Geochemistry International最新文献

Abstract

The contemporary scientific investigations delves into the mechanisms of migration, distribution, and fractionation of rare earth elements and yttrium (REE + Y) in aqueous environments characterized by humid and arid sedimentogenesis. This particular facet has been employed to solve numerous fundamental inquiries pertaining to lithology and paleolimnology. The manner in which REE + Y behave in water bodies within the cryolithozone remains largely unexplored. By evaluating the concentration levels and distribution patterns of REE + Y in the waters and sediments of the lakes situated in the Ukok Plateau and the Ulagan depression, under the prevailing conditions of cryolithogenesis as the primary geochemical process transforming rocks, soils, and sediments, we were able to augment the existing comprehension of nival sedimentation. In all the lakes examined, the content of REE + Y in the water exhibits a similar magnitude as that of small lakes in other landscape zones of the southern Western Siberia, while the content level of REE + Y is greater in the bottom sediments. The concentrations, distribution and fractionation of REE + Y vary from lake to lake and are determined by several factors: the chemical properties of REE + Y in solutions, the intensity of transformation of host rocks and the mineral forms of the REE + Y in the rocks (minerals). The concentrations, distribution and fractionation of REE + Y in bottom sediments vary slightly from lake to lake except for the contents of Eu, Y, La. The content of Eu, Y, La in the bottom sediment of the lake is influenced by a combination of factors, including their concentrations in the water, the redox conditions of diagenesis, and the mineral composition of the bottom sediment, particularly the amount of accessory minerals.

Abstract

The East Qinling Molybdenum Belt (EQMB), which is located on the southern margin of the North China Craton (NCC), is the largest Mo province in the world. This belt hosts a significant number of Mesozoic magmatic-hydrothermal Mo deposits and a small portion of pre-Mesozoic Mo deposits. Understanding the mineralization timing and mechanism of the unique pre-Mesozoic Mo deposits is essential to comprehend the evolution of the EQMB, the pre-Mesozoic Mo enrichment, and the Mesozoic Mo mineralization event. The recently discovered Zhaiwa deposit is a porphyry Mo deposit located in the Xiong’er Terrane of the EQMB. In this study, five molybdenite samples from the Mo-bearing quartz veins were analyzed for Re-Os isotopes composition. These samples yield an isochron age of 1794 ± 45 Ma, which represents the age of mineralization. The mineralization is mostly hosted within the biotite-amphibole plagiogneiss and granite porphyry. LA-ICP-MS U-Pb data of zircons constrain the crystallization age of the granite porphyry to be at 1791 ± 16 Ma. The close spatial and temporal association suggests that the granite porphyry is the causative rocks of the Mo mineralization. The δ³⁴S values of pyrite vary from 5.3 to 6.8‰, suggesting that the S was mainly derived from magmatic source. The intrusion of magmas and associated Mo mineralization are contemporaneous to the regional Xiong’er volcanism that occurred during the late Paleoproterozoic. The Xiong’er volcanism was triggered by partial melting of lithospheric mantle in an extensional setting. The results of our study provide robust evidence for a late Paleoproterozoic Mo metallogenic event along the southern margin of the NCC. Future exploration should also consider the potential of late Paleoproterozoic porphyry Mo mineralization existing in the EQMB, which is closely associated with the Xiong’er volcanism.

Abstract—

The layer-by-layer analysis of the variation dynamics of chemical parameters of snow at one of the observation sites (at the village of Yaksha) in the Pechora–Ilych state biosphere reserve in the winter of 2019–2020 has shown that the chemical composition of atmospheric precipitation is affected dominantly by long-range material transport. Features of the atmospheric circulation and the regions from which air masses are transferred control the saturation of the precipitation with certain chemical components. The calculation of the trajectories of reverse transport of air masses allowed us to identify regions where the air masses can be formed that come to the study area and carry material that potentially affects the chemical composition of the precipitation. The calculation of trajectories is demonstrated to make it possible to identify source regions of pollutants entering the atmosphere. This method of studying the chemical composition of snow is generally very informative and enables better understanding its formation factors.

Abstract—

Important problems of magma differentiation, formation of native metals, and ore-forming processes in the Earth’s crust are often related to participation of hydrogen. In this paper, new experimental data on the crystallization of andesite melts at high temperatures (900–1250°C) and hydrogen pressures (10–100 MPa) have been obtained, which clarify the possible role of hydrogen in the processes occurring in andesite melts in the Earth’s crust and during volcanism under strongly reduced conditions ((f{text{O}}_{2}) = 10^–17–10^–18). In the crystallization experiments, it was found out that the compositions of the crystals (pyroxenes and plagioclases) formed in experiments on crystallization of andesite melt under hydrogen pressure closely correspond to the crystal compositions of lava flows of Avacha volcano in Kamchatka. This result can be considered as an experimental confirmation of the participation of hydrogen in the volcanic process.

Abstract—

The paper presents data on the formation of K–Na richterite in the enstatite + diopside association with K₂CO₃–Na₂CO₃–CO₂–H₂O fluid at 3 GPa and 1000°C as a model for the formation of this mineral in peridotites of the upper mantle. Richterite formation depends on the (H₂O + CO₂)/(K₂CO₃ + Na₂CO₃) and K₂CO₃/Na₂CO₃ ratios in the starting material. A high concentration of alkaline components in the fluid leads to the decomposition of clinopyroxene, the formation of olivine, and a change in the component composition of the pyroxene and amphibole. Fluids with a high potassium concentration are favorable for the formation of K-richterite similar in composition to that formed in metasomatized peridotites of the upper mantle. In some cases, such a fluid leads to the decomposition of amphibole and stabilization of alkaline melt. An increase in the activity of the sodium component results in richterite similar in composition to richterite from lamproites. The clarified relations can be used to assess the activities of fluid components and conditions for the formation of K-richterite. To update the data bank of the Raman spectra of minerals, the largest and most homogeneous amphibole crystals of different compositions were studied.

Abstract—

New data on roméite (CaNa)Sb₂O₆F solubility in the NaF–H₂O system of P–Q type have been obtained within a wide range of sodium fluoride concentrations (from 0 to 25 wt % NaF). The concentration of antimony, in equilibrium with roméite and fluorite, in the range of NaF concentrations from 1 to 8 mol kg^–1 H₂O (25 wt % NaF), is in the range of 0.02–0.2 mol kg^–1 H₂O. According to the data, the concentration of antimony in phases L₁ and L₂ in the region of fluid immiscibility of the NaF–H₂O system at 800°C, 200 MPa and fO₂ = 50 Pa, specified by the Cu₂O–CuO buffer, is 0.4 and 2.1 wt % Sb, respectively. Our experiments were the first ever to produce skeletal fluorite crystals and the intermetallic compound Pt₅Sb, which belongs the hexagonal crystal system and has the following lattice parameters (LP): a = b = 4.56(4) Å, c = 4.229(2) Å, and α = β = 90°, γ = 120°. Pentaplatinum antimonide was formed on the inner surface of the Pt capsules at 800°C, Р = 200 MPa, and fO₂ ≤ 10^–3.47 Pa (Cu–Cu₂O buffer) in experiments on the incongruent dissolution of roméite, which causes a sharp decrease (more than 1000 times) in antimony concentration in the solution.

Abstract

The paper considers the current state of Platinum Group Elements (PGEs) geochemistry in the ocean. The behavior of PGEs in the aquatic environment is defined by their oxidation state, the ability to change it, and complexation. The difference in chemical properties leads to PGEs fractionation in the ocean. This is their characteristic feature, along with their ultra-low contents. The paper describes the sources of PGEs supply to the ocean, PGEs behavior in the river–sea mixing zone, and their distribution in seawater. The processes of PGE accumulation in sediments, seafloor sulfides, and ferromanganese deposits of the ocean are reviewed. Possible mechanisms of PGE accumulation on ferromanganese oxyhydroxides are discussed.

Natural Mg phyllosilicates are potential sources of divalent cations, which are necessary for the mineralization of CO₂ into carbonates. The influence of inorganic (({text{HCO}}_{3}^{ - })) and organic (oxalate and citrate) ligands on the dissolution kinetics of talc and serpentine was studied in experiments in a flow-through reactor at 25°C. The dissolution rates of natural silicates r (mol cm^–2 s^–1) in solutions of various composition were calculated at the stationary stage of dissolution after a rapid initial stage, which is characterized by the formation of a surface leached layer depleted in magnesium. The presence of ligands increases the dissolution rate of magnesium silicates due to the formation of surface complexes, which leads to magnesium separation from the surface and transfer into solution. The initial incongruent stage may be the most promising for the development of carbonation technologies, because the minimum removal of the network-forming elements prevents the undesirable formation of secondary minerals (for example, clays), which exclude divalent cations from the carbonation process and greatly reduce the permeability of rocks.

Abstract

The solubility of loparite (Na,Ce,Ca)₂(Ti,Nb)₂O₆) in aluminosilicate melts of various compositions was experimentally studied at T = 1200 and 1000°C and P = 2 kbar under dry conditions and in the presence of 10 wt % H₂O in a high gas pressure vessel with a duration of 1 day. The starting material was previously melted artificial glasses of malignite, urtite and eutectic albite–nepheline compositions, as well as natural loparite from the Lovozero massif. The dependence of the loparite solubility on the composition of the aluminosilicate melt (Ca/(Na + K), (Na + K)/Al) has been revealed. Partition coefficients of a number of elements (Ti, Nb, Sr, REE) between silicate melt and loparite crystals (K_i = ({{C_{i}^{{{text{melt}}}}} mathord{left/ {vphantom {{C_{i}^{{{text{melt}}}}} {C_{i}^{{{text{lop}}}}}}} right. kern-0em} {C_{i}^{{{text{lop}}}}}})) were estimated.

Abstract

Olivine grains from the Seymchan pallasite were studied using optical microscopy, Raman spectroscopy, and scanning electron microscopy (SEM). Olivine is characterized by the presence of hollow straight channels <1 µm wide and inclusions of hollow negative crystals of prismatic habit 1–2 µm thick. The channels are oriented parallel to [001] of olivine and developed along [001] screw dislocations. The elongation axes of negative crystals are also oriented parallel to [001]. In the channels, hollow segments alternate with segments filled with metallic iron. Negative crystals are crystallographically faceted voids in olivine; the largest of them contain inclusions of metallic iron. The rectilinear configuration and crystallographic orientation of the channels correspond to the characteristics of [001] screw dislocations, which allows us to consider [001] dislocations as channel precursors. The data obtained demonstrate for the first time the evolution of [001] dislocations in olivine as a result of the shock-induced reduction of divalent iron during the interaction of olivine with the host FeNi metal. A model is proposed for the transformation of dislocations with the formation of channels and hollow negative crystals in Seymchan olivine in accordance with one of the reactions:(begin{gathered} 2{text{F}}{{{text{e}}}_{{{text{host}}}}} + {{left( {{text{M}}{{{text{g}}}_{{1 - n}}}{text{F}}{{{text{e}}}_{n}}} right)}_{2}}{text{Si}}{{{text{O}}}_{4}} = 2n{{left[ {{text{FeO}}} right]}_{{{text{host}}}}} + {{left[ {n{text{Si}}{{{text{O}}}_{2}} + 2n{text{F}}{{{text{e}}}^{0}} + left( {1 - n} right){text{M}}{{{text{g}}}_{{text{2}}}}{text{Si}}{{{text{O}}}_{4}} + 2n{{v}^{{2 - }}} + 2n{{v}^{{2 + }}}} right]}_{{{text{ol}}}}}, 2{text{F}}{{{text{e}}}_{{{text{host}}}}} + {{left( {{text{M}}{{{text{g}}}_{{1 - n}}}{text{F}}{{{text{e}}}_{n}}} right)}_{2}}{text{Si}}{{{text{O}}}_{{text{4}}}} = 2n{{left[ {{text{FeO}}} right]}_{{{text{host}}}}} + {{left[ {n{text{MgSi}}{{{text{O}}}_{3}} + n{text{F}}{{{text{e}}}^{0}} + left( {1 - n} right){text{M}}{{{text{g}}}_{{text{2}}}}{text{Si}}{{{text{O}}}_{4}} + n{{v}^{{2 - }}} + n{{v}^{{2 + }}}} right]}_{{{text{ol}}}}}. end{gathered} )According to the model, at T > 1000°C the reduction process is accompanied by an increase in the concentration of Fe⁰ and associated vacancies (({{v}^{{2 - }}}) and ({text{ + }}{{v}^{{2 + }}})) in dislocation zones. Voids in channels and negative crystals are the products of the annihilation of anionic and cationic structural vacancies having opposite charges. Phase association formed in this solid-state transformation of olivine corresponds to either OSI (olivine → SiO₂ + 2Fe⁰) or OPI (olivine → pyroxene + Fe⁰) buffer equilibrium. The results can be used for the reconstruction of the thermal and shock histories of different types of pallasites.