Home – Home – Jornals – Russian Geology and Geophysics 2022 number 10

Xenoliths of high-alumina pyroxenites in the Quaternary basalts of the Sigurd Volcano of West Spitsbergen are spinel and spinel-garnet clinopyroxenites, spinel-garnet websterites, and websterites. The granoblastic texture with relics of subhedral magmatic texture, the change of mineral assemblages, and the signs of partial melting in the xenoliths reflect their multistage formation. The goal of our study was to determine the sequence and thermodynamic conditions of the change of mineral assemblages and to establish their age by Re-Os, U-Pb, Sm-Nd, and Rb-Sr isotope dating. It has been established that the primary assemblage in the pyroxenites, which included high-alumina Opx, Cpx, and Spl, was transformed in several stages: (1) Spl→Grt replacement with the formation of garnet-containing websterites and clinopyroxenites, (2) development of kelyphitic Opx-Spl rims over Grt grains, (3) formation of Amph, (4) exsolution with the formation of Cpx and Opx, and (5) partial melting. Comparison of the obtained results with published data shows that the primary assemblage is similar in Al₂O₃/MgO ratio to Opx + Cpx ± Spl cumulates resulted from the crystallization of basaltic melts at 1.2 GPa and the degree of crystallization of ~15%, i.e., in the lower crust (at 2.0 GPa, Grt and Cpx crystallize from the solution). The equilibrium parameters of the Grt-Opx assemblage in the pyroxenites are 1060-1310 ºC and 2.2-3.3 GPa; in the P-T diagram, their points are localized below the Spl→Grt phase transition curve, in the system CaO-MgO-Al₂O₃-SiO₂, thus corresponding to the model continental geotherm with a surface heat flow density of 60 mV/m² and somewhat higher. This indicates the Spl→Grt replacement and the formation of garnet-containing pyroxenites in the shallow-depth upper mantle. The formation of kelyphitic Opx-Spl rims over the Grt grains indicates a subsequent temperature and pressure decrease to values above the Grt→Spl phase transition curve. This is confirmed by the presence of exsolution structures in pyroxene, which formed when the temperature decreased by 100-150 ºC. The Re and Os isotope composition in the bulk samples of Spl-Grt websterites that did not undergo partial melting corresponds to an age of 457.0 ± 3.5 Ma, which reflects the time of transformation of the primary Cpx-Opx-Spl assemblage into a garnet-including one. A similar value (488.6 ± 5.9 Ma) was obtained by U-Pb dating of zircon from Spl-Grt websterite, also without signs of melting. Zircon crystals from Spl-Grt clinopyroxenite with clear signs of partial melting have typomorphic features of autochthonous magmatic zircons. They form a single age cluster of 310.7 ± 3.3 Ma, which marks the age of melt crystallization in the pyroxenites. Thus, Spl pyroxenites are, most likely, Opx + Cpx ± Spl cumulates, products of crystallization of basaltic melts in the lower crust. The subsequent Spl→Grt replacement and the formation of garnet-containing websterites and clinopyroxenites in the shallow-depth upper mantle can be regarded as an indicator of the delamination of the continental crust into the mantle, and the Re-Os isochron date of 457.0 ± 3.5 Ma is the most likely upper age bound of the crust delamination into the mantle. The subsequent uplifting of the Spitsbergen lithosphere, which was expressed as the formation of kelyphitic Opx-Spl rims over garnet, exsolution in pyroxene, and partial melting, was not far in time from the delamination stage and lasted ≤ 300 Ma.

The mineralogy of loess and loess-like sediments has important implications for the mechanisms of postdepositional alteration, as well as for environmental and geotechnical issues. The geotechnical properties of soils, their mechanical strength, and the bearing capacity of the soil foundations depend on the mineral species, their percentages, and relationships. Clay minerals control the mass transport processes which have bearing on pollution of ground and water. Loess covers vast territories on all continents, including southern Russia, West Siberia, southern East Siberia, the Russian Far East, Transbaikalia, and other regions. Loess and loess-like sediments have specific genetic, lithologic, geotechnical, stratigraphic, and geomorphic features. The mineralogy of Quaternary loess has been studied by X-ray diffraction, chemical, and particle size analyses, following the standard techniques, with reference to earlier published evidence. The study area comprises three watershed provinces in the southeastern Russian Plain (southern European Russia). The results have geotechnical and environmental applications for loess provinces.

The paper presents new data on the composition of organic matter and the molecular-weight distribution of saturated hydrocarbons in the thermal waters of the Annenskoe field. The TIC chromatograms of extracts of the thermal waters, obtained by capillary gas chromatography-mass spectrometry, revealed 98 organic compounds of biogenic, thermogenic, and technogenic origin. The technogenic components amount to no more than 5%. The geochemical odd-even preference indices of saturated hydrocarbons (CPI, ACL, OEP, and TAR_HC) are calculated. It is shown that organic matter, in particular, saturated hydrocarbons, results from two processes: thermogenic processes in the water-rock system and bacterial activity. Variations in the composition of organic matter in the Annenskoe thermal waters have been determined for the first time. They are characterized by minor fluctuations within 30 min and serious fluctuations over two years and are expressed as an increase in the portion of thermogenic alkanes and a decrease in the portion of nitrogen-containing components.

The demand for detailed bio- and lithostratigraphic research of the Prisayan Formation is significant because of the lack of its stratotype section. This leads to confusion about the meaning of the stratigraphic volume of the Prisayan Formation. On the basis of the results of a comprehensive research, a lectostratotype section that covers the whole section of the Prisayan Formation is proposed. The following three subformations are identified in the formation on the basis of lithological criteria: lower (Ust-Balei), middle (Idansk), and upper (Sukhov). Stratigraphic correlation of the Prisayan Formation with the coeval deposits of the West Siberian Plate is carried out. The common development trends of typical units of the flora of the Siberian paleofloral region are revealed in West Siberia and the Irkutsk Basin. The observed tendencies make it possible to compare the beds with Coniopteris murrayana , Sphenobaiera czekanowskiana from the lower subformation with the fossil plant assemblage of the Sharapovo Horizon; the beds with Coniopteris snigirevskiae from the middle subformation with the fossil plant assemblage of the Nadoyakh Horizon; and the beds with Raphaelia diamensis and Phoenicopsis irkutensis with the fossil plant assemblage of the Laidin Horizon of West Siberia. The Prisayan-1 palynoassemblage (PA) from the lower and middle subformations correlate with palynozones 5-7 of West Siberia. The Prisayan-2 PA corresponds to palynozone 8 of West Siberia. The paleobotanical and palynological data suggest that the stratigraphic volume of the Prisayan Formation is the upper Pliensbachian-Aalenian of the Lower-Middle Jurassic. This paper touches upon the history of accumulation of the Prisayan Formation, as well as the evolution of the paleorelief and vegetation.

The Permian-Triassic magnetostratigraphy of the central East European platform has been updated using evidence from the Zhukov Ravine, Slukino, and Okskiy Siyezd reference continental redbed sequences. The new data reveal five rather than four polarity subzones in magnetozone R₃P, which thus can be ranked up to orthozone RnP. The upper Permian strata of the Zhukov Ravine and Okskiy Siyezd sections contain intervals of anomalous paleomagnetic directions similar to those revealed earlier in roughly coeval rocks from the Nedubrovo section in the Vologda region. The anomaly may be of regional extent and may correspond to quite a long (thousands of years) span of geomagnetic field stability near the Paleozoic-Mesozoic boundary, which fits the subequatorial dipole field in the first approximation. The paleomagnetic results were used to constrain the position of three new Permian-Triassic paleomagnetic poles of the East European platform, corrected for inclination shallowing: PLat = 54.8°, PLong = 173.6°, dp / dm = 2.6°/4.0° (Zhukov Ravine); PLat = 47.6°, PLong = 164.3°, dp / dm = 3.0°/4.8° (Slukino); PLat = 52.7°, PLong = 153.6°, dp / dm = 3.7°/5.4° (Okskiy Siyezd).

Many attributes and inversion transformations solving the problem of extracting information on the elastic properties of a medium are based on the Zoeppritz equation or its approximations calculated for a boundary between two half-spaces. In a real medium, a seismic signal is in interference as a result of reflection from the top and bottom of individual thin layers or units whose elastic properties differ from those of the adjacent rocks. In such media, there is a discrepancy between experimental and theoretical dependences of the reflection coefficient amplitude developed for half-spaces. For a homogeneous thin layer, the change in the frequency response of the reflection coefficient is associated with the reflection coefficients of the layer top and bottom interfaces and the wave traveltime within the layer at the same time. A detailed analysis of the reflection coefficient amplitude of a homogeneous thin layer using the spectral decomposition technology allows us to isolate only the change in the reflection coefficients, regardless of the thickness. The proposed method for predicting the elastic properties of thin layers is based on the analytical derivation of the reflection coefficient of a homogeneous thin layer, according to which there is a parabolic relation between the squared reflection coefficient amplitude and the squared frequency. The approximation coefficient at the zero degree of the argument mainly characterizes the properties of a medium, unlike the first- and second-degree coefficients, which depend both on the properties and on the thickness of the layer. Therefore, it makes sense to use the approximation coefficient at the zero degree of the argument as an attribute of the wavefield to predict the elastic properties of thin layers in the interwell space. A good example of a relatively homogeneous thin layer is a narrow riverbed. Determining the properties of the rock filling the channel feature helps identify more sandy areas promising for further development of the field.

The methodology for building a 3D density model of the Earth’s crust and upper mantle for a crustal block in the South Kamchatka Peninsula is presented. The model is derived from the interpretation of the Bouguer anomaly values and data obtained by deep seismic soundings and electrical prospecting methods. 3D image creation can be viewed as a several phase procedure. At the first step, the area of interest is investigated through a network of intersecting profiles utilized for subsequent modeling depth density sections using the parameter scanning procedure. The a priori data include materials of geophysical surveys and geological, and petrophysical studies as well as results of depth estimation at singular points by the Euler deconvolution method. The next step involves formation of a 3D density matrix from the cross-section models. Coordinates in the cross-section planes are defined in the square grid nodes and converted to plane rectangular coordinates (the SK-42 reference system). A file of the cross-sectional database of density values is thus produced. For area shaving a sparse network of cross-sections, an additional database is derived from grid files of densities mapped horizon-wise. This ultimately results in the creation of the unified databases. A 3D network of cubic unit cells for three-dimensional (voxel) representation of density of the geologic medium is calculated. The model of density of unit cube with edge measuring 4 km was constructed for entire investigated block of the Earth’s crust. The model was detailed by decreasing successively the unit cube edges to 2.0, 1.0, and 0.5 km. The 3D modeling results are presented as block diagrams and density isosurface maps in horizontal slices at different depths. The block diagrams show isosurfaces with densities typical of deep subsurface boundaries and structures. The isosurfaces for densities of 2.75 and 2.90 g/cm³ reflect the morphology of the roof of the crystalline basement, and the lower-upper crust boundary, respectively, while the isosurfaces of 3.20 and 3.33 g/cm³ represent feasible shape of the crust-upper mantle contact. The lower-density zone ubiquitously observed in the investigated crustal block is caused by rock destruction along the deep fault. It is identified in the model with 1 km cubic unit cell side and encompasses the feeding systems of volcanoes, namely: Sopka Asacha (Asacha Volcano), Mutnovskaya Sopka (Mutnovsky Volcano), Gorelaya Sopka (Gorely Volcano), and Vilyuchinskaya Sopka (Vilyuchansky Volcano).The model with a 0.50 km cubic unit cell side shows a 2.85 g/cm³ isosurface, which delineates a column-shaped high-density region. A higher density revealed in the block is assumed to be associated with basite intrusions. 3D density models based on cross-section modeling provide additional information for geodynamic reconstructions, allowing a rough estimation of the geometry of concealed geologic features and their volumes, and can be utilized in 3D modeling to build initial models.