Mantle Matter Research Articles

Mechanisms of Crustal-Mantle Material and Energy Exchanges: Impacts on Hydrocarbon and Geothermal Resources.

The exchange of matter and energy between crust and mantle significantly influences the formation and development of oil, gas, and geothermal resources. Understanding how these exchanges impact these resources is crucial in geological science. In many oil-rich basins in China, significant accumulations of H2, CO2, geothermal energy, and other associated resources linked to deep mantle materials or geological processes have been discovered. Therefore, investigating the effects of crust-mantle material and energy exchanges on these resources is of utmost importance. This paper aims to systematically analyze the effects of mantle materials (e.g., H2, CO2, catalytic elements) and energy upwelling into basins. It synthesizes the impacts of various organic-inorganic interactions on hydrocarbon generation, evolution of organic source rocks, reservoir development, and the formation and accumulation of oil and gas. Finally, it proposes a mechanism detailing how interactions between mantle matter and energy influence specific resources. Simultaneously, this study employs numerical simulations to uncover the specific impact of magma intrusions on the geothermal field of surrounding rock formations. It demonstrates that magma chambers, continuously supplied with mantle energy, create regional thermal anomalies and facilitate the development of high-temperature geothermal resources. This underscores that mantle materials and energy not only significantly influence the generation of oil and gas but also govern the formation of geothermal resources and the evolution of thermal reservoirs. This research provides a theoretical foundation for understanding the subsequent formation of oil and gas resources under the influence of deep geological processes. Moreover, it furnishes a scientific basis for evaluating and exploring the symbiotic relationship between oil and gas resources and geothermal reservoirs.

To the problem of origin volcanoplutonic processes

In the article from the position on the Concept of the Dynamics of the Earth’s Crust Evolution (CDECE) the problems of the origin of volcanoplutonic processes and related endogenous ore formation are considered. It is established that geodynamic forces are created when the Earth rotates around its axis, all global geotectonic processes occur under the influence of these forces. These forces in the face of the Earth spread with certain regularities. Movements of lithospheric masses on the surface of the upper mantle occurs under the influence of these forces. It is known that the geospheres of the Earth have different physical characteristics. Therefore, they react differently to the Earth’s rotation. For this reason, mass shifts occur between the differently characterized geospheres. Because of these displacements between geospheres physical-chemical phase transformations form, where decomposition of mantle matter occurs, which is associated with the increase in the volume of mantle matter, that further, is associated with the formation of primary magma, which is associated with the manifestations of volcanoplutonic processes and earthquakes. These physical and chemical phase transformations theoretically occur in all zones between different geospheres of the Earth. However, the most intensive manifestation of them occurs between the Earth’s crust and upper mantle, associated with the increase in the radius of rotation of the Earth. The speed of movement of lithospheric masses is connected with the regularities of distribution of geodynamic forces, which have differential character of development this is connected with geometrical parameters of the Earth where the directions of movement of lithospheric masses, occur from west to east. In these conditions, the rate of movement in near equatorial zones have a maximum value, and in the direction of the poles reduce to zero. This is due to the fact that the Earth’s radius is located perpendicular to its axis of rotation. Therefore, the velocity of lithospheric masses movement in its equator have maximum values, which in the poles decrease to zero.

The origin, conditions and mechanism for the formation of alpine-type hyperbasites of the Lesser Caucasus

The problem of hyperbasites origin is one of the widely discussed topics in geology. This is because they often appear when no expected. Their development does not correspond to the general regularities of the geological complexes’ development. Therefore, when problematic instances of hyperbasites appear, discussion is inevitable. This is due to the imperfections of existing concepts, which are not without flaws. The essence of this concept lies in the fact that during the rotation of the Earth around its axis, geodynamic forces are formed. The hyperbasites complex by its nature belongs to deep igneous formations formed at the initial stage of development of volcano-plutonic processes, where the composition of magmatic products was not subject to decomposition. In general, the origin of igneous rocks is associated with deep anomalous processes, which were formed under the influence of geodynamic forces, where decompression of mantle matter occurs, causing a catastrophic increase in the volume of mantle matter, as well as the associated development of volcanoplutonic processes. Hyperbasites are formed both in divergent and convergent zones of the Earth’s crust. The main factor for their formation is high pressure – deep thermodynamic conditions, where there are no favorable thermodynamic conditions for the complete separation of magmatic melts by composition. The emergence of hypermafic rocks on the surface is associated with geotectonic or denudation processes. Denudation processes can expose only those hypermafic formations that are located at the site of formation. These zones include ancient platforms, sheets, terranes, etc., which were cut by deep erosion processes. As for those hyperbasic formations that are classified as alpinotype hyperbasites, they were moved to the structure of the Alps-Himalayan folded zone from the basement with a collision with subsequent geotectonic processes, where they formed in the bed of the Paleotethys Ocean, both in the process of divergence and convergence. The noted pattern of formation and the mechanism of formation of alpine-type hyperbasites clearly corresponds to the patterns of development of geodynamic forces in the face of the Earth, also with natural laws, which are the main factors in the evolution of the Earth’s crust. From the standpoint of KDEZK, the origin, mechanism of formation, as well as the form of distribution of alpinotype hypermafic rocks of the Lesser Caucasus occurred in the Paleotethys bed, under different thermodynamic conditions and at depths. Further, as a result of the collision, it participated in the formation of folded zones of the Lesser Caucasus. Within the Lesser Caucasus, two genetic types of hypermafic rocks are exposed. Some of them correspond to the convergent zone of the Tethys paleocean, while others correspond to divergent zones. In terms of ore content, the most promising are those hypermafic rocks that are genetically related to convergent zones.

Phlogopite phenocrystals with inclusions of chrome spinelides from non-diamond-bearing kimberlites: the case of An-195 body of the West Ukukit field

The article presents the results of a microprobe study of the composition of phlogopite macrocrystals with inclusions of chrome spinelides from An-195 kimberlite body of the West Ukukit field; Yakut kimberlite province. The purpose of this study was to establish the genesis of such phlogopites: whether they are representatives of deep mantle matter or their formation is directly related to the formation of kimberlites composing An-195. It was found that the inclusions of chrome spinelides are confined only to chromic non–zonal phlogopites or to chromic sites of zonal macrocrystals (Cr2O3 0.75–1.55 wt.%; TiO2 2.67-3.84 wt.%; Mg# 83-88). Based on a comparative analysis of the studied phlogopites and chromespinelides with the composition of analogues from the parageneses of these minerals in various mantle xenoliths of ultrabasic rocks; eclogites and rocks of the MARID series; it was concluded that these phlogopites most likely represent a phenocrystal association of kimberlite minerals. They clearly differ from phlogopites of the MARID series rocks with equal Mg# and TiO2 in higher Al2O3 contents; from phlogopites of ultrabasic mantle rocks xenoliths – generally with lower magnesia content. Phlogopites from eclogites have a generally higher Cr2O3 content. Inclusions of high–chromium spinelides in phlogopite (Cr2O3 44-50 wt.%; TiO2 3-5.5 wt.%; Fe3+ < 0.35 f.e.) correspond in composition to spinelides from cataclysmic dunites and are products of their recrystallization under the influence of high-temperature mantle fluid. Inclusions of less chromic spinelides (Cr2O3 30-44 wt.%; TiO2 4-7.5 wt.%; Fe3+ > 0.35 fe) probably crystallized simultaneously with phlogopite macrocrystals and were captured by them during long-term co-growth.

Tectonics and gravity field structure of Central Kazakhstan

Purpose. Identification of the nature of the manifestation of tectonic elements of different ages in Central Kazakhstan in gravitational fields based on the results of the calculation of regional, intra-crustal and local transformants. Methodology. Synthesis and analysis of the data on integrated interpretation and modeling of gravitational, geomagnetic, geothermal fields, the latest movements of the Earth’s crust and parameters of the seismic regime, tectonics and stratigraphy of rocks. Findings. Regional, intra-crustal and local heterogeneities in the lithosphere manifest themselves differently in blocks of Precambrian rocks, Early and Late Caledonides, Early and Late Hercynides. They may be associated with the processes of Paleozoic intracontinental rifting, with the rise of mantle matter and its emplacement into the Earth’s crust, followed by the manifestation of Late Paleozoic orogenesis, doubling of the thickness of the Earth’s crust, outpourings of magmatic formations. Originality. It is established that large gravitational minima are distinguished in areas with Hercynian folding, characterized by abnormally high amplitudes in the movement of the Earth’s crust. In the regions of the Caledonian folding, the values of gravity field anomalies of intermediate intensity and increased amplitudes of the latest movements of the Earth’s crust are manifested. Areas with Pre-Paleozoic folding have relative maxima of gravitational anomalies and minimum values of the latest movements of the Earth’s crust. Earthquake sources are concentrated in the consolidated crust at the junction of areas with different ages of basement consolidation, in gradient zones of geothermal, geomagnetic and gravitational anomalies. According to the variations of the intra-crustal transformant, it was found that a wide range of changes in the values of the gravitational field corresponds to areas with minimal temperature values, whereas in areas with increased temperature values, the range of changes in the values of gravity anomalies is reduced. The distribution of the local transformant of the gravitational field indicates the existence of highly variable anomalies, which reflects the high-frequency gravitational effect of near-surface objects of the Earth’s crust. Practical value. The distribution of inhomogeneities in the lithosphere with various density, geomagnetic and geothermal anomalies of geophysical fields, the nature of the seismic regime and the latest movements of the Earth’s crust predetermined the formation of geostructures with different types of mineralization, each of which is recommended to be searched and explored by a specific rational set of geophysical methods.

Трансформация геосистем Байкальской природной территории

The article presents the main provisions of the concept of transformation of geosystems of geodynamically active territories. The object of the study is the geosystems of the Baikal natural territory. The interaction of the Siberian Platform, the East Sayan Mountain region, the Baikal rift zone, the Angara-Vitim batholith influenced the formation of various conditions for the transformation of geosystems. Modern ideas about the influence of endogenous processes on the transformation of the lithosphere and climate are considered. The transformation of geosystems is understood as the restructuring of their structure due to transformative dynamics and evolution, which develop under the influence of the modification of material-energy flows and information connections. Examples of transformation of geosystems of the Baikal natural territory under the influence of endogenous heat and mantle matter, changes in information connections of components of geosystems and their subordination to geosystems of physical and geographical areas are given.

Структурные и геохимические особенности золоторудных районов Приамурской провинции

The article discusses structural confinement and geochemical features of localization of three historically formed gold ore districts in the Amur gold-bearing province: Solovyovsk, Gonzha, and Tokur, presenting the data on the gold deposits in the districts, from which, since 1890, the main amount of gold has been extracted. It is shown that the structural elements, determining the position of gold ore districts, are the zones of ore-controlling faults: South Tukuringra, North Tukuringra, and Jeltulak. The distinct spatial connection of the native gold and cinnabar dressing halos with the North-Tukuringra and South-Tukuringra fault zones has been established. The significant amounts of mercury in the native gold of the Amur province, as well as high gold content in cinnabar and native mercury are noticed, which may indicate the genetic proximity of the gold and mercury mineralization of the province. It is assumed that the main source of gold and mercury is the mantle matter, initially rich in these components. It is established that, in the Amur gold-bearing province, the principal geochemical feature of ore-controlling fault zones is gold and mercury mineralization confined to them.

TWO STAGES OF THE CENOZOIC ALKALINE-BASALT VOLCANISM IN THE DARKHAD DEPRESSION (NORTHERN MONGOLIA) – GEOCHRONOLOGY, GEOCHEMISTRY, AND GEODYNAMIC CONSEQUENCES

The isotopic data showed that there are two stages distinguished in the Cenozoic history of the Darkhad depression volcanic activity, the Late Oligocene initial stage (~28.0–26.6 Ma) and the final Late Miocene – Early Pliocene stage (~5.8–4.2 Ma). It has been stated that the rocks of the initial stage are only represented by trachybasalts; however, among the final-stage basaltoids there are series of shield-volcano hawaite-basanite-phonotephrite rocks and compex trachybasaltic "valley" lava flows, the formation of which is the last stage in the territorial volcanic evolution. It has been shown that the initial-stage trachybasaltic andesites are characterized by their enrichment of TiO2, P2O5, Sr, Zn, Ga and low concentrations of Al2O3, MnO, CaO, Sc and HREE (La/Yb=27.2–30.2). Basaltoids of the final stage have a similar rare-element distribution and show an increase in the contents of TiO2, Al2O3, P2O5, LILE, HFSE, Th, U and in the degree of fractionation of REE (La/Yb from 12.2 to 20.9) towards the rocks alkalinity enhancement. Modeling of eclogite, pyroxenite and peridotite melting processes in the La/Yb – Sm/Yb system shows that trachybasaltic andesite melts could be formed at ~7–8 % melting of eclogitic matter or at ~10–11 % melting of Grt-containing pyroxenites, with trachybasalt formed at ~3 % melting of Grt-containing peridotites. The composition distribution of rocks in coordinates (Mg# – Fe/Mn) indicates that the parental magmas are the initial-stage trachybasaltic andesite magmas as well as the Early Pliocene trachybasaltic "valley" lava flows. Sr, Nd, Pb isotope characteristics of the Darkhad depression basaltoids show significant shift of isotopic ratios in time towards the relatively enriched mantle as compared with the depleted MORB mantle. The initial formation of trachybasaltic andesite melts occurred in the Late Oligicene at the pre-rift stage of the territory development involving metasomatized mantle matter, with the pyroxenite or eclogite component contained in the magma formation source. The origin of trachybasalt magmas of the final stage is associated with the processes of decompression melting of peridotites in a weakly metasomatized lithospheric mantle at the rift stage of the Darkhad structure development.

Late Mesozoic–Cenozoic Tectonics and Geodynamics of the East Arctic Region

Abstract Tectonic and geodynamic models of the formation of the Amerasian Basin are discussed. The Arctic margins of the Chukchi region and Northern Alaska have much in common in their Late Jurassic–Early Cretaceous tectonic evolution: (1) Both have a Neoproterozoic basement and a complexly deformed sedimentary cover, with the stage of Elsmere deformations recorded in their tectonic history; (2) the South Anyui and Angayucham ocean basins have a common geologic history from the beginning of formation in the late Paleozoic to the closure at the end of the Early Cretaceous, which allows us to consider them branches of the single Proto-Arctic Ocean, the northern margin of which was passive and the southern margin was active; (3) the dipping of the oceanic and, then, continental lithosphere took place in subduction zones southerly; (4) the collision of the passive and active margins of both basins occurred at the end of the Early Cretaceous and ended in Hauterivian–Barremian time; (5) the collision resulted in thrust–fold structures of northern vergence in the Chukchi fold belt and in the orogen of the Brooks Ridge. A subduction-convective geodynamic model of the formation of the Amerasian Basin is proposed, which is based on seismic-tomography data on the existence of a circulation of matter in the upper mantle beneath the Arctic and East Asia in a horizontally elongated convective cell with a length of several thousand kilometers. This circulation involves the subducted Pacific lithosphere, the material of which moves along the bottom of the upper mantle from the subduction zone toward the continent, forming the lower branch of the cell, and the closing upper branch of the cell forms a reverse flow of matter beneath the lithosphere toward the subduction zone, which is the driving force determining the surface kinematics of crustal blocks and the deformation of the lithosphere. The viscous dragging of the Amerasian lithosphere by the horizontal flow of the upper mantle matter toward the Pacific leads to the separation of the system of blocks of Alaska and the Chukchi region from the Canadian Arctic margin. The resulting scattered deformations can cause a different-scale thinning of the continental crust with the formation of a region of Central Arctic elevation and troughs or with a breakup of the continental crust with subsequent rifting and spreading in the Canadian Basin.

Морфология и текстурно-структурные особенности хромититовых залежей Главного рудного поля Кемпирсайского массива (Южный Урал, Казахстан)

In the paper data of morphology, textural and structural features of chromitites from deposits of south-east part of Kempirsay massif (South Urals, Kazakhstan)are summarized. It is showed that formation of unique chromium deposits is closely related with formation processes of wall dunite-harzburgite association and that chromitite localization occur abidingly in olivine monomineralic rock – dunite. Superimposed low-T processes altered primary mineralogical composition of wall peridotites completely but these affected weakly their structure on the micro and macro scale. Mesh serpentine replaced olivine and pyroxene grains but pseudomorphosis of both are survive. Addition, significant displacements of mineral aggregates in the massive peridotite blocks are not observed and it allow to study textural and structural characteristic of chromitites and primary wall ultramafic rocks. We have found some major features of building of ore-bearing associations as follow: (i) increasing chromite grain size according to increasing concentration of chromite, (ii) widespread of deformational structures – ore folding and boudinage, extrusion of solid dunite into massive chromitite, break of ore veinlets. We have performed retrospective analysis of papers about Kempirsay chromitite which in present day are not available for wide readers. Based on this analysis and our observations, we propose a modified dynamic model of chromitite formation as result rheomorphic differentiation of upper mantle matter during its upwelling from deep zone of rift structure with later transformation in the upper mantle of fore-arc setting.

ГЕОХИМИЯ НЕОГЕНОВЫХ ВУЛКАНИТОВ СЕВЕРНОЙ ЧАСТИ ЦЕНТРАЛЬНО-КАМЧАТСКОГО ВУЛКАНИЧЕСКОГО ПОЯСА

New geological and geochemical data on the age and composition of Neogene rocks in the northern part of the Central Kamchatka volcanic belt are presented. K-Ar dating of volcanic rocks determined the age of all studied volcanic rocks to be Late Miocene. The geochemical characteristics of the rocks show that the northern part of the Central Kamchatka volcanic belt is characterized by a complex geological history, in which magma generation processes are governed by the type of mantle matter. The “earliest” among the Late Miocene rocks that make up the Umuvayam and Tolyatovayam complexes (6.7–9.0 Ma) are represented by typical island-arc rocks. Whereas, “later” (5.4–7.7 Ma) volcanic rocks of the Alney and Veemgetver complexes are characterized by an association of rocks with a high proportion of enrichment from the OIB-type source. The magma source corresponded in composition to spinel lherzolite. The enrichment of volcanic rocks in large-ion lithophile elements is explained by the role of fluids introduced into the melts during the melting of the supra-subduction mantle wedge, which probably also underwent modification as a result of supra-subduction metasomatism in the geological past.

Evolution of the Late Mesozoic Magmatism of the Omulevka Terrane of the North Part of the Verkhoyansk–Kolyma Orogenic Region

This article presents the results of a study of Late Mesozoic intrusive formations of the Omulevka terrane of the Verkhoyansk–Kolyma orogenic region. The research area covers the Selennyakh block of the Omulevka terrane and the territory adjacent to the south. The compositions of rock-forming, accessory and restitic minerals and geochemical features of intrusive rocks are considered. The methods of optical microscopy, microprobe, silicate and spectral analyses were used. There are the following several stages in the evolution of magmatism: (1) the Late Jurassic supra-subduction (gabbro, dolerites), (2) the beginning of the Early Cretaceous-transitional from supra-subduction to marginal-continental (gabbro-diorites, diorites, granodiorites), (3) the Early Cretaceous of active continental margin (granodiorites, granites), (4) the Late Cretaceous postorogenic or continental-riftogenic (alkali-feldspar granites of A-type), (5) the Late Cretaceous continental riftogenic (subalkaline gabbroids and basaltoids). In the process of evolution from stage one to stage four, there was an increase in the silicic acid content, total alkalinity and ferruginousity of rocks with the movement of magmogeneration levels to higher and higher horizons of the lithosphere (calculated pressure from 1.6–1.4 GPa to 0.6–0.9 GPa). At the same time, the preservation of high temperatures of magmogeneration (1000–1150 °C) and crystallization implies the supply of additional heat from an external (deep) source during the formation of granitoid melts. The magmatic activity is completed by the intrusion of subalkaline derivatives of a deep hearth, formed by metasomatized lherzolites. All the studied igneous rocks are either direct mantle fusions, or bear signs of the participation of mantle matter in the generation of parent melts in crustal substrates: the presence of tschermakite in gabbroids, nonequilibrium structures, the composition of early generations of biotites corresponding to biotites of mantle and crust-mantle derivatives, the presence of pyroxenes and accessory minerals characteristic of mantle magmas in granitoids. In the diagram Al-Na-K-2Ca–Fe + Ti + Mg, the composition points of the studied intrusive rocks tend to the mixing trend. In general, the research results suggest that the evolution of the Late Mesozoic intrusive magmatism of the studied territory and the specific matter of rock compositions were caused by the crust-mantle interaction as a result of the rise of mantle diapirs in the crust from a long-existing deep hearth of the main melt.

Density inhomogeneity of the Earth’s crust of the Black Sea and adjacent territories from three dimensional gravity modelling. Part II. Density sections

To obtain a more complete picture of the deep structure and density heterogeneity of the main tectonic structures of the Black Sea megadepression and adjacent territories, density sections were constructed that intersect structures along geotraverses, DPS profiles and illustration profiles. Based on the results of three-dimensional gravity modelling, the Scythian plate has a block structure, more complex than the East European platform. It is also characterized by a thick layer of deformed sediments, with the largest compaction of the crust is being in its western and southern parts. The pre-Dobrudzha Trough consists of two parts: the denser one, which adjoins the Serpentine Rise, and the less dense one, adjacent to Zmeiny Uplift. The Karkinitsky Trough is characterized by compaction of the middle and lower parts of the Earth's crust in the south and west, that is associated with tectonic processes during riftogenesis, namely, the introduction of mantle matter into the lower and middle parts of the Earth's crust. There is no «granite» layer under the Black Sea-Kalamitsky Swell, the «basalt» layer is represented by a crust-mantle mixture. The deep domain of the Indolo-Kuban Trough is divided into two blocks along the axis: denser (south, west and southwest) and less dense (northeast, east and north). In Crimea, in all density sections a thick layer of dislocated sediments is observed, the crystalline crust is characterized by significant compaction (especially the southern part), the absence of a «granite» layer, the occurrence of the subsurface «basalt» layer and the presence of the thick crust-mantle mixture. The West Black Sea Depression is characterized by the absence of a granite layer in the central part, the occurrence of sediments on the «basalt» (and sometimes on the «diorite») layer. In this case, the crust is of the oceanic type. In the East Black Sea basin there is a thin layer of dislocated deposits with a density close to the density of crystalline rocks. According to the density value, this layer can be attributed to very compact dislocated deposits that suggests the oceanic or rifted continental crust. The crystalline crust of the Central Black Sea Rise is heterogeneous in composition and structure. The Andrusov and Arkhangelsk Ridges, which echelonedly displaced to each other, have different density characteristics of the crystalline crust, the depth of its bottom and density on it. Collectively, these features give evidence of the heterogeneous history of the formation and development of these structures. The Sinop Depression is a large Neogene graben which is underlain by a crust-mantle mixture of different thickness. The Sorokin Basin is characterized by an increased thickness of the sedimentary cover, the absence of a «granite» layer and a rise in the roof of the «diorite» layer to a depth of 2―3 km in the eastern part, as well as the presence of a «granite» layer with a thickness of over 10 km in the western. The northwestern part of the Shatsky Uplift is characterized by decompression of the entire Earth's crust, and in the southeastern part, decompression is observed only in the middle part of the section.

Unsolved problems of the global tectonics and possibility of their solutions

The deep geological and geophysical studies of the continents and oceans have revealed a number of well-defined new regularities in the structure of the crust and upper mantle that do not find a clear explanation in the modern geodynamic concepts. The regularities are the following. The Earth is divided into two hemispheres with different structure of the lithosphere: the Pacific and Indo-Atlantic hemisphere. The Pacific hemisphere is surrounded by a ring of the tectonically active zones with high seismicity (Benioff Zones). The system of the mid-ocean ridges with approximately equal distances between them, 90°, is symmetrical relatively to the South Pole. The crust in the oceans is different in age and composition, it identified the remnants of an ancient (Archaean) crust and large areas of subcontinental crust. The continents are characterized by the large thickness of the lithosphere (more than 200 km), composed of the lower density depleted matter. Experimental data on petrophysical properties of the crust and upper mantle matter at high pressure and temperature, the data on deep xenoliths, geochemical studies of natural gases have showen a large role of deep energy-intensive fluids in the formation of the sialic crust and depleted mantle rocks. These data give possibility to explain the continents and oceans origin. The irregular in space the Earth degassing results in different lithosphere types formation: the thick granite-gneiss crust and the lower density depleted mantle of the continents were created in the areas of the higher deep fluids flows; in the areas of the lower fluids advection the primary oceanic crust was preserved and only some separate spots of the transition crust appeared. The lower density of the continental «roots» was the main factor in the formation of continents: the lower density lithosphere led to its emerging in respect to the oceanic lithosphere. Two hemispheres with different lithosphere structure were formed may be due to the elliptical form of the orbit, causing periodic changes of the planet accelerations. The structural symmetry of the global rift system relative to the planet poles gives possibility to explain its origin by the expansion of the planet.

Mantle Flow and Determining Position of LAB Assuming Isostasy

The assumption of isostatic equilibrium is often used in geophysics; e.g., the depth of the lithosphere–asthenosphere boundary (LAB) is determined using data regarding the gravity field and topography, together with the assumption of isostasy. However, isostasy implies a hydrostatic state, which is contrary to the mantle convection hypothesis, which states that most of the mantle matter is in motion. We therefore discuss herein the question of when the assumption of isostasy can be used. It is suggested that isostasy may be used for parts of oceanic plates (except for subduction zones, hotspots, and oceanic ridges) and for many continental regions (except for postglacial regions and regions of intensive volcanic or tectonic activity). Moreover, using the results of a numerical model of convection and calculations of dynamic topography, it is shown that some generalization of isostasy is possible in the form of deep dynamic isostasy (DDI). It is also indicated that, for some regions without isostatic equilibrium (e.g., postglacial regions), it is possible to use the “isostatic” method with some corrections to the results.

Dispersion of group velocities of Rayleigh and Love waves and anisotropic properties of the Asian continent

We have studied the structures of the Earth’s crust and upper mantle of the Asian continent using a representative sample of dispersion curves of group velocities of fundamental-mode Rayleigh and Love waves for more than 3200 seismic paths. Maps of distributions of variations in group velocities with periods of 10 to 250 s over a spherical surface were calculated by the 2D tomography method. The maps reflect the deep structure of the Earth’s crust and upper mantle of the study area and give a tentative idea of the horizontal distribution of the anisotropic properties of the mantle matter. The obtained data are confirmed by the calculations of the velocity profiles of SV- and SH-waves for the entire Asian continent and for its regions. Vertically, anisotropy is observed to the depths of ~250 km, with its maximum in the depth range from the bottom of the crust to 150 km.

The Mesoproterozoic mantle plume beneath the northern part of the Siberian craton

The study of the Mesoproterozoic (1473 ± 24Ma) dolerites of the Olenek uplift of the Siberian craton basement has shown their petrologic and geochemical similarity to typical OIB produced with participation of a mantle plume. The dolerites are characterized by variations in the geochemical composition explained by different degrees of melting of the same source. A conclusion is drawn that the parental melts of the rocks were slightly modified by crustal contamination, as evidenced from their Nd isotope composition (£Nd(T) = +0.6 to −0.8) and the presence of inherited zircons of four ages (2564, 2111, 2053, and 1865Ma). Since the Siberian craton in the structure of the Nuna supercontinent (Columbia) was located relatively close to the Baltic continent and the Congo and Sao Francisco cratons, we assume that the Early Mesoproterozoic mafic intrusions (1500–1470Ma) of all these cratons belong to the same large igneous province (LIP). The province formation was related to the activity of superplume (or mantle hot field), which supplied mantle matter to the lithosphere basement. The superplume core was probably located beneath the northern part of the Siberian craton, where basites are compositionally most similar to the primary mantle source.

Toward postplate tectonics

Half a century ago, principles of the theory of lithosphere plate tectonics, or plate tectonics, were first formulated. Since then, the theory has become substantially more complicated and tectonic processes and phenomena have been identified that are not described by the theory. This relates to certain types of vertical movements, primarily to the newest uplifts that led to the formation of modern mountain systems. Comparison of geological processes (both those described by the theory of plate tectonics and those unexplained thus far) with data of the seismic tomography of the mantle has made it possible to outline a new tectonic model, according to which the source of observable tectonic manifestations is lateral flows of upper mantle matter, propagating from superplumes—flows of matter and energy rising from the mantle’s bottom. These lateral flows not only move lithospheric plates but also determine structural–substantial transformations of the lithosphere and the upper underlithospheric mantle, which lead to vertical movements and mounting building.

Isotope fractionation of carbon during diamond crystallization in model systems

A systematic experimental study of fractionation of carbon isotopes during diamond crystallization in model systems near the IW and CCO buffers helped to estimate the effective partition coefficients of carbon isotopes between diamond and crystallization medium. In the systems Fe(Ni,Co)-C, near the IW buffer, diamond is heavier than the solution of carbon in metal melt by 4.5%c at 5.5GPa and 1400-1500°C. In the system (Na2CO3CO2)-C, near the CCO buffer, diamond is lighter than the carbonate fluid by 2.6%c at 7.5GPa and 1400-1700°C. The values of fractionation are close but not equal to calculated equilibrium values and decrease as the rate of diamond crystallization increases. With regard to the low effectiveness of carbon isotope diffusion in diamond, the effective partition coefficients of carbon isotopes obtained during real diamond crystallization are the most informative for interpretation of data for natural diamonds. Based on the experimental results, we propose a scheme of the primary isotope specialization of diamonds. Isotopically heavy diamonds (513CVPDB of 0 to -5%c) crystallize in zones of metal melts (in the case of isotope depletion, 513CVPDB decreases to -10%c or lower). Isotopically light diamonds (513CVPDB of -7 to -10%c) crystallize in more oxidized mantle zones. The interaction of different types of mantle matter with contrasting redox characteristics causes wide variations in the carbon isotope composition of diamond and in the composition of diamond-hosted inclusions.

Geochemistry of metabasites of the Kolpakov Group of the Sredinny crystalline Massif in Kamchatka

Metabasites (amphibolites, garnet amphibolites, and basic crystalline schists) compose numerous sheeted bodies (often highly boudined) from a few to 100 meters thick in the plagiogneisses and migmatites of the Kolpakov Group. Chemically, they are reconstructed as basalts and picrites that were metamorphosed, as host terrigenous rocks, under the kyanite-sillimanite subfacies of the amphibolite facies (t = 620–650°C, Ps = 5.9–6.9 kbar). Metabasites are dominated by amphibolites and basic crystalline schists distributed throughout the entire section of the Kolpakov Group, whereas garnet amphibolites are more typical of the upper parts of the group, where they are intercalated with amphibolites, basic crystalline schists, plagiogneisses, and quartzites. Metaultrabasites (plagioclase-free amphibolites) occur much more rarely as small boudins up to few meters in size. According to U-Pb SHRIMP zircon dating, the plagiogneiss protolith age corresponds to the end of the Early-Late Cretaceous (90–100 Ma), which is similar in age to the weakly metamorphosed terrigenous deposits of the Kikhchik Group of the Sredinny Range. This allows us to consider the terrigenous rocks of these groups as isofacial sedimentary rocks. The same age (Early-Late Cretaceous boundary) was taken for protoliths of metabasites forming interbeds among metaterrigenous deposits of the Kolpakov Group. The interval of 100−90 Ma coincides with the beginning of the formation of the Okhotsk-Chukotka volcanogenic marginal-continental belt in East Asia. It is shown that the Kolpakov Group possesses the geochemical features of tholeiitic basalts of different geodynamic settings and comprises both typically island arc (low-Ti) and oceanic (moderate to high-Ti) tholeiites associated with ultrabasic volcanic rocks—picrites. Such a chemical peculiarity of basic rocks is typical of the marginal-continental extension zones (pull-apart basin) that were initiated on the sialic crust. It is obvious that similar geodynamic setting of the basite magmatism existed for the Sredinny Range of Kamchatka. The ascent of the mantle matter beneath the extension zone of the continental crust of the sedimentary basin and its intersection by faults that formed simultaneously with the Okhotsk-Chukotka volcanogenic belt served as the beginning of the basite volcanism in the sedimentary basin. They provided an intense fluid effect and a temperature increase in the crust with subsequent granitization and metamorphism of volcanogenic-terrigenous deposits and, finally, the development of the modern structure of the Sredinny Kamchatka Massif. The intense Late Cretaceous basite volcanism and associated granitoid magmatism in Kamchatka were presumably caused by the ascent of mantle plumes bearing hydrogen fluids.