Geochemistry International最新文献

The problem of identifying scarce PGE–Cu–Ni-bearing intrusions among the huge array of barren mafic bodies in the northwestern Siberian Platform has been faced by researchers for several decades. Its solution is usually based on the geological and geophysical methods. Geochemical studies including modern elemental and isotopic analytical data are much less frequently applied for this purpose. We applied such an approach to some Norilsk complex bodies containing sulfide mineralization. Using the Maslovsky deposit located in the Norilsk syncline as an example, we have demonstrated the characteristic features of ore-bearing rocks that can be used in the search for new promising targets. The rocks of the Maslovsky deposit were studied in two sections from boreholes OM-4 and OM-24. Their geochemical parameters fall within the ranges of ε_Nd = 1.0 ± 1.0 and (La/Lu)_n = 2.3 ± 0.8, which differ the magmatic bodies of the Norilsk district with unique sulfide ores from barren massifs. The ⁸⁷Sr/⁸⁶Sr ratios in the representative gabbroic rocks from the vertical cross-sections of the Maslovsky deposit vary from 0.7056 to 0.7069. As PGEs are accumulated in the rocks, the Pd/Pt ratio increases from ~1 at clarke contents to ~3 in rich ores. No evidence of in situ assimilation by melts of silicate rocks was found.

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.

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.

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.

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.

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.

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.

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.

This paper documents the zircon U–Pb ages, whole-rock geochemistry, and Sr–Nd–Pb isotopes of the Mesozoic granites in the central part of the North Qilian Orogenic Belt to provide information on the tectonic evolution and crustal accretion process of the Qilian Orogenic Belt. Zircon U–Pb dating yields an age of 215.3 ± 3.1 Ma, indicating that the Beidaban monzogranites formed from Late Triassic. They are characterized by high contents of SiO₂, Al₂O₃, and K₂O; are slightly peraluminous (A/CNK = 1.08–1.15); and have mineralogical assemblages of primary biotite and ilmenite, illustrating that they are shoshonitic and peraluminous S-type granite. The Beidaban monzogranites have initial (⁸⁷Sr/⁸⁶Sr)_i values ranging from 0.71456 to 0.71867 and εNd(t) values ranging from –12.9 to –8.5 with two-stage Nd model ages of 1.69–2.04 Ga, suggesting that they originated from partial melting of the Paleo-Mesoproterozoic (Longshoushan Group) continental crustal metasedimentary rocks. Initial Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 19.44–21.80; ²⁰⁷Pb/²⁰⁴Pb = 15.76–15.89; ²⁰⁸Pb/²⁰⁴Pb = 39.62–41.26) and geochemical features such as high Th/Ta (9.3–67.4, 37.4 on average) and Rb/Nb (12.5–17.1) are consistent with recycled crustal components. Combined with previous geochronological and geochemical data, we suggest that the Mesozoic granites probably formed in a post-collisional tectonic setting and that the North Qilian Orogen Belt experienced comprehensive intracontinental orogenesis after the closure of the Qilian ocean.