Geochemistry International最新文献

The GEOCHEQ_Isotope software package, previously developed to calculate chemical and isotopic equilibria of carbon and oxygen in hydrothermal and hydrogeochemical systems by minimizing Gibbs energy, was extended to the simultaneous calculation of isotopic effects of carbon, oxygen, and iron (the main objective of the study). As for carbon and oxygen, the β-factor formalism was used to develop algorithms and a database for the calculation of iron isotopic effects. According to the developed algorithm, the Gibbs energy G*(P, T) of formation of a rare isotopologue was calculated through the Gibbs energy of formation of the main isotopologue taking into account the value of the ⁵⁶Fe/⁵⁴Fe β-factor of this substance and the mass ratio of ⁵⁴Fe and ⁵⁶Fe isotopes. The approximation of ideal isotope mixture was used. The temperature dependence of the β-factor is unified in the form of a third-order polynomial by inverse even degrees of absolute temperature. Based on a critical analysis of currently available data on equilibrium isotopic factors obtained by different methods (elastic and inelastic γ-resonance scattering, isotope exchange experiments, and ab-initio calculations), the main result was obtained: for the first time, internally consistent database on iron β-factors of minerals and water complexes was developed. To develop the database, minerals and aqueous complexes for which the estimates of the equilibrium fractionation factors of iron isotopes obtained by different methods exist and consistent within the error of the methods have been identified: metallic iron (α-Fe), hematite, magnetite, siderite, pyrite, and the aqueous complexes ({text{Fe(III)(}}{{{text{H}}}_{{text{2}}}}{text{O)}}_{6}^{{3 + }}) and ({text{Fe(III)(}}{{{text{H}}}_{{text{2}}}}{text{O)}}_{6}^{{2 + }}). The values of the iron β-factors for these minerals and aqueous complexes, accepted as reference ones, formed the “mainstay” of the developed database. Considering that the equilibrium isotopic shifts of iron between minerals and water complexes are estimated much more accurately within the framework of one method rather than the corresponding β-factors, the database was made consistent by linking the ln β values for minerals and water complexes to the reference ln β values. The application of the GEOCHEQ_Isotope software package to the closed carbonaceous hydrothermal system H₂O–CO₂–Fe₂O₃–FeO–CaO (T = 200°C, P = 16–50 bar) has shown the possibility of its use for the calculation of changes in mineral composition and isotopic effects on oxygen, carbon, and iron.

The structure of borosilicate glass and glass-ceramic materials of two compositions with different Cs/Na ratios was studied using Raman spectroscopy. The materials were synthesized in two different modes. The anionic environment of cesium in the glass and the structural rearrangements of the network during the formation of crystalline phases have been studied. X-ray diffraction patterns of glass-ceramic samples made it possible to determine the only crystalline phase CsBSi₂O₆, the structure of which was not clearly determined. The glass-ceramics of the studied composition can be used to immobilize cesium by incorporating it into the crystalline phases of the CsBSi₂O₆ composition, while sodium is retained in the glassy matrix. These studies showed that the composition of the crystalline phase does not depend on the initial ratio of alkali cations, while the proportion of the ordered and amorphous phases is controlled by the kinetics of the melt cooling process.

Bog and lake ecosystems of the boreal region are recognized as important parts of the global biogeochemical carbon cycle. At the same time, many aspects of the gas regime dynamics of bog lakes remain understudied. The paper presents data on the seasonal dynamics of dissolved CH₄ and CO₂ concentrations in the bog lake located in the ridge-lake complex of the Ilassky bog, a typical raised bog in the northern taiga of northwestern Russia, and results of analysis of the seasonal vertical distribution of greenhouse gases in the water column and the dynamics of surface concentrations with increased time resolution. The reasons for and patterns of their variability are considered, including those in relation to the characteristics of the bottom sediments. Concentrations of CH₄ and CO₂ in the water column vary during the year within wide ranges: from 4 to 652 µg/L and from 0.19 to 19 mg/L, respectively. CH₄ concentrations in the surface layer are at approximately the same level from May through August, with values measured in the water (5.9 to 11 µg/L) more than one hundred times higher than the concentrations in equilibrium with the atmosphere (0.04 to 0.05 µg/L), indicating a methane flux to the atmosphere. The CO₂ concentrations decrease throughout the open water period and become lower than the equilibrium concentrations with the atmosphere by the end of August, indicating a change in the flux direction and uptake of CO₂ from the atmosphere. The results showed that, depending on the season, a bog lake can be not only a source but also a sink for atmospheric carbon, 90–99% of which is CO₂ according to literature data.

The geochemical characteristics (REE, trace elements) and Sr and Nd isotopic composition of apatite from corundum-bearing metasomatites of the Khitoostrov occurrence (Belomorian Mobile Belt), associated plagioclasites, and host rocks represented by garnet amphibolites and kyanite–garnet–biotite gneisses of the Chupa sequence have been studied. Apatites from the corundum-bearing metasomatites and kyanite–garnet–biotite gneisses are enriched in medium REE and have a negative Eu anomaly (Eu/Eu* 0.20–0.35). Apatite from the corundum-bearing rocks differs from apatite from the gneisses of the Chupa sequence in the increased content of Sr, LREE, decreased content of HREE, as well as a lower ⁸⁷Sr/⁸⁶Sr(t) ratio and an increased ɛ_Nd(T) value: 0.70 865–0.70 896 and –9.3 ± 0.2 compared to 0.72 533 and –8.1, respectively. Apatite from the garnet amphibolites is enriched in MREE, lacks Eu-anomaly (Eu/Eu* 0.98), and has a low ɛ_Nd(T) = –9.3 and the lowest ⁸⁷Sr/⁸⁶Sr(t) ratio of 0.70 560. The Sm-Nd age estimate of apatite is 1.80 ± 0.15 Ga, which coincides with the Svecofennian metamorphism in the Belomorian Mobile Belt. The geochemical features of the apatite and ⁸⁷Sr/⁸⁶Sr(t) ratios indicate that the metasomatic alteration of the gneisses was caused by the lower crustal fluid and was accompanied by the influx of LREE and the removal of HREE. The slightly lower Eu anomaly and higher Ce vs Th and REE vs La/Sm relations reflect the fact that apatite from the corundum-bearing metasomatic rocks was formed in a more oxidizing environment than apatite from host rocks. Neither the corundum-bearing metasomatites and plagioclasites, nor the host rocks revealed any Sr-isotopic and REE-geochemical traces of interaction with surface (meteoric) waters.

The first data on the composition of organic compounds in thermal waters have been obtained from deep boreholes in the Paratunka geothermal field in Kamchatka. A variety of organic compounds belonging to eleven homological series were identified by capillary gas chromatography–mass spectrometry and solid phase extraction. The thermal waters were found out to be strongly dominated by aromatic and aliphatic hydrocarbons (HC), which were formed in relation to both thermogenic processes (transformation of organic residues under the effect of high temperatures and pressures) and bacterial activity. The Karymshina thermal waters are characterized by a specific molecular mass distribution of HC and contain only even-normal alkanes. It is shown that the composition of organic compounds of medium volatility in the Paratunka geothermal field is similar to the composition of organic matter (OM) in other thermal water occurrences of the Kamchatka Peninsula (Mutnovka and Uzon geothermal fields), which have been previously studied using the same methodology: all the waters are characterized by the prevalence of aliphatic and aromatic HC over other identified compounds.

Several pelagic sections in North-Eastern/Central Tunisia and neighboring basins have recently been the subject of detailed δ¹³C and δ¹⁸O stratigraphy. These studies have been utilized to create a composite Mesozoic (Triassic to Cretaceous) δ¹³C and δ¹⁸O curves, which is useful for high-resolution stratigraphic correlation. The long-term trend of the created composite δ¹³C and δ¹⁸O profiles change on variable time scales (100’s of years), and show the major δ¹³C positive and negative excursions around the key Mesozoic timelines: the Carnian, Toarcian, Barremian-Aptian, Cenomanian-Turonian, upper Cretaceous and Cretaceous/Paleogene. Short-term fluctuations vary also on time scales of similar magnitudes, and dominated the δ¹³C signal in the Mid-Cretaceous. Generally, the observed changes in the Carbon-isotopic record have commonly been attributed to changes in the organic carbon flux to the sedimentary reservoir in response to eustatic sea-level change. However, it is also suggested that carbonate production and burial rate may have influenced the δ¹³C signal. During Triassic and Jurassic, increased inorganic carbon burial led to a shift towards more negative values in the ¹³C/¹²C ratio, which is probably linked to the expansion of pelagic carbonate deposits in epicontinental seas (e.g. Carnian and Toarcian carbonate). However, short-term variations in δ¹³C could either be due to local sea-level changes in the North African epicontinental sea or changes in oceanic ¹²C storage due to variations in different water properties circulation. In addition, oxygen isotopic data can serve as a useful tool for estimating paleotemperatures in instances where diagenesis is not a hindrance. These data can record changes in paleoclimate associated with cooling or warming during the Mesozoic.

New geological, geochemical, and geochronological (U-Pb zircon) data obtained on the greenstone rocks of the Kichany structure from the Archean Tiksheozero greenstone belt made it possible to clarify and supplement the previously proposed stratification schemes. The composition of the identified sequences, the order and duration of their formation have been specified. The Archean supracrustal rocks are divided into three sequences. The lower sequence (previously not identified) is represented by a bimodal series: tholeiitic metabasalts and felsic metavolcanics, with subordinate metagraywackes. It has been formed for over 20 million years (from 2788 ± 5 to 2766 ± 9 Ma). Sm–Nd data obtained on basaltic metaandesites (Sm–Nd model age 2.86 Ga and ε_Nd = 2.92) indicate their mantle nature. Metarhyolites from the lower sequence with a Sm–Nd model age of 2.89 Ga and ε_Nd = 2.59 were generated from a source with a short residence time. The differentiated volcanic series of the upper sequence (from basalts to dacites) has been also formed for about 20 million years (2738 ± 7–2716 ± 7 Ma). The parental melts for the intermediate–felsic metavolcanics of the upper sequence are variably enriched in ancient crustal matter. The oldest rocks with a Sm–Nd model age of 2.84 Ga and ε_Nd = 2.67 were formed during the Early Neoarchean crust-forming event. The younger rocks have a different contribution of ancient crustal material: significant contribution for dacites (Sm–Nd model age of 3 Ga and ε_Nd = 0.4) and less significant contribution for dacitic andesites (Sm–Nd model age of 2.89 Ga and ε_Nd = 1.73). In the Paleoproterozoic (from 1786 ± 11 to 1796 ± 6 Ma), the supracrustal rocks of the Kichany structure underwent metamorphic transformations.