Geochemistry International最新文献

Phase relations and the distributions of rare earth elements (REE), Sc, Y, and Li between aluminofluoride and aluminosilicate melts in the model granite system Si–Al–Na–K–Li–F–O–H were experimentally studied at 700°C, 1 and 2 kbar, and water contents of 3 to 50 wt %. Our original and available literature experimental data on phase relations in the granite system saturated with water and fluorine and containing trace elements are compared with the mineral assemblages of rare-metal cryolite-bearing granites from the Zashikhinsky, Katugin, and Ulug-Tanzek deposits in eastern Siberia. Liquid immiscibility between granite and salt aluminofluoride melts, which occurs at high contents of fluorine and lithium in the system, is proved to facilitate the accumulation of rare elements in salt cryolite-like melts. At a temperature of 700°C and pressures of 1 and 2 kbar, aluminofluoride melt in the granite system crystallizes and forms cryolite. Fluorine-bearing minerals of trace and rare earth elements, such as pyrochlore and gagarinite, occur at these deposits in association with cryolite and lithium micas. Comparison of experimental data and natural observations provides arguments in support of the hypothesis that liquid immiscibility should play an important role in the formation of cryolite. Cryolite is thought to be able to serve as a reference mineral for rare metal-rare earth mineralization in granites with high lithium and fluorine content.

The paper reports the first comprehensive physicochemical study of minerals of the amblygonite LiAlPO₄F–montebrasite LiAlPO₄(OH) series. An EPR spectroscopic and calorimetric study of montebrasite LiAlPO₄(OH)_0.9F_0.1 from the Shuk-Byul rare-metal granite pegmatites (Sangilen Highlands, Tuva) and amblygonite LiAlPO₄F_0.5(OH)_0.5 from pegmatites of the Voron’i Tundras (Kola Peninsula) was carried out. Using the EPR method, the radiation-sensitive paramagnetic O^– centers were discovered. They were formed without participation of impurity elements in the regular sites of crystal lattice. The possibility use of minerals of the amblygonite–montebrasite series for EPR geochronometry has been demonstrated for the first time. Using the method of high-temperature melt dissolution calorimetry on a Tian-Calvét microcalorimeter, the enthalpies of formation from elements ({{Delta }_{f}}H_{{{text{el}}}}^{0}) (298.15 K) = –2326.3 ± 2.2 kJ/mol for montebrasite with the composition LiAl(PO₄)(OH)_0.9F_0.1 and for amblygonite with the composition LiAl(PO₄)F_0.5(OH)_0.5 (‒2347.9 ± 3.1 kJ/mol) are obtained; and the values of this parameter are calculated for the end members with an ideal composition of the series: for montebrasite (–2315.5 ± 2.2 kJ/mol) and for amblygonite (–2401.6 ± 3.1 kJ/mol). The values of the standard entropy S⁰ (298.15 K) and the Gibbs energy of formation ({{Delta }_{f}}G_{{{text{el}}}}^{0}) (298.15 K) for intermediate and end members of the amblygonite–montebrasite series are estimated.

In the northern regions, seasonal temperature fluctuations cause cyclic thawing and freezing of soils and rocks, which is accompanied by dangerous natural phenomena, including landslides. Special conditions arise on the mountain slopes of many reservoirs due to the fluctuations of water levels and the interaction of humified waters with rocks. The paper presents the results of an experimental study of the biotransformation of sodium humate (SH) and changes in the microstructure of the surface of silicate rock that is a part of the Bureya landslide body under various conditions of cyclic freezing–thawing (CFT). Freezing of samples was carried out at a temperature of –18°C, and thawing at different temperature ranges (+4 and +23°C). The role of biogenic factor was played by four strains of microorganisms isolated from the surface and bottom waters in the Bureya Reservoir above and below the landslide body. The nature of the SH biotransformation during CFT was studied by spectral methods at different wavelengths (254, 275, and 465 nm). Analysis of the composition of water-soluble forms of chemical elements in an aqueous environment during rock destruction was carried out by ICP-MS, and changes in the surface microstructure was studied using scanning electron microscopy. It was found that the aromatic component of SH was subjected to the most intense transformation. After CFT, the strain Actinomyces sp. 45VD isolated from bottom waters showed maximum activity at a thawing temperature of +4°C relative to SH and leaching of elements from rock (Al, Ca, Mg, Mn). Its activity was comparable to the intensity of element leaching by a natural microbial consortium of non-sterile rock. Rocks subjected to CFT in deionized water were used for comparison; in this case, the content of water-soluble forms of many chemical elements (Fe, Ni, Cu, Zn, As, Mo, Ag, Cd, Tl, Bi, As) was below the detection limits (<0.001). The high biogeochemical activity of Actinomyces sp. 45VD is confirmed by the analysis of SEM-images of the microstructure of the silicate rock surface and the formation of a wide variety of isomorphic biominerals.

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.

Abstract

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.