Vol 489, No 1 (2019)

New data on the composition and structure of the Sezym Formation of the Lower Permian on the western slope of the Polar Urals are presented. This formation rests disconformably on the Middle Carboniferous shallow marine limestones and is conformably overlain by the Artinskian deep marine clastic deposits. New data are important for the reconstruction of the Late Paleozoic paleogeography and the geodynamic setting of the northeastern sector of the European Craton.

The following U–Pb ages (Ma) were obtained for zircons from volcanic rocks and granitoids representing various complexes of the Nembonda Depression (the Anadyr segment of the Okhotsk–Chukotka Belt): 147.7 ± 2; 123.8 ± 1.7; 119.3 ± 1.7; 95.8 ± 0.5; and 80.55 ± 0.96. These results support the hypothesis of a significant Aptian igneous event in the Verkhoyansk–Chukotka region and provide the basis for the revision the boundaries of the Okhotsk–Chukotka Belt.

An Ediacaran complex of dike rocks has been identified for the first time in the Yenisei Ridge orogen. These igneous rocks are represented by basic, intermediate and acidic variants and formed in the conditions of the transform active continental margin at the stages of its development before and during the interruption of subduction and slab breakoff. The rocks range from dikes of a picrodolerite–olivine dolerite–dolerite–quartz diorite–leucogranite association with ages ranging 626‒623 Ma (U‒Th‒U isotope data for zircons, SHRIMP-II method, this work) up to adakite–gabbro-anorthosite magmatism at the end of the Ediacaran (Vernikovskaya et al., 2017).

Temperature relations of β-factors for ¹⁸O/¹⁶O fractionations in TiO₂ polymorphs have been determined using the density functional theory:

1000 ln β_rt(¹⁸O/¹⁶O) = 6.93039x – 0.08158x² + 0.00116x³ + 0.08305*P

1000 ln β_ant(¹⁸O/¹⁶O) = 7.34275x – 0.09906x² + 0.00153x³ + 0.08027*P

1000 ln β_brk(¹⁸O/¹⁶O) = 7.19088x – 0.09157x² + 0.00139x³ + 0.07601*P,

x = 10⁶/T(K)², P is pressure (GPa). The deduced relations can be applied for isotope thermometry if combined with β-factors of coexisting phases.

The phase and chemical composition of drop-shaped inclusions in a directionally crystallized solid solution was studied. The initial melt contained (mol %): Fe, 31.79; Cu, 15.94; Ni, 1.70; S, 50.20; Sn, 0.05; and As, 0.04, along with Pt, Pd, Rh, Ru, Ag, Au, Se, Te, Bi, and Sb each of 0.03. The results of experiments testify to the simultaneous separation of two types of liquids under cooling of the initial sulfide melt within the (Pd, Au, Ag)–(Bi, Sb, Te) and Cu–(S, Bi, Sb, Te) subsystems, respectively. The solidification of these liquids resulted in the appearance of inclusions subdivided by the authors into four classes. Class I was characterized by a eutectoid structure with the matrix of the Pd(Bi, Sb)_хTe_1 – х solid solution and Au crystallites with admixed Ag, Cu, and Pd. Class II was constituted of sulfosalts with Bi and Au inclusions. The inclusions of sperrylite Pt(As, S)₂ were ascribed to class III. Class IV was formed by composite inclusions consisting of fragments of classes I–III. The experiment described showed the more complicated behavior of noble metals and metalloid impurities under crystallization of compound sulfide–metalloid melts against the data of isothermal experiments described earlier.

Marbles of the Kochumdek complex contain pyrrhotite, rasvumite, alabandine, würtzite, galena, and acanthite. The diversity of sulfides is caused by efficient crystal chemical fractionation of trace elements under metamorphism of spurrite–merwinite facies. The isotopic–geochemical features of sulfides reveal their formation at the expense of trace elements and sulfur budget of the sedimentary parent rocks.

In the northern part of the Siberian Platform, east of the Anabar Shield, several massifs of alkaline rocks with carbonatites identified (Tomtor, Bogdo, Promezhutochnyi) and projected according to geophysical data (Bualkalakh, Chuempe, Uele) form a large alkaline–carbonatite province. The first data on the composition of alkaline rocks of the Bogdo massif indicate that the latter belong to a group of feldspathoid rocks of basic composition: rischorrites, biotite–aegirine liebenerite syenites, carbonatized pseudo-leucite nepheline syenites with symplectites, and nepheline–feldspar aggregates. Sphene grains were extracted from various rocks of the Bogdo massif, and their U‒Pb age was determined using the SHRIMP-II secondary-ion microprobe. The calculated U‒Pb age is 394.4 ± 3.2 Ma, which is similar to the age of the Tomtor massif and the age of the rocks of the Kola alkaline province. One of the reasons for the manifestation of alkaline plume magmatism on this territory may be the effect of the peripheral zone of the African Large Low Shear Velocity Province (“Tuzo”) on the Baltic area and Siberia during the Devonian age.

High-precision dating of granitoids is of key significance for age identification of the main stages of crust formation in various blocks of the continental crust. Here we demonstrate geochronological results for of the Kedrovskii diorite–granodiorite massif localized within the South Muya block of the Baikal–Muya accretionary terrane (BMT) (Eastern Siberia) among Neoproterozoic gabbroids of the Kedrovskii complex and metasedimentary rocks of the Kedrovskaya Formation. The results of U–Pb (ID TIMS) and ³⁹Ar/⁴⁰Ar geochronological studies of the Kedrovskii massif are discussed. The crystallization of the massif (781 ± 3 Ma) occurred at an early stage of formation of the Proterozoic continental crust of the BMT during accretion of Baikalids with the Siberian craton margin. A later thermal event (626 ± 11 Ma) that took place at the Ediacaran stage of the BMT evolution resulted in the formation of a series of transverse biotite–quartz–feldspar veins cutting through the granitoids of the Kedrovskii massif. The geochronological data obtained show that the gold mineralization of the Kedrovskoe deposit (273 ± 4 Ma) was not generated by granitoids of the Kedrovskii massif.

The generalized equations of solution motion are given both in porous media and in cavities taking into account the osmotic effect. In certain cases, commonly small relevant corrections play a key role since they are mainly causing the solution motion. Based on the generalized equations, a number of problems have been solved that are of practical value. Owing to the limited volume of this paper, only a simplified example is given.

It is generally accepted that PKP precursor waves arriving ahead of PKP-waves observed in real data are explained by scattering on small-scale inhomogeneities in the lower mantle. In this paper, a stable analytical solution (without interference) was obtained for the field of longitudinal waves in a layered (discrete) ball of planetary size. The calculations of the total wave field, rays, and travel-time curves of longitudinal waves for the spherical model of the Earth AK135 with a carrier frequency of 1 Hz are presented. The analytical solution showed that, at angles smaller than 145°, low-amplitude waves with a higher frequency of about 1.3 Hz arrive ahead of the PKP-waves. These high-frequency oscillations indeed appear similar to the waves scattered on a certain object. The ray pattern and the travel-time graph show that these high-frequency oscillations are due exclusively to the spherical geometry of the Earth. This could be explained by the interference of refracted and reflected longitudinal waves in the bottom of outer core. This field persists even at smaller angles due to the interference of diffraction waves.

Observations of heat flow along nine geotraverses across mid-oceanic ridges in the Atlantic, Pacific, and Indian oceans are analyzed statistically. It is found that there is significant heat flow distribution asymmetry: opposite sides of the axis of the ridges differ in the average heat flow. Geotraverses in the Southern Hemisphere of the Earth exhibit higher average heat flow on the western flanks of the mid-oceanic ridges, while geotraverses in the Northern Hemisphere are characterized by higher average heat flow along the eastern flanks. Various tectonic factors are taken into account when explaining this phenomenon; however, it is suggested that the Coriolis force must be considered the main factor responsible for this asymmetry, causing redistribution of magmatic material in the asthenospheric reservoir.

This study analyzes the set of parameters of geophysical fields and geodynamic events before the very strong Sea of Okhotsk earthquake of May 24, 2013, with M_w = 8.3 and a focal depth of 630 km. In the time neighborhood of the earthquake (May 19–20), temporally synchronous geodynamic processes were observed in the areas of Kamchatka Peninsula that were separated by hundreds of kilometers. The simultaneous occurrence of anomalies in the behavior of tilts and in the dynamics of subsoil radon flow, conditioned by the deformation of remote foreshock activation in Avacha Bay, argues for a united earthquake-preceding geodynamic process of a regional scale. A possible reason for the observed synphase geodynamic phenomena is a “geodeformation wave” that emerged at the last stage of preparation prior to the Sea of Okhotsk earthquake.

Microinclusions of organic matter were dated using acceleration mass spectrometry in seven samples from the upper part of the thick Pleistocene syngenetic ice wedge exposed in the outcrop of the Batagay yedoma located in northern Yakutia along the upper Yana River (67.58° N, 134.77° E). The dated fragment veins were formed within 22 760–29 910 radiocarbon years BP (or 27.1–33.8 cal BP) ago. Using detailed isotope data, the January average air temperature in the Late Pleistocene (27–30 cal BP) was calculated for the Batagay section along with a series of reference sections in northwestern Yakutia. It was shown that the minimum January average air temperature (–51°C) was characteristic at that time for the area of the Batagaika crater, with values 4–5°C higher in areas 500–600 km further north. This effect was caused by the occurrence of the Yakutia anticyclone in winter during the Late Pleistocene, which was just as pronounced as that at the present time.

In this paper, we analyze the applicability of the land degradation neutrality (LDN) concept of the UN Convention to Combat Desertification to the Russian boreal forests. In this regard, it is necessary to adapt three global LDN indicators (land cover, land productivity, and carbon stock) to the assessment of land degradation processes of boreal forests in Russia and around the world. According to the research results, landscapes with different types of forest restoration dynamics can be viewed as an object of forest land dynamic studies. A set of LDN indicators adapted for boreal forests conditions has also been suggested in the course of our research. In order to assess LDN proxies, we calculated the retrospective and projected net carbon balance in the middle taiga zone of the Noshulskoye forest domain (Komi Republic, Russia) using the CBM CFS model. We explored three scenarios of forest net carbon balance under three different felling regimes. The net carbon balance should not be applied as an independent LDN indicator, because it does not take into account changes in species diversity and primary productivity. It is suggested that industrial felling should imitate natural types of restoration dynamics in order to achieve LDN targets. It can be reached through minimization of forest felling at sites with fireless types of succession, which accumulate maximum stocks of dead phytomass matter and serve as forest refuges supporting biodiversity.