration works using the common depth point method (CDP-2D) along a 170 km long profile in the lower reaches of the Vitim River. The research was carried out according to the original methodology developed in the Siberian branches of the GS RAS, using the Baikal instruments recording seismic signals continuously (in contrast to traditional cable bottom or ground-based systems with time-limited recording). In this case, patented Malysh air guns were used in the water for the excitation of elastic waves, which have been recorded by ground registration method on the riverbank. To this day, the archives store primary materials from each receiving point (about 7000, with a distance of 25 m between them) in the form of a continuous hours-long digital recording of seismic signals and noise. These materials make it possible to generate seismograms of significant duration - up to 23 s, equal to the time interval between wave generations, in contrast to 6-10 second seismograms traditionally used to construct sections of the upper part of the Earth’s crust. In addition, the 24-bit Baikal recorder makes it possible to record signals with an amplitude two orders of magnitude lower than the amplitude of seismic noise. On the seismic sections constructed by the processor at arrival times of up to 13-14 sec, due to an increase in the stack fold of signals up to 1000-2000, we can select low-amplitude waves, reflected from boundaries in the middle and lower parts of the Earth’s crust to the Moho boundary. High fold is achieved by increasing the size of the stacking site (bin) by several times. By processing seismograms, generated from archival materials, for the first time, we have a complete vertical section of the Earth’s crust along a 170 km long profile, using low-amplitude signals. The profile passes through the junction zone of the Angara-Lena monocline and the Bodaibo-Patom folded system. The proposed approach can be used to obtain preliminary (and relatively cheap) information about the deep structure of the Earth’s crust along profiles performed using the CDP-2D river seismic exploration method.

The paper presents theoretical results on mathematical modeling and numerical inversion of transient electromagnetic data for the monitoring of the state of the cryolithozone. Solutions to the direct and inverse problems are considered based on the vector finite element method, the Sumudu integral transform and the apparatus of artificial neural networks. We show the capability of spatial localization of thaw zones (taliks) with transient cross-borehole exploration systems in a geoelectric model that takes into account the dispersive properties of frozen rocks. The performance and accuracy of the developed algorithms are assessed.

This paper describes a technique for hierarchical neural networks based on the nearest neighbor method with preliminary clustering of the original training dataset and construction of a search cluster decision tree. This method is a promising alternative to neural network technologies with deep learning and has quite a few advantages: high learning rate, identification of objects with a low degree of similarity, and the ability to generalize and retrain. As shown by testing the hierarchical neural network method on real data from the West Siberian oil and gas province, predicting the oil saturation in the Vikulov suite interval is much faster and more efficient than inversion approaches to quantitative interpretation of seismic data while achieving fairly similar geological results. This characterizes the proposed method of hierarchical neural networks as an effective tool for the quantitative interpretation of seismic data to solve geological problems.