Geochemistry International最新文献

The Don terrane, which is extensively reworked by metamorphism and granitoid intrusions, is part of the Volga–Don orogen stretching along the eastern margin of the Sarmatian segment of the East European Craton. The terrane consists of gneiss-granites of the Pavlovsk complex, metavolcanic rocks, ranging from basaltic andesites to dacites (amphibolites and gneisses), and metasedimentary rocks (marbles and calc-silicate rocks) of the Don Group. The volcanic rocks are typically enriched in LILE and LREE and show negative HFSE anomalies, indicating fluid-assisted melting of the mantle wedge in a subduction zone. The Nd isotopic composition (ε_Nd2200 = –1.2 to +3.4, model age 2180–2550 Ma) and Hf isotopic composition (ε_Hf = –4.3 to +3.3, model age 2290–2640 Ma) indicate an enriched mantle or a mixed crustal–mantle source of the parental melts of the volcanics and a juvenile source for sediments of the Don Group. The U–Pb zircon metamorphic age of the gneisses and amphibolites is 2047 ± 7 Ma, and that of the thermal effect of the granitoid batholith on the host rocks is 2060 ± 4 Ma. According to isotope geochemical and geochronological data, the Don Group is underlain by Archean rocks, contains Archean detrital zircons, but the age of this group is no older than 2300 Ma. A facies and age analogue of the Don Group is the Temryuk Formation of the Central Azov Group of the Ukrainian shield. In the Paleoproterozoic, the eastern margin of Sarmatia was likely a continental arc, which was nearly coeval with the island arc–backarc basin system of the Losevo Group.

The granitic basement rocks of central and western India, which are overlain by the Deccan Traps, are important for understanding early Earth processes and crustal evolution. The Alirajpur region presents a unique opportunity to study the complete sequence of basement granites, overlain by the marine Turonian Bagh beds. These granitic basement rocks are mainly composed of orthoclase, quartz, plagioclase, and biotite as rock-forming minerals. Abundant zoned zircons are hosted within biotite and hornblende. The whole rock geochemistry is calc-alkaline with a prevalence of potassium over sodium. The Alirajpur granitoids exhibit low REE with positive Eu anomaly exhibiting typical lower crust signatures. A detailed petrological-geochemical comparison of the granitic basement rocks from the Koyna and Alirajpur basement, separated by ~500 km, indicates that they are genetically related and provide important clues about the extent of the Precambrian basement underlying the ~500 000 km² of Deccan Traps.

The paper considers how different maturity criteria reflect the overall degree of approach to the chemical equilibrium state of OM. The material for this study was OM from carbonate, siliceous–carbonate, carbonate–siliceous, and siliceous rocks of the northern and central regions of the Volga–Ural area (more than 100 samples). The raw data were processed using the apparatus of nonparametric correlation analysis (paired correlation coefficients between 27 parameters and partial correlation coefficients). The strengths of relations between maturity criteria based on reactions of different type (for example, reactions of C–C bond cleavage and isomerization reactions of aromatic compounds) were studied. It is shown that none of the 266 correlation coefficients corresponds to the values characteristic of a functional dependence. The partial correlation coefficients show that there are only thirteen pairs in which the parameters determined by reactions of different type are directly interrelated, and the strength of the relation significantly affects the values of both parameters. Thus, the values we measured in carbonate and silicite rocks cannot be to used to characterize the general approach of their OM to chemical equilibrium. Although the concept of “maturity” may have, technically speaking, the meaning of a general tendency toward equilibrium, it should be admitted that no methods are available so far to measure it. No data can be used to identify the only single parameter whose value is controlled only by the maturity value. Moreover, it is quite probable that there is no such a parameter at all. Nowadays the degree of approach to chemical equilibrium can be characterized only by using a set of parameters determined by reactions of all four types identified in the work.

In order to crystallographically reconstruct the growth processes of mineral crystals and to establish fundamental patterns in crystal growth at a nanoscale, the effects of various factors on the characteristics of layer-by-layer crystal growth from solution were modeled using atomic force microscopy (AFM). In an experiment on growth in the area of a scratch, it was shown, using an original method of AFM data processing, that the average rate diagrams indicate a situation of a self-organization process: stable auto-oscillations in the growth rate. Comparison of the results with data on the growth of similar uninfluenced hillocks leads to the conclusion that giant fluctuations and the phenomenon of simultaneous growth and dissolution in local areas are caused by nanoindentation, when the strain from artificially formed defects strongly influences the evolution of the surface. In an AFM experiment on the trapping of foreign solid particles by a growing crystal at the nanoscale, the process of formation of a screw dislocation initiated by a foreign inclusion particle was registered. To theoretically explain the process, a three-stage mechanism is proposed that involves strain relaxation around the inclusion particle by the formation of one or more dislocations prior to the sealing of the inclusion during the first stage, the attachment of edge dislocations to them during the time of overgrowing in the second stage, and the development of a resulting dislocation after the particle has been completely sealed during the third stage. In studying growth in a flow cell, the mechanism of nanoscale reorientation of the growth hillock in the direction of the flow was established at a nanoscale, and the phenomenon of a change in the dominant hillock was registered. The resulting dissolution patterns in the channel are a clear demonstration of Curie’s Symmetry Principle, according to which only those symmetry elements of a body in an environment can be preserved that are shared by the body and the environment.

In the framework of the eutrophic Ob fen study, methane fluxes were measured by the chamber method, and water was sampled for the analysis of general chemical composition, content of biophilic elements (C, N, and P) and δ¹³C of the dissolved inorganic carbon (δ¹³C-DIC). Samples were taken from open and forested areas within the fen, with the latter having long received discharged domestic wastewaters. The methane emissions positively correlate with the concentrations of nitrogen compounds and dissolved organic carbon (DOC). In both areas, higher methane emissions were observed from waterlogged microdepressions than from dry elevated microlandscape features. The largest emission was observed from the microdepressions in the forested area near the wastewater discharge point, where the extreme values of CH₄ fluxes were almost 30 times higher than the background ones. However, the methane fluxes decrease to the background level at 160 m from the wastewater discharge point, together with the concentrations of almost all nitrogen and carbon compounds dissolved in the fen water. This led us to conclude that wastewater pollution significantly affects the intensity of methane fluxes by increasing ebullitive methane emission near the pollution source. The isotopic composition of the dissolved inorganic carbon (DIC) was heavier in the forested area (δ¹³C-DIC = –9.64 to –9.21‰) than in the open one (–12.83 to –11.24‰). In the open area of the fen, DIC isotopic composition became heavier away from the dry upland, correlating with the increase in the methane fluxes. The obtained data indicate that methane-generating processes are more active in the forested area of the fen than in the open one and become more active in the open area with increasing distance from the dry upland. This highlights the potentialities of using δ¹³C-DIC as a fingerprint of wastewater pollution, at least in the Ob fen area.

The article presents the results of study of ferruginous mineral waters. The waters under consideration are discharged on the territory of Western Transbaikalia and belong to the anoxic sulfide-free and acidic types. The peculiarities of the formation of gas, major and trace elements, and dissolved organic substance composition have been established using modern methods. It has been shown that the chemical composition of the waters is greatly influenced by acid–base conditions. Acidic ferruginous waters contain large amounts of heavy metals; organic matter is mainly represented by low molecular weight organic compounds. The only metals present in significant amounts in ferruginous waters are manganese and zinc. Dissolved organic matter is represented by diverse types of high-molecular weight compounds that are formed as a result of biotic processes.

This study presents the sulfur isotopic characteristics in baryte from carbonatites of the Sallanlatva massif and sulfides (mainly pyrite and pyrrhotite) from carbonatites, phoscorites and products of their contact interaction with the host silicate rocks of most carbonatite-bearing complexes of the Devonian Kola Alkaline Province (KAP). For some complexes (Ozernaya Varaka, Kontozero), these characteristics are reported for the first time. The determined range of δ³⁴S variations of sulfides in one complex does not exceed 4‰, but reaches 20‰ for the entire Kola Alkaline Province. This may be related to the evolution style of carbonatites and associated rocks. It is shown that the δ³⁴S value in sulfides decreases from (1) the least evolved volcanic carbonatites of the Kontozero complex (δ³⁴S_avg. = –1.3‰) through (2) carbonatites and phoscorites of the Kovdor, Ozernaya Varaka, Sokli, and Salmagora massifs towards (3) the rocks of Seblyavr, Vuoriyarvi, and, finally, the carbonatites of Sallanlatva (δ³⁴S_avg. = –14.7‰) massifs, where sulfides differ from those of other KAP carbonatites in their exceptionally low δ³⁴S values. The carbonatite volcanics of Kontozero are almost barren of REE mineralization; carbonatites of the second group contain accessory amounts of REE minerals; the third group is peculiar in the abundance of late carbonatites, where REE carbonates are frequently major minerals. Thus, the greater the volume of REE minerals in carbonatites of the complex, the lower the δ³⁴S value in sulfides from its carbonatites and associated rocks. For the first time in the KAP, the sulfur isotopic composition of associated baryte–pyrite pairs was studied in the Sallanlatva carbonatites. The sulfur isotopic characteristics are shown to correspond to the final low-temperature (250–350°C) stage of carbonatite evolution in oxidized conditions, which satisfies the parameters of baryte crystallization. Since the studied samples of the Sallanlatva carbonatites are explosive breccias, the oxidized composition of fluids may indicate their phreatomagmatic nature, i.e., formation due to the interaction of intruded hot matter (melt/fluid) with meteoric waters.

The present study focuses on the petrographic studies, XRD patterns, whole-rock geochemical data and Nd isotopic data of the clastic rocks of the Paleoproterozoic Mahakoshal Basin which includes meta-argillites, meta-greywackes and quartzites to characterize their provenance, tectonic setting, weathering intensity, and paleoclimate condition. Geochemically, the rock samples of meta-argillite and meta-greywacke in the bivariate plot Log Na₂O/K₂O vs. Log SiO₂/Al₂O₃ are classified as litharenite and greywacke. The samples of quartzite fall in the fields from the sublitharenite to quartzarenite. The weathering indices such as CIA, (alpha _{{{text{Ca}}}}^{{{text{Al}}}}), (alpha _{{{text{Na}}}}^{{{text{Al}}}}), (alpha _{{text{K}}}^{{{text{Al}}}}), (alpha _{{{text{Sr}}}}^{{{text{Al}}}}), (alpha _{{{text{Ba}}}}^{{{text{Al}}}}) and Th/U suggest that the clastic rocks have undergone moderate to intense chemical weathering and show K-metasomatism. Chondrite normalized REE patterns, discriminant diagrams of La–Th–Sc and Ni–V–10*Th and various transition elements ratio such as Cr/Th, Th/Co, Th/Sc, and La/Sc support the mixing of the mafic, intermediate and felsic sources. The Eu anomaly is highly variable and ranges from 0.29 to 1.08 and supports that the sediments have been derived from the heterogeneous source. T_CHUR model ages are consistent with 1.98–2.92 Ga possible provenance. εNd(t) and f(Sm/Nd) values overlap with the arc and the Archean crust signatures indicating that the sediments are derived from a Paleoproterozoic arc and older Archean crust (TTG gneisses and Archean granite). The opening of the Mahakoshal Basin is characterized by the retreating accretionary orogen and the closure of the basin is due to switching retreating accretionary orogen into advancing accretionary orogen which is contemporaneous to the assembly of the Columbia Supercontinent (∼1.8 Ga).