Home – Home – Jornals – Russian Geology and Geophysics 2020 number 10

We have studied volcanogenic diamonds in the context of a discussion of their genesis, including some assumption on their artificial origin. The carbon isotope composition of diamonds collected from the eruption products of Tolbachik Volcano (δ¹³C_VPDB from -22 to -29 ‰) is within the range of the δ¹³C_VPDB values of natural diamonds, including those from kimberlites. The ¹⁵N_Air values of the Tolbachik diamonds, measured for the first time (-2.58 and -2.32 ‰), correspond to δ¹⁵N_Air of volcanic gases and differ from that of atmospheric nitrogen (δ¹⁵N_Air = 0 ‰), which may be expected in synthetic diamonds. In the studied volcanogenic diamonds, as in synthetic ones, the nitrogen impurity is unaggregated. However, such an unaggregated form of nitrogen is specific to many natural diamonds (e.g., variety II diamonds, according to Orlov’s classification). Impurity elements (Cl, F, O, S, Si, Al, Ca, and Na) are locally concentrated in volcanogenic diamonds; they are a constituent of micro- and nanoinclusions in them. The high contents of F and Cl in the studied diamonds are correlated with the composition of volcanic gases; there is no reason to expect a similar correlation in synthetic diamonds. Moreover, the studied cube-octahedral Tolbachik diamonds have a number of accessory forms, some of which are not observed in synthetic diamonds. Their surfaces are frequently covered with films composed of Mg-Fe and Ca-Mg silicates, aluminosilicates, sulfates, metal alloys, and native Al. Mineral inclusions in the studied diamonds are Mn-Ni-Si alloys and silicides varying in composition from (Mn,Ni)₄Si to (Mn,Ni)₅Si₂, Mn5Si2, and pure Mn silicide MnSi. Summing up the obtained data, we conclude that volcanogenic diamonds form in a strongly reducing environment, in which silicides and native metals and their alloys are stable. The predominant cube-octahedral morphology of these diamonds and the unaggregated nitrogen impurity point to their short-term residence under high-temperature conditions. This makes them similar, to some extent, to synthetic diamonds. There are, however, clear differences as well. Volcanogenic diamonds are similar in compositional peculiarities, including isotope compositions, to natural diamonds that form under most unfavorable conditions, such as cuboids, balases, carbonado, and some diamonds of the eclogite paragenesis. They also resemble diamonds found in situ in harzburgite and chromitite of ophiolites. This suggests a specific mechanism of formation of both volcanogenic and ophiolitic diamonds in the oceanic lithosphere.

The paper discusses the mechanisms of formation of high-purity quartzites from the Bural-Sardyk deposit. Based on a stadial analysis of the sedimentary-metamorphic sequence, we interpret the succession of postsedimentary transformations of high-silica rocks. Comprehensive studies using geological and geochemical methods and isotope research data reveal the role of tectonics, magmatism, metamorphism (regional and contact), and dynamic metamorphism in the formation of high-purity quartzites. Two main varieties of tectonites have been identified and described: stress-metamorphic tectonites and cataclasites. The paper presents a new geological map of the Bural-Sardyk deposit on a 1:10,000 scale.

We present results of geochemical and Sr-Nd isotope studies of rocks and of local dating and determination of the Lu-Hf isotope composition of zircons from late Vendian-early Cambrian and Cambrian-Ordovician intrusive associations (granitoids and gabbroids) of the Kaa-Khem and East Tannu-Ola batholiths in Eastern Tuva. The wide ranges of the ε_Nd values (6.9 to 0.5) of rocks and the ε_Hf values of magmatic and inherited zircons reflect the diversity of the magma sources of late Vendian-early Paleozoic intrusive associations formed at the island arc and accretion-collision stages. Late Vendian (572-562 Ma, Kopto and Buren massifs) and early Cambrian (522-518 Ma, East Tannu-Ola batholith) island arc tholeiitic and calc-alkalic plagiogranitoids resulted from the melting of the Vendian-early Cambrian island arc crust without the contribution of a more ancient crustal material. The subalkalic gabbro-monzodiorite-granosyenite association of the Zubovka massif (510 Ma) formed from a mantle source depleted isotopically but enriched in incompatible elements, with the participation of an island arc crust material; this process took place in the early phase of plume activity at the accretion-collision stage. Island arc complexes were the main source of Cambrian-Ordovician accretion-collision calc-alkalic plagiogranitoids (500-450 Ma, Terektyg-Cheder, Karaos, Tapsa, Baisyut, and other massifs). Variations in their composition were due to the melting of thick crust, whose isotopic heterogeneity was caused by the different contributions of a more ancient crustal source. The crust of the Tuva-Mongolian terrane made the main contribution to the formation of the potassic granitoids of the Bren’ massif (450 Ma), marking the completion of accretion-collision processes in this region. The isotope parameters of the Vendian-early Paleozoic granitoids are indicators of the crust formation and evolution in the course of subduction and accretion-collision processes.

The evolution of the eastern part of the Mongol-Okhotsk Orogenic Belt marks the late Mesozoic accretion-collision processes involving the Siberian and Sino-Korean plates. Tectonic restructuring in the region was accompanied by the formation of igneous complexes, which replaced each other almost without a temporal break. Andesitic magmatism was widely manifested in the southern framing of the eastern part of the belt. Its study is complicated by the isolation of volcanic fields and their confinement to the Amur-Zeya depression. Study of the geochemical and geochronological characteristics of igneous complexes there made it possible to recognize and systematize the temporal stages and substantiate the geodynamic formation of these rocks. However, there are controversial geologic objects, such as the Isikan volcanic field. We present the first data on the isotopic age and chemical and isotope-chemical compositions of the rocks of this field. ⁴⁰Ar/³⁹Ar dating showed that the integrated age of the dacitic matrix is 113.0 ± 2.6 Ma and the plateau age is ??121.0 ± 1.6 Ma. In chemical composition the studied volcanic rocks are similar to rocks of the Poyarkov volcanic complex developed within the Bureya-Jiamusi superterrane, which formed under subduction in the period 120-105 Ma. In geochemical and isotope-geochemical characteristics the volcanics are similar to rocks of igneous belts of Andean-type active continental margins.

A high-amplitude negative δ¹³С anomaly (-11 ‰ V-PDB) has been found in the carbonate horizons of the Chistyakovka Formation of the Taseeva Group in the type sections of the southern Yenisei Ridge. The δ¹⁸О values (-2.7 to -5.4 ‰ V-PDB) and the lithologic features of carbonates indicate that these are primary sedimentary rocks and their isotope parameters reflect the primary isotope composition of the paleobasin waters. The negative δ¹³С excursion in the Chistyakovka Formation is close in stratigraphic position and amplitude to similar isotope excursion in the Vendian strata of the southern Siberian Platform and to the global middle Ediacaran Shuram-Wonoka anomaly. According to the data on clastic zircons, the age of the Chistyakovka Formation is no older than 580 Ma, which agrees with the earlier estimated age of the Shuram-Wonoka event. As follows from the data obtained, the age of the basement of the Taseeva Group is no older than 600 Ma. This provides a more accurate correlation of the Vendian complex along the southwestern margin of the Siberian Platform. The different stratigraphic scales of the sediments indicate that the marginal trough here began to form at different times (from late Riphean to late Vendian), which explains its segmented structure.

Deep and ultradeep oil and gas have been the prominent direction of international exploration in recent years. The deep oil and gas resources are distributed predominantly in three domains of carbonate, clastic, and igneous rocks. The carbonate rock reservoirs contain about 40 % of global oil and gas reserves owing to their superior reservoir properties. In recent years, a number of large oil and gas fields have been discovered in deep marine carbonate rocks in China, especially in the Sichuan and Tarim basins, which shows a vast oil and gas exploration potential in these rocks. In this paper, the genesis and source of deep natural gas in the Tarim and Sichuan basins have been systematically studied. The results show that the natural gas in deep marine carbonate rock reservoirs is generated not only from the later deep-source rocks. The substantial cracking of crude oil and pyrobitumen in the reservoir under conditions of great burial depth and high geotemperature can also provide abundant natural gas sources for deep reservoir. Accordingly, the contribution of gas from oil cracking should be considered in the evaluation of deep natural gas resources.

The bottom sediments of lakes with different mineralization located in the basins of inland areas are high-resolution archives of climate and environmental changes. We present results of study of the Late Pleistocene-Holocene sediments of freshwater Lake Baunt, one of the lakes of the Baunt (Tsipa) depression in the permafrost zone in northern Buryatia. The sediments were studied by XRD, IR spectroscopy, laser granulometry, elemental analysis, AMS dating, etc. Mineral analysis of the bottom sediments with an age of ~18 ka has revealed predominant phyllosilicates, quartz, and feldspars. Mathematical modeling of complex XRD spectra made it possible to identify chlorite, muscovite, illite, mixed-layer illite-smectite and chlorite-smectite, and kaolinite among the phyllosilicates. We have determined their structural and crystal-chemical features and quantitative proportions in the section (800 cm long core), which vary in accordance with the climate cycles and lake level fluctuations. The results obtained helped to reconstruct the evolution of the Lake Baunt basin controlled by the regional climate throughout the Late Pleistocene-Holocene. This paper continues a series of our publications concerned with the reconstruction of the Late Pleistocene-Holocene climate in East Siberia by comprehensive studies of the mineral composition of sedimentary sections of small lakes.

Structural and geological research has shown that the extension of the Russian Plate caused a neotectonic transition of the deformed material from the Voronezh and Volga-Kama anteclises to neighboring syneclises, primarily the deepest Peri-Caspian. The resulting geologic structures and landforms morphologically resembled landslides, but their large (up to many hundreds of kilometers) lateral sizes excluded the influence of exogenous processes and made it necessary to treat this event as a gravitational collapse involving a significant crustal mass. This phenomenon was previously considered for orogens only, but the conditions for its appearance here existed too: In the late Cenozoic, the East European Platform experienced a collisional pressure from the southern boundary of the Eurasian Plate, which led to a gravitationally unstable uplift. The collapse was best manifested in the zones of junction of anteclises and syneclises, where lenses of the thickened continental crust are thinned beneath anteclises. This fact, along with the published modeling data, gave grounds to state that the platform collapse was favored by the lenticular crustal structure: The light crustal lenses forming anteclises floated up under compression and diverged under subsequent decompression. Gravitational extension and subsidence in the anteclise apexes was compensated by compression near the syneclises, where folded dislocations and neotectonic ramparts formed. It has been established that a gravitational collapse results in a specific paragenesis of platform structures and morphostructures, which can help to identify its manifestations in other places and to make an unbiased assessment of the relationship between the relief formation and tectonics. In general, the data obtained suggest a relationship between the recent activity of passive continental margins and the gravitational collapse of the largest lenses of the continental crust.

The paper considers the basic regularities of inductively induced polarization (IIP) manifestation in the transient electromagnetic method. When calculating inductive transient responses of a polarizable ground, we assumed that the electrical conductivity of geologic materials is described by the Cole-Cole model. To present and analyze the results of computer-based simulation of the IIP phenomenon, we used a normalized transient response. It is defined as the ratio of the transient electromotive force (EMF) response of a polarizable ground to the EMF response of a ground that differs from the polarizable one only in that it has zero chargeability. The coordinates of the minimum of the normalized transient response are useful for an overall assessment of IIP manifestation. We show, by the example of a homogeneous polarizable half-space, how the inductive transient response is affected by the Cole-Cole model parameters and the size of transmitter and receiver loops. For a two-layer ground, IIP manifestation depends also on whether the base or upper layer is polarizable, as well as on the layer thickness. Inductively induced polarization is most pronounced when a conductive polarizable layer overlies a resistive nonpolarizable base. In this case, at a certain thickness of the layer, the IIP effect far exceeds that observed in the presence of a thick polarizable layer and even of a homogeneous polarizable half-space.

Marine geoelectromagnetic sounding with artificial sources is strongly hindered by the influence of a conductive seawater layer. There is only one known wide successful application of electrical prospecting in this field - Controlled Source Electromagnetic Method (CSEM). However, this method has unfortunate limitations: the need to submerge an electromagnetic probe to the bottom of a deep (more than 1000 m) sea and the great rafting (~15 km). The method is not applicable in an ice-covered sea. Deep sounding from the sea surface and, hence, from the ice surface is possible if the TM polarization field is used. This field is generated by a vertical electric line (VEL) or a circular electric dipole (CED). The former has drawbacks even when it is used at sea. At the same time, a CED is efficient in one-dimensional and three-dimensional media in frequency and time modes. We have developed a three-dimensional mathematical tool for the CED field in the Born approximation, which is quite adequate in a conductive section with deep local inhomogeneities. The research is carried out within the framework of a geophysical project using the Arctic drift ices.