Geochemistry International最新文献

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.

Granulites exposed in high-grade regional metamorphic belts and exhumed as xenoliths in basaltic pipes are considered as window into the deep crust thus play a key role in constraining models of crustal processes and evolution. Here we present a detailed investigation of the tectono-metamorphic history of the two-pyroxene mafic granulite located in the southern region of the Sonapahar area. This involves conducting monazite chemical dating, analyzing petrological and geochemical characteristics, applying geothermobarometry, performing phase equilibria modeling, and tracing a pressure-temperature (P-T) path. Metamorphic P-T conditions estimated for the mafic granulite using conventional thermobarometer and winTWQ shows temperature in excess of 800°C and pressure of about 8.6 kbar, stand for high temperature granulite facies metamorphism. The metamorphic evolution path obtained from P-T pseudosection suggest a clockwise P-T evolution path, thus signify isothermal decompression and indicate rapid upliftment. Geochemical study of trace and rare earth elements (REE), suggest protolith is of tholeiite basalt in nature that is derived from back arc basin setting near to subduction zone. Additionally, the analyzed rock was examined using primitive mantle-normalized trace element spider diagram. The results indicate an enrichment in large-ion lithophile elements (Th, U, K, Pb) and a depletion in high field-strength elements (Nb, Ta, Ti). The presence of negative anomalies in Nb and Ti, coupled with elevated values of Th, K, and Pb, suggests the possibility of crustal contamination. Monazite chemical data from the studied rock reveals a peak metamorphism age of 521.3 ± 4.20 Ma, which corresponds to the Kuunga Orogeny in the later phase of global Pan-African collision.

A geochronological and isotope–geochemical study of alkaline basalts from three areas of young magmatism within the northeastern part of the Arabian Plate (Southeastern Turkey), Batman, Kurtalan and Alemdağ, was carried out. The obtained isotope data have indicated that the volcanism in the studied region developed over a 5-Ma period from the end of Miocene to the middle Pleistocene during four pulses separated by breaks in magmatic activity: 6.1–4.9 Ma (Batman area, hawaiites), ~3.0 Ma (Alemdağ plateau, phase I, basalts), 2.0–1.9 Ma (Alemdağ plateau, phase II, tephrites), and 1.5–1.3 Ma (Alemdağ plateau, phase III, basalts; Kurtalan area, basalts). A comparison of spatial–temporal changes of magmatic activity evolution in the studied part of the Arabian Plate and within the largest basalt plateau of Arabian foreland, Karacadağ Plateau, located to the west, was carried out. The results of Sr–Nd–Pb isotope–geochemical studies show that the development of young basalt volcanism in the Arabian Plate was characterized at different time by the contribution of various mantle sources in magma generation under this region. Initial pulses of magmatic activity are associated with melting of Arabian subcontinental lithospheric mantle (SCLM). The processes of fractional crystallization combined with crustal assimilation (AFC) have played an important role in the petrogenesis of lavas as well. Later, a deep mantle source (PREMA) with a depleted isotopic composition played a leading role in the formation of basaltic magmas of increased alkalinity. The melts generated by this source were mixed with the SCLM material in various proportions at different stages of magmatism with a limited participation of AFC processes in the petrogenesis of the rocks. It was concluded that young basalt volcanism of increased alkalinity in the northeast of the Arabian Plate is not related to the collision of the Eurasian and Arabian plates genetically, but presumably manifested here as a result of the migration of the initial rift geodynamic setting from the Red Sea basin to the north along Levantine and East Anatolian transform faults due to directed convection flows in the lower part of mantle under this part of the Earth.

Pargasite stability was experimentally studied in IHPV at ({{P}_{{{{{text{H}}}_{{text{2}}}}{text{O}}}}}) = 2 kbar and temperatures of 1000 to 1100^oC, with equilibrium approached from above and below. Calcic amphibole was used to experimentally model processes that occur in a volcanic chamber at pressures up to 5 kbar. The phase diagram of pargasite has been refined. It has been established that the stability of pargasite is controlled by three reactions. (1) At low water pressures of less than 1 kbar, the dehydration reaction Prg = Fo + Sp + Di + Ne + An + H₂O proceeds. (2) At water pressures higher than 1.2–1.5 kbar and a temperature of about 1100°C, the decomposition of pargasite is controlled by its incongruent melting Prg = Fo + Sp + {Di + Ne + An}^L + H₂O. (3) The third reaction Prg + L = Fo + Sp + Di + {Ne + Pl}^L + H₂O occurs within the same pressure range as the previous one but at lower temperatures of about ~1050°C. The reaction controls the pargasite liquidus and is caused by interaction between amphibole and coexisting melt. The liquidus of pargasite seems to most strongly depend on the activity of silica ({{a}_{{{text{Si}}{{{text{O}}}_{{text{2}}}}}}}) in the melt.