Geochemistry International最新文献

As a result of a study of igneous rocks of the basalt - andesite series, dredged on the Shaka Ridge in the South Atlantic, it was found that they differ from the basalts of mid-ocean ridges and ocean islands, and have an age of 183.8 ± 2.2 Ma, comparable to the time of manifestation of the Karoo-Maud mantle plume in central Gondwana. Geochemical and Sr–Nd–Pb isotopic features of the studied igneous rocks show their similarity with the Jurassic mafic complexes of the Ferrar province in Antarctica and the Falkland Islands, formed during the intrusion of the Karoo-Maud plume and under the influence of paleo-Pacific subduction. However the supply of ice rafted debris into the study area due to ice transportation is considered unlikely. Based on the all data obtained, it was concluded that the Shaka Ridge is a continental block that was moved during the opening of the South Atlantic in the Early Cretaceous-Early Miocene from the continental margin of Africa along an extended transform fault into the present Bouvet triple junction area.

Twelve hydrothermal autoclave experiments were conducted with Domanik oil shale from the Ukhta region (Chut River) at temperatures of 250–375°C and run duration of 24 h (6 experiments), 72 h (5 experiments), and 48 h (1 experiment). The composition of hydrocarbon gases C₁–C₅ was studied for each experiment and quantitative data on their yields were obtained. Based on these data, the distributions of generation potential of individual gaseous hydrocarbons by activation energy were established under hydrothermal experimental conditions. The character of the kinetic spectra of individual alkanes C₂–C₅ is virtually identical; their main narrow maximum corresponds to E_a 55 kcal/mol with an Arrhenius factor of 1 × 10¹⁴ s^–1. The distribution of the methane generation potential by activation energies is distinguished by the fact that a significant part of its generation potential falls within the region of activation energies of 60–70 kcal/mol and by the uncertainty of the distribution character in this region.

The conditions of the formation of K-cymrite in volatile-rich pelite and partially devolatilized mica quartz–muscovite–chlorite schist were experimentally investigated at pressures of 5.5, 6.3, and 7.8 GPa and temperatures ranging from 900 to 1090°C corresponding to hot subduction geotherm. Experimental samples at these P–T conditions formed assemblage of solid phases (Grt + Coe + Phe + Cpx + Ky, with accessory Po + Ru + Zrn ± Mnz) and water-enriched supercritical fluid–melt. Analysis of the obtained data indicates that the stability of phengite and its potential replacement by K-cymrite depends on the P–T conditions and the amount of volatiles in the metasediment. In samples of volatile-rich pelite and mica schist at 5.5 GPa and 900°C, as well as at 6.3 GPa and 1000°C, phengite remains stable in equilibrium with 3–13 wt % of the fluid–melt. With increasing pressure up to 7.8 GPa and temperature up to 1090°C, the fraction of supercritical fluid–melt in pelite reaches 20 wt %, while phengite disappears. Only 5 wt % supercritical fluid–melt are formed in the schist at 7.8 GPa and 1070°C, while most part of phengite is preserved. For the first time, phase assemblage with phengite and K-cymrite (±kokchetavite) was obtained in the pelite and schist samples at 7.8 GPa and 1070°C. The assemblage was identified using Raman mapping. At stepwise devolatilization (with removal of fluid–melt portion forming in equilibrium with volatile-bearing minerals that are stable at P–T conditions of experiments), phengite has been preserved up to 7.8 GPa and 1090°C, but K-cymrite is not formed in the absence of fluid–melt. It was concluded that the most effective transport of volatiles (first of all, water) in the metasediment to depths over 240 km may occur during its partial and early (before the formation of supercritical fluid–melt) devolatilization. In this case, almost all phengite may reach depths of 240 km during metasediment subduction and then transform into water-bearing K-cymrite, or, in the presence of nitrogen in the metasediment, into nitrogen-bearing K-cymrite, thus facilitating the further transport of LILE (large-ion lithophile elements), water, and nitrogen. However, the formation of a significant portion of supercritical fluid–melt leads to the complete dissolution of phengite with increasing P–T conditions, making further transport of LILE, water, and nitrogen impossible. During deep multi-stage devolatilization, phengite remains stable up to depths of 240 km; however, during further subduction, it likely transforms into an anhydrous K-hollandite (KAlSi₃O₈).

The phase composition of native gold was examined in an insufficiently studied part of the Au–Ag–Cu system in the range between pure gold and Au₃Cu. In this region, a miscibility gap has been established for the Au–Ag–Cu solid solution, which is decomposed into Au–Ag–Cu and Au₃Cu phases. These results in combinations with previously obtained and literature data made it possible to construct a complete phase diagram of the Au–Ag–Cu system in the gold-rich region for low (about 100°C) temperature. The diagram demonstrates the field of a homogeneous Au–Ag–Cu solid solution, and two-phase fields (Au₃Cu and Au–Ag–Cu solid solution) and (AuCu and Au–Ag–Cu solid solution), which are separated by a three-phase field (Au₃Cu, AuCu, and Au–Ag–Cu solid solution).

The Early Proterozoic gabbros of the Velimyaki intrusion of the Northern Ladoga region contain titanomagnetite ore, which has been mined as early as the end of the 19th century. Titanomagnetite horizons are enriched in phosphorus in form of apatite reaching 10 vol %. Isotopic Pb–Pb dating indicates that apatite was likely redeposited during superimposed metamorphism that was significantly separated in time from the magmatic stage of gabbros and clinopyroxene–titanomagnetite ores. Mineralogical, petrological, and isotope-geochemical criteria for the superimposed nature of the mineral formation with apatite recrystallization are the relationship of this mineral with the formation of other metamorphic minerals (hornblende, biotite, sodic plagioclase), the isotopic age of apatite (1790 ± 5 Ma), and the lower temperature (620–710°C) of its formation compared to the crystallization temperatures (900–1260°C) of magmatic minerals. The Pb–Pb age of apatite coincides with the age of metamorphic minerals from other rocks of Late Svecofennian region, as well as with the Rb–Sr ages of biotite and amphibole from host supracrustal rocks. Based on the data obtained, it was concluded that recrystallization of apatite and resetting of the U–Pb system occurred during the Late Svecofennian regional metamorphism.

The speciation of chemical elements in the waters and its dependence on the dissolved organic matter were studied by a complex of methods, involving thermodynamic calculations and experimental fractionation. The waters were studied at the abandoned and flooded Herberz Mine in the Pitkäranta district, Karelia, Russia. The regional natural waters are typically highly humified. In combination with the unique metallogeny of the rocks, this makes the mine suitable for solving the formulated problems. The eastern shaft of the Herberz Mine was sampled to a depth of 20 m to trance the changes induced by changes in the redox conditions. One of the geochemical characteristics of the waters is their relatively high concentrations of trace elements and a low salinity (TDS, total dissolved solids). All water samples from the Herberz Mine contain elevated concentrations of Zn, Fe, Mn, Cu, Ni, As, and W. Experimental fractionation and thermodynamic simulations of the speciation of chemical elements led us to identify metals whose accumulation most strongly depends on organic matter (OM). Both methods have demonstrated that U, Th, Cu, Ni, and Y show a high chemical affinity to OM. Metals (Cd and Fe) weakly bonded to the functional groups of natural OM, with the predominance of electrostatic bonding and a higher proportion of carboxyl bonds, are most susceptible to transformations with changes in geochemical conditions.

As a large gold-bearing province, the Yenisei Ridge does not show elevated background gold concentrations. All types of its sedimentary, metamorphic, and igneous rocks, except only the carbonaceous black shales, contain concentrations of the noble metal at the level of its Clarke values. All local gold deposits are constrained within the regional Central Metallogenic Belt, in which geological–geochemical conditions occurred that were favorable for the deposition of gold and gold–uranium ore mineralization: most of the deposits are constrained within a trough structure, the area was affected by several pulses of plume magmatism, which introduced, redistributed, and concentrated gold and uranium, and the developing ore-concentrating and ore-controlling systems formed economic deposits and associated zones of hydrothermal metamorphism with geochemical aureoles of Pb, Zn, Ag, Au, Bi, and As.

SEM-EDS and SIMS in situ methods were used to study the trace element composition of zircon from rapakivi granites of the Wiborg massif: wiborgites of the second phase, trachytoid granites of the third phase, as well as aplitic granites from their contact zone. All three rock varieties are available for study in the building stone quarry of the Vozrozhdenie deposit (Karelian Isthmus), where the granites of the Gubanov intrusion are mined. Zircon composition from all rock types shows traces of active fluid impact. This impact is manifested both at the level of zircon internal structure (dark zones and areas on BSE-image) and in the contents of trace and rare-earth elements, which significantly increase in the altered zones that differ in the BSE color. The total REE content in the studied zircon reaches 9400 ppm. Zircon from granites of the third phase show an opposite slope of LREE and HREE distribution pattern, i.e., “bird wings” profile (Sm_N/La_N < 1). In the discrimination diagrams, the majority of the analyzed spots falls into the field of hydrothermal zircon. It is possible to assume that a source of fluid that affected zircon in all types of granites was fluid-saturated melts that produced trachytoid granites of the third phase.