Volume 68, Nº 3 (2023)

The Uyandina–Yasachnaya volcano-plutonic belt (UYVB) is one of the largest structures of this type in Northeast Asia. It strikes in the north-west direction for 900 km from the upper reaches of the Kolyma River to the upper reaches of the Selenyakh River. The belt is characterized by the strong facies variability and zoning, which is expressed in a change from mafic volcanic rocks (Ilin’-Tass zone) in the northeast to felsic volcanic rocks (Darpir zone) in the southwest. Poor knowledge of the UYVB results in an ambiguous interpretation of its geodynamic nature and volcanic evolution. The paper presents new detailed geochemical, isotopic, and geochronological data on the volcanic rocks of the Indigirka section of the Darpir zone of the UYVB. The U-Th-Pb SIMS zircon dating showed that volcanic rocks of the section previously attributed to the Oxfordian–Tithonian stages have the younger age within 150 ± 2–152 ± 2 Ma, which corresponds to the Tithonian. Andesites from the section base show Ta–Nb depletion, indicator element ratios Th/Nbpm, La/Nbpm, and La/Smpm > 1, and extremely low εNd = (–8), which may indicate a crustal contamination of mantle sources. Felsic volcanic rocks overlying the andesites have the postcollisional signatures. They are also characterized by the wide variations of εNd from –2.4 to –6.5 and model ages. The systematic position of the rocks in the section as well as upward increase of alkalinity and silica content from basaltic andesites to rhyolites in the upper parts of the section, which is typical of island-arc buildups, suggest that suprasubduction melts were generated beneath the collisional crust. Source of these melts could be a mantle domain that was enriched in fluid during previous subduction event.

The unusual shape of the Matryoshka diamond, which is a diamond crystal with a cavity containing a diamond crystal freely moving in it, continues to attract keen interest of many researchers in the context of its seemingly paradoxical origin for a mantle mineral. The discovery sparked lively discussions and multiple attempts to explain the nature of the unique shape of this crystal. A comprehensive mineralogical and crystallographic analysis of the unusual specimen, as well as other analogous diamond crystals, suggests that it was formed as a consequence of the mutual disorientation of the crystals during their growth and the presence of a twin of diamond subindividuals that formed both the core (inclusion) and the sheath (host) diamonds. The twinning planes (111) in the contacting crystals of the inclusion and the host were in a sub-perpendicular position to each other during their simultaneous growth. The captured diamond of the inclusion prevented the normal development of the diamond that became the host. The diamond host rapidly grew along the direction of its own twin boundary and constantly generated new growth layers, which eventually converged around the small captured twin crystal cluster during metric selection. Analysis of diamond crystals of similar shape from the Nyurbinskaya pipe and from elsewhere worldwide confirms the ontogenic model of their origin as a consequence of the capture of diamond inclusions that hampered the rapid growth of the twin crystal (spinel-law twins) in the direction of the twin boundary.

Gas seep and fluid flows from the seabed are an environment-forming factor of the aquatic environment, mainly due to their influence on the dissolved gases in the water, including dissolved oxygen. During the summer seasons from 2019 to 2021 in the area of shallow water gas emission site off the southern coast of the Heracles Peninsula, series of vertical probing profiles were carried out to determine hydrological parameters of the water: dissolved oxygen concentration (O2), temperature (T), salinity (S), and flow velocity (U). The study area is an underwater ledge with faults in the form of three canyons composed of dense limestones, two of which contained bubble gas emissions. Significant variability in O2 was identified in canyons where gas emissions are observed: from 1 to 80% saturation in the bottom layer, in contrast to normoxia at the background sites. Hypoxia was observed in the bottom layer above the emission sites in the absence of turbulence at temperature stratification. The values S decreased with depth, and the maximum difference reached 0.4‰. The bubble gas was dominated by methane (68.5–75.5%), and the carbon isotope composition of the bubble methane gas varied from –67.9 to –59.8‰ VPDB in 2019 and 2020, respectively. This generally indicates that the CH4 is of predominantly microbial genesis, was formed under different conditions, and matured in various periods of research during the monitoring period. Bacterial mats (mostly sulfur-oxidizing bacteria) were found in the areas of gas emissions.

Sedimentary rocks with different contents of organic matter (oil shales of the Bazhenov Formation and sandstones of the Tyumen Formation) from the West Siberian oil and gas province were used to demonstrate the simultaneous study of the mineralogical and structural-group composition, as well as semi-quantitative evaluation of mineral and organic matter contents in rocks by Fourier transform infrared (FTIR) microscopy. The following bands characterizing the stretching vibrations were used: Si–O and Al–O–Si of clay minerals (990–1090 cm–1), Si–O–Si of quartz (798 cm–1), CO2−3 of carbonates (1460 cm–1), as well as aliphatic C–H stretch (2800–3000 cm–1) and C=C ring stretch (1600–1650 cm–1) of organic matter. The obtained results are in good agreement with data obtained by traditional methods of bulk rock analysis: programmed pyrolysis, X-ray diffraction (XRD), and X-ray fluorescence (XRF) analyses. FTIR-ATR (attenuation total reflection) microscopy also provides the possibility of the distribution analysis of sample surface with color mapping, demonstrating the heterogeneity of the rock composition using the Bazhenov Formation as an example of unconventional reservoir. In addition, the method makes it possible to estimate the organic matter maturity and the rock residual generation potential based on the ratio of stretching band intensities of aliphatic and aromatic fragments.