Vol 68, No 7 (2023)

The potential of the stepwise dissolution analytical procedure to obtain chemostratigraphic and geochronological information is demonstrated by a case study of Riphean carbonate rocks of the Kamovskaya Group of the Baykit Uplift (Eastern Siberia). The procedures suggested for studying Rb–Sr and U–Pb systems in carbonate rocks included (1) selection of samples with the lowest 87Sr/86Sr ratio on the basis of preliminary Rb–Sr analysis of the collection using the routine procedure of bulk leaching in 0.1N CH3COOH; (2) detailed study of the Rb–Sr system in selected samples using two-step dissolution in 0.2N CH3COOH and derivation of L(Rb–Sr)1 and L(Rb–Sr)2 fractions; (3) analysis of Pb isotope composition of the selected samples to derive the preliminary value of their age; (4) detailed study of U–Pb system of carbonate rocks using the stepwise dissolution in 0.5N HBr, resulting in six dissolved fractions L(U–Pb)1–L(U–Pb)6 for each of the selected samples; and (5) a final calculation of the Pb–Pb age of the studied rocks, based on the results from the obtained fractions, with the exclusion from the calculation of the initial fractions of this dissolution containing epigenetically altered carbonate material. The values of the primary 87Sr/86Sr ratio in L(Rb–Sr)2 fractions of carbonate rocks of the Kamovskaya Group are as follows: limestone of the Madrinskaya Formation, 0.70490; dolomites of the Yurubchenskaya Formation, 0.70495–0.70503; and dolomites of the Kuyumbinskaya and Vingoldinskaya formations, 0.70580 and 0.70521, respectively. These values characterizing the least altered carbonate material correspond to 87Sr/86Sr ratios in the Early Archean and can be used for chemostratigraphic calculations. Taking into account the Rb–Sr data, the U–Pb age of the Yurubchenskaya Formation carbonate rocks dissolved in six steps was calculated, starting from L(U–Pb)3. The slope of the resulting isochron in the coordinates 206Pb/204Pb–207Pb/204Pb corresponds to 1501 ± 23 Ma, which supports the conclusion that the carbonate rocks of the lower part of the Kamovskaya Group of the Baykit Uplift were formed in the Early Riphean.

A model of the thermal evolution of the lithosphere of the West Siberian Basin in the area of Superdeep Well SG-6, which was drilled through the Koltogor–Urengoi graben, is used to numerically estimate the generation of various hydrocarbon (HC) fractions by the Triassic and Jurassic source rocks. The thermal model assumes the emplacement of a sill into the subsurface layers of the basement in the Early Jurassic and hydrothermal activity in the Late Pliocene–Lower Pleistocene, which noticeably impacted the conversion history of the HC potential of the Triassic and Jurassic source rocks. For the Triassic Pur formation, the emplacement of the sill into the subsurface layers of the basement in the Lower Jurassic drastically intensified the conversion of the HC potential to 84% and the degradation of more than 97% of the generated light oil mass. Calculations show that the heavy oil generated by the rocks of the Pur, Togur, and lower horizons of the Tyumen formations has degraded almost completely as a result of secondary cracking, while heavy oil predominates among the generated HC fractions in the upper horizons of the Tyumen Formation and in the rocks of the Bazhenovo Formation. The light oil remaining nowadays in the matrix of the source rocks has completely degraded in the Triassic rocks but makes up much of the HC products in rocks at the bottom of the Togur Formation and the top of the Tyumen Formation. This oil is the dominant in the upper horizons of the Togur Formation and in the bottom part of the Tyumen Formation. According to calculations, gas hydrocarbons account for a significant proportion of HC products in the Togur and Tyumen formations, and they dominate in the Middle Triassic Pur Formation. At the relatively low initial potential of HC generation and a low content of organic matter in the rocks of the Pur, Togur, and Tyumen formations, no primary expulsion threshold of liquid HC has been reached, and the generated liquid HC probably did not leave the rock matrix, while the gaseous HC likely migrated. The threshold of primary expulsion of liquid HC for the Bazhenov rocks was calculated to be reached at about 65 My.

Two sodalite samples (sample I is Na8Al6Si6O24Cl2⋅0.4H2O from the Kovdor alkaline ultramafic massif with carbonatites in the Murmansk region, Russia, and sample II is Na8Al6Si6O24Cl2⋅0.2H2O from the Bayan Kol nepheline syenite and miaskite massif in the Republic of Tyva) were studied by thermal and electron-microprobe analyses, powder X-ray diffraction, photoluminescence, and by IR, Raman, and ESR spectroscopy. Solution melt calorimetry was applied to determine the enthalpy of formation from elements for water-bearing sodalite samples: −13536 ± 10 (I) and −13503 ± 19 (II) kJ/mol. The enthalpy of formation of sodalite of the theoretical composition Na8Al6Si6O24Cl2 was evaluated at ΔfH0(298.15 K) = −13446 ± 11 kJ/mol. The data obtained on the enthalpy of formation of sodalite and literature data on its S0(298.15 K) were used to calculate the standard Gibbs energies of formation of anhydrous and of water-bearing sodalite.

The paper discusses modeling the dynamics of nickel concentration in soils, water, and the bottom sediments of lakes caused by atmospheric emissions from the Pechenganickel plant, Kola Peninsula, throughout its whole operation period. The applied technology of balanced identification makes it possible to use a mathematical description of heterogeneous geochemical processes in ecosystems to combine heterogeneous experimental data and build up a computer model with an optimal balance of its complexity and fitting quality of the data. The model is used to analyze the spatial and temporal variability of natural objects in the zone of distribution of atmospheric pollution (nickel) from the Pechenganickel plant. The paper presents and discusses results of this study, including estimates of the retrospective state of the simulated objects (before the start of the intense studies) and a forecast of their dynamics until 2030. According to the model calculations, the intensity of Ni accumulation in the soil and bottom sediments was 2.35 and 4.48 mg/(m2 year) during the maximum deposition periods (1980–2005), whereas the model predicts a decrease in the intensity of Ni accumulation in the bottom sediments (0.23 mg/(m2 year)) and slow Ni leaching from the soil (0.19 mg/(m2 year)) after the shutdown of the plant.