Home – Home – Jornals – Russian Geology and Geophysics 2023 number 4

Experimental modeling of the interaction of garnet of eclogite and lherzolite parageneses with CO₂ fluid was carried out on a multi-anvil high-pressure apparatus of a “split sphere” type (BARS) in Pt ampoules with inner graphite capsules, using a buffered high-pressure cell with a hematite container, at a pressure of 6.3 GPa in the temperature range 950-1550 ºC. It has been established that the main interaction processes at 6.3 GPa and 950-1250 °C are partial dissolution, recrystallization, and carbonation of garnet, which result in the formation of magnesian carbonate, kyanite, and coesite, a decrease in Mg contents in the recrystallized garnet, and the formation of carbonate, silicate, and oxide inclusions in it. Under these conditions, crystallization of metastable graphite and growth of diamond on a seed at ≥1250 °C were observed. In the temperature range 1350-1550 °C, the experimental garnet underwent partial dissolution and recrystallization in the CO₂ carbonation fluid; no carbonation took place. These processes were accompanied by a decrease in the portion of the grossular component in the garnet and by the enrichment of the fluid phase with calcium. We have established the indicative characteristics of garnet that interacted with CO₂ fluid: zoning, with low contents of CaO and MgO in the peripheral zones of crystals relative to the cores, and the presence of inclusions of carbonates, kyanite, coesite, and CO₂. The compositions of the produced garnet and carbonates are consistent with the data on these minerals in mantle peridotite and eclogite parageneses and in inclusions in diamonds, which suggests a significant role of metasomatism involving CO2 fluid in the evolution of deep-seated rocks and in the diamond formation. In this experimental study, we have first studied the processes of diamond crystallization and determined the boundary conditions for diamond growth in the system silicate-carbonate-CO₂ simulating natural diamond-forming media. In general, the established regularities can be regarded as potential indicators of mantle metasomatism and mineral formation involving CO₂ fluid.

Modeling the compositions, properties, and conditions of generation of natural agents of mantle metasomatism is one of the most topical subjects in experimental petrology. Particular attention is paid to the study of C- and S-bearing metasomatic agents and their role in the global carbon and sulfur cycles and in the processes of natural diamond formation. Experimental studies aimed at the estimation of sulfur solubility in carbonate melts under lithospheric mantle conditions were carried out on a multianvil high-pressure apparatus of the “split-sphere” type (BARS) in the carbonate-sulfur ((Mg,Ca)CO₃-S) and carbonate-pyrite ((Mg,Ca)CO₃-FeS₂) systems at 6.3 GPa and 1050-1550 °C for 20-60 h. It has been experimentally established that the main processes occurring in the carbonate-sulfur system are the recrystallization of carbonate in a sulfur melt (1050-1350 °С) and the generation of a high-calcium carbonate melt with dissolved sulfur (5.0-6.5 wt.%) (1450-1550 °С) as well as graphite recrystallization and the initial stage of diamond growth (1550 °С) in this melt. The work demonstrates that the carbonate-pyrite interaction is accompanied by the recrystallization of carbonates and pyrite (1050-1250 °С) and the generation of two immiscible melts (sulfide one with dissolved oxygen and carbonate one with dissolved sulfur (1.7-2.5 wt.%) (1350-1550 °С)) along with the formation of graphite and the growth of diamond on seed crystals (1550 °С). It has been found that the solubility of sulfur in carbonate melts increases with temperature, which goes together with a decrease in CaO (±FeO) concentrations and an increase in MgO concentrations. The present study has shown for the first time that melts of alkaline-earth carbonates are capable of dissolving up to 6.5 wt.% sulfur and they are probable sulfur concentrators under the conditions of the lithospheric mantle.

V-Cr spinel of wide composition range is present as a rock-forming or accessory mineral in the parametamorphic rocks of the Sludyanka Complex (South Baikal area, Siberia, Russia). The spinel can be subdivided into two large groups: magnesian spinel and ferroan spinel. In this paper we describe ferroan spinel. It is subdivided into three groups: (1) Zn-V-bearing and vanadium spinel (up to 5 wt. % ZnO and up to 19.5 wt. % V₂O₃); (2) V-bearing zinc spinel and vanadium zincochromite (up to 6 wt. % V₂O₃ and 5.0-20.3 wt. % ZnO); and (3) Zn-V spinel and zincochromite (>6 wt. % V₂O₃ and 5.0-22.2 wt. % ZnO). A separate small group of ferroan spinel is represented by ternary chromite-coulsonite-magnetite solid solution. In addition to the Slyudyanka spinel, we present a summary of metamorphic chromium spinel and vanadium spinel from other deposits. The behavior of Cr, V, and Zn in metamorphic and magmatic spinel is discussed.

A 6-meter core of the Holocene sediments in Lake Dukhovoe is studied. The material composition of the core is heterogeneous and has clear bedding. Lacustrine sediments are represented by the upper 293 cm. Each bed of the sediments is subjected to microbiological analysis. A high total number of heterotrophic bacteria is revealed in the upper (0-15 cm) and deeper (110-150 cm) sapropel intervals. Sulfate-reducing bacteria are identified only in Chrysophyceae cysts. The CaO/Fe₂O₃ ratio shows the degree of “carbonate content” of bottom sediments. It is low in the sapropel of the lake (0.2), which indicates the geochemical specificity (carbonate-free sapropel). The Na, Mg, Al, Si, Al, Ca, Fe, Mn, Zr, and Cr concentrations in sapropel are within the average concentrations for the Earth’s crust, shale, and oceanic pelagic clays, while U and Mo exceed them. Phytoplankton is enriched in phosphorus (biogenic element) and chalcophile elements (Zn, Cd, Sn, Sb, Hg, Pb, and Cu), which characterize the pollution of the modern atmosphere of the Baikal region. Diagenetic processes result in the transformation of the pore waters, namely an increase in mineralization as compared to lake water, an increase in the concentrations of biogenic components (HCO₃^-, NH₄⁺, NO₃^-, and PO₄^3-) and a decrease in SO₄^2-. Pyrite framboid accumulations are revealed in organomineral sediments below the horizon of 200 cm, and iron phosphates represented by vivianite are identified in sapropel.

The paper provides the results of experiments with sediment traps in the deep-water part of the South Basin of Lake Baikal (depth of 1366 m), installed from March 2015 to March 2016 in order to study recent sedimentation within the lake. We present new data on total fluxes of particulate matter and fluxes of biogenic components (SiO_2biog, С_org, and N_tot) at different depths of the water column both for the whole year and for individual periods of the year. Diatom analyses were carried out for all obtained samples. The total flux of sedimentary material averaged 94.9 g/m²/y; the average fluxes of SiO_2biog, С_org,and N_tot were 23.9, 11.6, and 0.94 g/m²/y, respectively. The molar C/N ratio varies from 11 to 21 and indicates a predominance of allochthonous material in almost all samples. Maximum fluxes of sedimentary matter were recorded from 20 June to 20 July 2015. This period corresponds to the bloom of diatoms of the species Synedra acus . This species amounts to >94% of the total diatom content in all samples taken during this year. The recent predominance of Synedra acus in the water column, as well as in the surface bottom sediments of South Baikal, is probably due to the climate warming.

Sources are sometimes approximated by magnetic dipoles or current loops. The usefulness of such sources is not obvious at the initial stage of the study of objects. In order to investigate them properly, study them in detail, volume magnetic field sources are needed. In this paper, they are represented by magnetized prisms. Such a model is valid due to the equivalence of current and magnetized objects. As there should be no real magnetization in the core, this property of a virtual prism to generate a magnetic field strength is referred to as virtual or effective magnetization (EM), which is determined for each prism as by solving the inverse problem via the adaptive method. The initial data for solving the inverse problem are the Z components of the vectors of the main magnetic field of the IGRF-2005 model in the geocentric coordinate system. Based on the effective magnetization and known formulas, the distribution of bulk currents, their density, and the magnetic moments of prisms of a two-layer core model is obtained. Their sum coincides with the magnetic moment of the virtual central dipole of the nucleus, but, as many scientists assumed, the central dipole is not actually distinguished. At the same time, four global inhomogeneities are identified in the core, which create the Canadian, Siberian-Asian, Australian, and negative South Atlantic global anomalies on the Earth’s surface. Based on the analysis of the obtained results, assumptions are made that the current generating the magnetic field is the movement of a positively weakly charged liquid of the core. The fluid motion is created by the rotation of the Earth and the decelerating gravitational forces of the Moon and the Sun. Arguments are given to support these assumptions. The results are illustrated in figures.

We consider the possibility of using circular polarizations, which make the field vector in a wave rotate clockwise or counterclockwise, as independent polarizations of the primary wave during magnetotelluric sounding (MTS). This approach has been proposed for the first time to search for possible manifestations of the Hall effect under natural conditions during MTS; in this case, if the effect is small, waves with circular polarization become normal modes in a horizontally layered medium located in the constant magnetic field of the Earth. Standard impedance relations are replaced by the corresponding apparent resistance impedances and curves for waves with circular polarization (mode). In the search for manifestations of the Hall effect during MTS, it becomes necessary to determine differences in the mode apparent resistivity curves due to the presence of lateral inhomogeneities of the medium. This study deals with the specific features of MTS in the case of representing the primary source field by circular polarization waves. Results of the theoretical analysis of mode impedances are demonstrated. Numerical simulation is used to investigate the differences in the mode impedances and curves depending on the nature of the distribution of lateral inhomogeneities in the medium. Mode apparent resistivity curves obtained by processing the experimental data of the MTS in Gorny Altai are presented. It is shown that, in comparison with standard curves, mode curves are subject to less distortion by lateral inhomogeneities of the medium owing to the specific features of distributions of the charges and currents excited by circular polarization waves in them.

The study is aimed at the theoretical development and scientific substantiation of a high-resolution transient electromagnetic (TEM) geophysical technique for monitoring the state of permafrost. The electromagnetic-monitoring configuration includes spatially distributed sounding systems for the surface and borehole locations of the field sources and receivers. Changes in the electrophysical properties of the earth can be traced for the cases of both ground thawing and freezing. We developed the theory of TEM signals modeling, which served as the basis for an algorithm for rapid and accurate numerical simulation of the electromagnetic field. The signals are scrutinized from various sounding systems in the basic earth models as a function of recording time. We show how the features associated with the spatial arrangement of frozen rocks appear in the emf diagrams. It follows that a set of the measured data enables visual detection of the boundary between frozen and thawed rocks.

The paper focuses on the wide-azimuth, broadband, and high-density (WBH) seismic data application methodology, which was used to complete a more detailed structural interpretation of the K oilfield, to identify a number of low-relief structures in its periclinal parts, and to detect the potential residual oil zones (ROZ) in the oilfield. The obtained wide-azimuth, high-density field data and the results of broadband inversion are the main factors that increase the degree of ROZ prediction. A comprehensive analysis has shown that potential ROZ are arched faulted-nose structures in the periclinal parts of the oilfield, low-relief anticlines in the periclinal parts of the oilfield, and lithologic pinchout zones. Technical support has been provided for identifying ROZ in the given oilfield, and the basis has been laid for predicting the residual oil distribution in analogous oilfields with high productivity and high water cut that are at the middle and late stages of development.