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Vol 70, No 12 (2025)
Articles
CROZET RISE, INDIAN OCEAN: PETROLOGY, GEOCHEMISTRY AND GEODYNAMIC FEATURES OF FORMATION
Abstract
One of the widely discussed issues of oceanic rise formation is the possibility of their formation by the interaction of hot spots and mid-ocean ridges. The Crozet Rise (less than 9 million years old) is located in the western Indian Ocean and consists of a group of volcanic islands with alkaline lavas. The paper studies basanites of the largest island of the archipelago, Possession Island. As a result of the conducted petrological and geochemical studies, the use of geophysical data and calculation models, the conditions of the formation of the lavas of Possession Island were determined, as well as the relationship between the features of the origin of the Crozet Rise and other rises of the western Indian Ocean located near southern Africa. The olivine-clinopyroxene-spinel phenocryst association observed in the lavas of Possession Island corresponds to deeper crystallization conditions compared to typical oceanic magmas, for which the olivine-plagioclase-clinopyroxene association is common. Fractionation of melts could have occurred in the intermediate chamber as a result of the intrusion of different portions of magma at pressures of 8–10 kbar and temperatures of 1200–1300 °C during the formation of the volcanic edifice 9 million years ago. The geochemical characteristics of the Possession Island lavas, including the isotopic composition of Sr, Nd, and Pb, indicate an enriched source, possibly with an admixture of a HIMU-type component (with high primary U/Pb and U/Th ratios), and are close to the composition of enriched magmas of the Indian Ocean uplifts – Crozet, Marion, and Bouvet, but differ from the composition of basalts of the Conrad, Afanasy Nikitin, and Mozambique Ridge uplifts located in the western part of the Indian Ocean. The enriched HIMU-type source is associated mainly with the substance of the ancient continental Gondwana mantle. The melting process may have involved fragments of continental crust or oceanic mantle enriched during the early stages of evolution, since the Crozet Rise is an intraplate structure recently formed under the influence of a hotspot on relatively ancient oceanic lithosphere. In this case, fractionation of alkaline magmas occurred at a depth of ~25–30 km. The formation of the eastern part of the Crozet Rise occurred under the influence of the Crozet-Marion hot spot, which can be considered a satellite of a large African plume, which significantly influenced the entire history of the development of the Southern Ocean.
Geochemistry International. 2025;70(12):943-965
943-965
FORMATION OF HYDROCARBONS DURING THERMAL MATURATION OF KAA-KHEM COAL
Abstract
The article examines coal from the Kaa-Khem deposit from the point of view of its oil and gas generation potential. Coal from the Kaa-Khem deposit was subjected to hydrothermal action at 350°C/24 hours to simulate maturation of organic matter. To assess the kinetic characteristics of the organic matter of Kaa-Khem coal, it was studied by the Rock-Eval pyrolysis method at three heating rates. The kinetic characteristics of individual C1-C5 hydrocarbons were additionally studied by the dry, stepwise pyrolysis. Changes in the composition of hydrocarbon biomarkers before and after hydrous pyrolysis were also analyzed, and it was found that hydrothermal action mainly affected the distribution of n-alkanes and aromatic hydrocarbons. The composition of the sterane and hopane biomarkers are not significant changes. The study of the coal composition by the pyrolytic gas chromatography-mass spectrometry made it possible to record the loss of long alkyl chains in the studied range of thermal maturity. The conducted studies allow us to estimate the hydrocarbon potential of Kaa-Khem coal as high. However, the possibility of its implementation and the forecast of the phase composition of fluids require further studies.
Geochemistry International. 2025;70(12):966-977
966-977
GEOCHEMICAL AND GRAIN-SIZE DIFFERENTIATION OF THE LAPTEV SEA SHELF SEDIMENTS DURING MARINE TRANSGRESSION
Abstract
The Laptev Sea belongs to the polar climate region, characterized by minimal influence on geochemical weathering processes. The dominant role in the formation of sedimentary rocks in high latitudes is played by mechanical sorting of sedimentary material. Using 15 sediment cores located along 120–130° E from the Lena Delta to the continental slope and in the Sadko Trough as an example, a close relationship between the grain-size and geochemical composition of marine sediments was revealed. The results of the study showed that shelf sedimentation occurs in at least two stages: (I) accumulation of sediments in coastal area, the area directly adjacent to the sources of sediments, and (II) the subsequent mechanical sorting of sedimentary material on the shelf itself. At the first stage, the finest clay fraction is partially separated from the primary terrestrial sedimentary material. At the second stage, as one move away from the coastal area, the sand and silt fractions are separated. This sedimentation pattern is determined by the combination of the Lena Delta morphology and the Siberian Coastal Current. In conditions of an unstable hydrodynamic regime of the shallow shelf, predominantly sandy sediments (SiO2/Al2O3 > 4.5) accumulate at a rate of < 0.5 g cm−2 yr−1. These sediments differ from the sediments of the terrestrial area by a higher content of Si and a lower content of Ti, Mg, Fe and Ni. In conditions of a calm hydrodynamic environment of the Sadko Trough, predominantly silty sediments (SiO2/Al2O3 < 4.5) accumulate at a rate of > 0.5 g cm−2 yr−1, characterized by a higher content of Ti, Mg, Fe and Ni and a lower content of Si. The contents of K, Rb, Cr and Ca in shelf sediments do not show a clear correlation with grain-size distribution and are thus not informative for reconstructing sedimentation patterns.
Geochemistry International. 2025;70(12):978-996
978-996
STRONTIUM SPIKE CALIBRATION IN THE ISOTOPE DILUTION METHOD: NEW APPROACHES AND SOLUTIONS
Abstract
The article discusses and provides theoretical verification of methods for determining the parameters of a spike solution (the content of the element under study and its isotopic composition) using strontium as an example. The study of its isotopic composition in rocks and minerals is widely used in geochemical and isotope-geochronological studies. The authors propose a new method for certifying a strontium spike — determining its parameters based on the results of isotopic analysis of two mixtures of the tracer with a reference solution. In this case, the ratios between three stable isotopes (84Sr, 86Sr, 88Sr) are used and precise knowledge of the Sr concentration in the spike solution is not required. The isotopic composition of one of the mixtures is specified by the average geometric value of the isotopic compositions of the reference solution and the spike, and the isotopic composition of the other is the average arithmetic value of these indicators. The combined use of these mixtures makes it possible to obtain all the parameters of the spike — the Sr concentration and its isotopic composition. The authors believe that the most correct way to determine these parameters is to use all four stable isotopes of Sr. One of the promising ways to determine the parameters of the spike solution is to use a reference solution enriched with the isotope 87Sr, in which the natural ratios of the remaining isotopes are preserved. The proposed methods are given a detailed theoretical justification and a graphical representation.
Geochemistry International. 2025;70(12):997-1012
997-1012
Хроника
Outstanding Scientist of Our Country. On the 130th Anniversary of the Birth of Academician A. P. Vinogradov
Geochemistry International. 2025;70(12):1013-1022
1013-1022