Doklady Rossijskoj akademii nauk. Nauki o Zemle

For the first time, the Pb-Pb age of phonolites from the Mar volcanic field located within the Uvat uplift of the Biryusa ledge of the Siberian craton has been determined. A direct estimate of the age of volcanic rocks was obtained using the stepwise leaching method for the whole rock. The obtained age of phonolites, equal to 1513±11 million years, determines the time of activation of alkaline volcanism associated with the extension of the continental crust within the Biryusa protrusion. The Mesoproterozoic age of alkaline volcanism on the southwestern margin of the Siberian craton coincides with the stage of the formation of the Kuonamskaya igneous province on the Anabar shield on the northeastern margin of the Siberian craton.

Purpose

The aim of the study was to reconstruct the conditions of bottom sediment formation within the degassing development site on board a large paleodoline in the southwestern part of the Kara shelf and identify the potential contribution of the permafrost degradation process to fluid intake.

Methods

The paper uses bathymetric data obtained on the Reson Seabat 8111 multipath echo sounder with a signal frequency of 100 kHz on flight 52 of the R\V Akademik Nikolaj Strakhov in 2021. Acoustic profiling was performed on a Parasound P-35 profiler with a signal frequency of 4-8 kHz (sedimentary cover) and 18-21 kHz (water column). A comprehensive core analysis was also performed, which included: 1)Isotope analysis was performed on a Picarro L2140i laser infrared spectrometer, 2) granulometric analysis on an Analysette 22 MicroTec Plus laser diffraction granulometer, 3)Measurement of magnetic susceptibility with a portable SM-30 kappa meter and determination of thermal conductivity with a Tempos device/

Results According to seismic and acoustic data, the sampling station is located in the area of the development of gas-saturated precipitation and gas outlets into the water column. The selected bottom sediment layer is divided into four layers according to its granulometric composition and texture. The upper three (layers 1,2 and 3) are represented by olive- and greenish-gray pelitic silts with an admixture of fine-grained sand, homogeneous and lenticular–layered texture, the lower (layer 4) - grayish-olive pelitic clay with lenticular-layered texture. The granulometric spectra of the studied precipitation are unimodal with maxima in the middle siltstone region. The isotopic composition of pore waters in bottom sediments differs significantly from both the waters of the open spaces of the Kara Sea and the fresh waters of the adjacent land. The revealed weighting of the water composition (δ18O to 5.51% and δD to -9.78%) can be associated with the degradation of permafrost and the destruction of gas hydrates contained therein. 

Conclusions

The available data suggest that the accumulation of Holocene marine sediments within the paleodoline in the southwestern part of the Kara shelf at depths of about 100 m occurred under conditions of fluid influx as a result of the degradation of the permafros and the destruction of the gas hydrates contained in them. The continuous nature of the influx of fluids to the bottom surface and into the water column, which was recorded according to seismic data in 2021 and 2023, suggests an ongoing degassing process that affects the isotopic composition of the pore waters of the upper horizons of the sedimentary section.

The paper presents the first data on the isotopic U-Pb dating of zircon (LA-ICP-MS method) form the intrusive rocks of the large Zirabulak granitoid pluton (Uzbekistan), which the Ingichke, the largest one in the Central Asia (about 100 Kt WO₃), and the Meliksu tungsten-polymetallic skarn deposits are associated with. These deposits, together with other deposits of tungsten, gold and other metals are part of the largest gold-tungsten metallogenic belt of Tien Shan. They are represented by bodies of hydrothermally-altered skarn with scheelite and sulfides (pyrrhotite, chalcopyrite), and subsequent within- and near-intrusion stockworks of late post-skarn metasomatites (propylitic and phyllic alteration) with polymetallic-scheelite mineralization. The concordant isotopic U-Pb zircon age dates of zircon from rocks of the Zirabulak pluton (289 ± 3.8 Ma for granodiorite of the main intrusive phase and 288 ± 3.3 Ma for granite) and the Meliksu stock (285.8 ± 2.5 Ma and 283 ± 2 Ma for granodiorite of the main intrusive phase) were obtained; they indicate the intrusion emplacement in the Early Permian. This is significantly younger than the known dates (some 300-310 Ma) of granodiorites but is coincident with the dates of granites in the other monzodiorite-granodiorite-granite intrusive massifs of the region, which the significantly smaller tungsten but locally large gold (with subordinate tungsten) deposits (Jilau, Yakhton, Kabuty) are associated with. Such age and metallogenic peculiarities of the productive intrusions can be correlated to the specific features of the geodynamic evolution of the region in the Late Carboniferous-Early Permian.

A comprehensive study of graphite from the Pestpaksha deposit (Lapland Granulite Belt, Kola Region) was conducted, including Raman-based temperature determination and carbon isotopic characterisation. C and O isotopic compositions of carbonates from rocks adjacent to graphite ores have also been studied. The crystallisation of graphite was found to have taken place at a temperature of about 600 °C during the retrograde stage of the metamorphic event. Large variations in carbon isotopic composition of graphite (δ¹³C_PDB from –18.5 ‰ to –29.0 ‰) suppor its crystallisation during C-O-H fluid reduction. The parent fluid probably had a carbonic acid composition with an organogenic isotopic signature (δ¹³C_PDB of about –25 ‰), and the large variations in graphite isotopic composition may reflect dynamic carbon depletion of the fluid by the Rayleigh depletion mechanism. The anomalously isotopically heavy carbonates identified (δ¹³C_PDB ~ +14.5 ‰, δ¹⁸O_PDB ~ –12.5 ‰) appear to be the product of a residual fluid enriched in the heavy carbon isotope.

This paper presents the results of U-Pb (ID TIMS) geochronological studies of columbite-tantalite and zircon from the pegmatite vein of the Aleksandrovskoye ore field, which is part of the East Sayan rare-metal pegmatite belt. As a result of U-Pb geochronological studies of columbite, age estimates of 1825 ± 3 Ma were obtained for the first time for the quartz core of the pegmatite vein and 1817 ± 5 Ma for the zone of intense alteration of amphibolites in its exocontact. The age estimate of 1810 ± 5 Ma was obtained for metamict zircon from the boundary zone of the quartz core. The studies indicate that the age of rare-metal pegmatites of the Aleksandrovskoye ore field of the East Sayan pegmatite belt differs significantly from the age of spatially conjugated granitoids of the Sayan complex (1.86-1.90 Ga).

Based on the method of melt inclusion study, the magma composition and formation conditions of melts involved in the formation of urtites of the Chik intrusion of alkaline rocks (southeastern Tuva) are studied. The mechanisms of their formation are identified. Primary melt inclusions are studied in nepheline and garnet from the urtite. Amphibole, sphene, garnet, sodalite, banalsite, calcite, and zeolite are identified among the daughter minerals of the melt inclusions. It was found that nepheline crystallized in the temperature range of 1100 - 1120°C. It is shown that the glasses of homogenized mеlt inclusions in nepheline are characterized by wide variations in SiO₂ contents, from 37 to 46 wt. %, Al₂O₃ – from 10 to 18 wt. %, CaO – from 5.6 to 19 wt. %, and increased concentrations of alkalis (11-15 wt. %), FeO (11-13 wt. %), TiO₂ (up to 3.4 wt. %) and P₂O₅ (up to 2.5% wt. %). The chemical composition of the glasses of homogenized melt inclusions in garnet is similar to the most differentiated composition of the glasses of melt inclusions in nepheline. It contains 47 wt. % SiO₂, 1 wt. % TiO₂, 18 wt. % Al₂O₃, 9 wt. % FeO, 7 wt. % CaO and 16 wt. % of alkalis (Na₂O + K₂O).The results of the study of melt inclusions in urtite minerals, as well as the analysis of the composition of various rocks of the Chik massif (urtites, melteigites, and ijolites, including their calcite and garnet-containing varieties), suggest the involvement of gravitational and clystallization differentiation of magma in their formation. The data obtained refute the hypothesis of the metasomatic nature of the urtites of the Chik intrusion of alkaline rocks (southeastern Tuva) and provide compelling evidence for their magmatic  genesis.

The structure of a new mineral, isostructural to lindquistite, with the idealized crystal chemical formula (K, Ba)Fe³⁺(Mg,Fe)₄(Al,Cr³⁺,Ti⁴⁺)₁₃O₂₇, has been studied by single-crystal X-ray diffraction analysis. This mineral was identified in a polyphase inclusion in alumochromite from a garnet-spinel lherzolite xenolith from the Obnazhennaya kimberlite pipe (Kuyokskoe field, Yakutsk kimberlite province).

The parameters of the hexagonal cell were obtained: a = 5.81093(15) Å, b = 5.81093(15) Å, c= 32.2003(10) Å, Ve.i. = 941.63(4) ų, pr. gr. P63/mmc, Z = 2. The structure of the mineral is layered of a mixed type. It can be described as a sequence of 14 cubic and hexagonal densely packed layers formed by oxygen atoms.

In each 7^th layer, large cations X (K⁺ and Ba²⁺) are placed in 1/4 oxygen positions, which occupy position 2b and are surrounded by 12 oxygen atoms. O^2- and X ions having close radii form the densest packing, while T (Mg²⁺, Fe²⁺), M (Al³⁺, Cr³⁺, Ti⁴⁺), and A (Fe³⁺) ions occupy voids. It was found that the new mineral is isostructural to the minerals lindquistite and hexaferrite of the W-type.

The decoding of the mineral structure as a polytypic magnetoplumbite does not reflect the reversal of the layers. The data obtained can be used to justify the identification of a separate lindquistite group, since the new mineral studied in this work is the second find in this group.

The lower part of the Burubaytal Formation that encompass the Upper Cambrian and Lower Ordovician cherts in the in the Southwestern Balkhash region of Southern Kazakhstan is enriched by organic detritus. This detritus consists of numerous fecal pellets composed of para- and protoconodont elements, as well as spherical acritarchs, individual acritarchs, and conodont elements. Additionally, there are scattered carbonaceous, finely dispersed organic matter and clumps of this matter. These findings indicate a significant contribution of pelagic organisms in the formation of a vertical flow of organic matter from the pelagic zone to the deep-sea bottom and an episode of increased burial of organic matter at the beginning of the Batyrbaian Stage in the Late Cambrian corresponding to the TOCE (Top of Cambrian Excursion) isotopic event.

Spatial heterogeneity of structural and rheological properties of a fault is one of the key factors determining the dynamics of its deformation. The paper analyzes spatial and temporal patterns of seismic localization in several segments of the San-Andreas fault zone. Groups of background and clastered events are distinguished. It is shown that background events exhibit structural features of the fault. Asperities in the fault plane appear as dense clusters of background events. Foci of strong earthquakes with magnitudes exceeding the maximum magnitude of background events 1.5-2 times completely rupture at least one contact spots. The fault area located between the asperities controls the slow postseismic slip and, consequently, postseismic aftershock activity. The number of aftershocks is maximal at the boundary of the earthquake's focal area and decreases monotonously with distance from the hypocenter. In the area between the asperities, the speed of aftershock migration is 0.01–10 km/day

The paper presents a comprehensive analysis of the seismic and geodynamic conditions before and after the March 28, 2025, M7.7 earthquake in Myanmar. The results of a global test of the M8 algorithm for predicting earthquakes with a magnitude of 7.5 and greater for this region are considered. It is analyzed how expected the earthquake was in terms of long-term seismic hazard based on seismicity data alone. A geodynamic analysis is performed to assess the seismogenic potential of the Sagaing fault before and after the earthquake. The risk of aftershocks is assessed. A model of the earthquake source is constructed to test the supershare property of the rupture and interpret the anomalously large length in the US Geological Survey model.

A quantitative assessment of the relationship between variations in bottom pressure created by wind waves and micro-displacements of the upper crust layer is given based on data from a laser hydrosphere pressure variation meter and two laser strainmeters. Based on experimental data, the dependence of the coefficient of transformation of bottom pressure into elastic oscillations of the upper crust layer on periods of wind waves is derived.

A new approach to diagnostics of the regional carbon balance is presented, which involves optimization of parameters of an ecosystem carbon exchange model using satellite observations of the column-average dry-air mole fraction of CO₂ in the atmosphere (XCO₂) and a mesoscale chemistry-transport model. The potential of the suggested approach is demonstrated through its application to estimate ecosystem CO₂ fluxes in Central Siberia using XCO₂ data from the OCO-2 satellite.

Britholite, a Ca-REE silicate sample with an apatite structure, was produced by cold crucible induction melting. Britholite has a simple composition, high capacity for actinides, and is stable in heated aqueous solutions. For the first time, the leaching rates of Nd (imitator of minor actinides) with water and brine at 200 – 250 °C were determined to be 10^-3 – 10^-5 g/(m²×day). which corresponds to a congruent dissolution of the matrix at a rate of 1 micron per 3–300 years.