Geochemistry International最新文献

Widespread granites in the Wuzhuerhada area within the Tataleng granitic batholith in NW China show notable U, F, W, B, Rb, Be, and Sn geochemical anomalies. However, the petrogenesis and mineralization potential for them are unexamined for now. This study investigated the major and trace element compositions, zircon U–Pb ages, and zircon Hf isotope and trace element compositions for the Wuzhuerhada rapakivi granite and the granite dyke in it. Two samples for the rapakivi granite yielded weighted mean ²⁰⁶Pb/²³⁸U zircon ages of 430 ± 2 and 433 ± 4 Ma, respectively, and one sample for the granite dyke yielded a weighted mean ²⁰⁶Pb/²³⁸U zircon age of 433 ± 4 Ma, indicating comparable emplacement ages of the Early Silurian granites. Geochemically, the Wuzhuerhada rapakivi granite and the granite dyke are slightly peraluminous, with relatively high ASI values, high alkali contents, and negative Eu anomalies, suggesting I-type granites. Geochemistry and zircon ε_Hf(t) values of –14.2 to –4.9 for them suggest that they were all derived from the partial melting of igneous rocks within the Paleoproterozoic relatively shallow mafic reworked lower crust in a typical syn-collisional setting. Minus ΔFMQ value, low quantitative oxygen fugacity (fO₂), and low Ce⁴⁺/Ce³⁺ ratios for zircons suggest that the studied granites have relatively reduced redox states. But the granite dyke shows fractionated whole-rock geochemistry and contains some hydrothermal-affected zircons, indicating that the granite dyke has more Sn–W–(U) mineralization potential than the rapakivi granite or other neighbour granites. Our contributions would help delineate possible Sn–W–(U) ore-prospecting areas where fractionated granite was emplaced in the South Qilian belt.

The paper presents quantitative analyses (ICP-MS) of surface, snowmelt, and rainfall waters for 30 elements and analyses of the bottom silts for 49 elements for mountain lakes in the Iolgo Range, Altai Mountains; and data on physicochemical characteristics of the water bodies (pH, redox potential (RP), and electrical conductivity). The surface waters are ultrafresh (mineralization (total dissolved solids, TDS) 5.7–43 mg/L) and slightly alkaline to alkaline (pH 7.6–9.0), and their RP corresponds to an oxidizing geochemical environment (166–225 mV). The water of all the lakes belongs to the calcic group. The predominant cations in the snowfield are Na, K, and Ca, and those in the rainfall are K, Si, and Ca. The characteristics of the water bodies are assumed as background ones for the territory. The content of pollutants does not exceed the maximum allowable concentration (MAC) in drinking water. The low concentrations of humic acids in the lean stony soils, long-lasting contact of the waters with Na-rich weathering products of the underlying sedimentary–volcanic rocks, and low CO₂ concentrations in the regional atmosphere are favorable for the formation of alkaline lake waters in the upper part of the Karakol valley. The main factors in the formation of the most alkaline and mineralized waters of all of the lakes studied in the upper Karakol valley are geochemical features of the bottom silts of these lakes and their stagnant nature, which facilitates the accumulation of elements over a long period of time. The lake silts inherit geochemical markers from the underlying volcanics, indicating the geodynamic setting of their formation. The PM-normalized multielemental patterns of the bottom silts show enrichment in LREE and LILE, depletion in HREE, and clearly discernible Nb, Ta, and Ti minima, which indicates that they contain weathering products of subduction-related rocks (which formed on an active continental margin). The study area is located within the Saganyy-Kylay Pb–Zn ore region with Pb and Cu ore occurrences and with W and Zn lithochemical anomalies. Elevated contents of these and associated metals are found both in the watercourses of the Tura and Ugul river basins and in the bottom silts of the lakes. Contents of Ni, Zn, and Pb in the surface waters and concentrations Be, V, Cu, Zn, Mo, W, and Pb in the lake silts exceed the Clarke values for lacustrine waters and the Earth’s crust, respectively, which corresponds to the metallogenic specifics of this region.

Kimberlite and lamproite are rare alkaline rocks that exhibit high concentrations of alkalis and volatile contents. These rocks are unique in their geochemical characteristics and are highly susceptible to alteration processes which can result in the depletion of mobile elemental (such as K, Na, and LILEs) concentrations. Despite geochemical similarities, kimberlites and lamproites have certain differences and do not plot within their respective fields when using the extant discrimination diagrams. To address this issue, here we develop new discrimination diagrams based on HFSEs (such as Zr, Nb, Hf, Ta, Ti, Y), Ce and Yb. HFSEs have high affinity to alkaline rocks, and also remain immobile during various magmatic and post-magmatic processes. Although their concentration or geochemical behaviour varies distinctively between lamproites and kimberlites, which make them ideal for a precise classification of kimberlites and lamproites. The newly proposed discrimination diagrams are effective in discriminating kimberlite from lamproite with 92 to 97% accuracy despite their variable potassium and titanium contents, altered/unaltered and contaminated/non-contaminated nature. These new discrimination diagrams based on HFSEs offer a promising solution to the existing challenge of accurately classifying these rocks with the combination of other geochemical parameters such as petrographic, mineral-chemistry, and petrochemical investigations.

Halogens play an essential role in mantle petrology, but no data on halogen content in diamonds are available to date. Concentrations of fluorine and chlorine in diamond were determined quantitative for the first time, using quantitative SIMS analysis based on external standards prepared by the ion-implantation of halogens. Fluorine concentrations in diamond vary from 0.018 to 0.036 at ppm (3.2‒6.3 × 10¹⁵ at/cm³); chlorine concentrations are similar, from 0.014 to 0.034 at ppm (2.4‒4.5 × 10¹⁵ at/cm³). Most likely, F and Cl are related to microinclusions in diamonds, although one cannot exclude their position in the diamond lattice. The source of halogens in the studied diamonds is complex. A part of F and Cl is juvenile, remained from their primary concentrations. Another part, forming the halogen repository in the deep Earth, comes to the mantle via subduction. Fluorine may form the fluorine–vacancy (F–V) complex in the diamond structure, F and Cl may be compositional parts of microinclusions in diamonds as well. The F/Cl ratio in the studied diamonds (1.00–1.82) is similar to F/Cl ratios in kimberlites (0.38‒1.68). It differs from the estimates for the Earth’s mantle (0.62–0.68) but is close to enstatite chondrite values (1.16–2.77).

The results of our studies of iron-bearing phases in various meteorites using Mössbauer spectroscopy with a high velocity resolution were briefly reviewed. Examples of obtained Mössbauer spectra of meteorites and their fitting were considered and demonstrated advantages of this technique for revealing spectral components related to various iron-bearing phases that could not be observed in the spectra recorded by conventional Mössbauer spectroscopy. It was shown that the obtained results can be used for the phase identification, phase analysis, estimation of variations in the local microenvironment and parameters of ⁵⁷Fe hyperfine interactions, determination of cation ordering in silicate crystals, and calculation of the temperatures of equilibrium cation distribution as well as for the systematization of ordinary chondrites.

Focused on the significant scientific question regarding the chemical reaction pathway of methane in high maturity gas-pools generation from C₂–C₄ hydrocarbons, the fluid inclusions trapped within hydrothermal quartz which developed in Dengying Formation were analyzed. The paleo-oil and gas pools of the Dengying Formation exhibit high maturity, with the crude oil having transformed into methane-dominated gas pools completely. The selected fluid inclusion assemblage (FIA) consisted of one pyrobitumen inclusion and less than 20 methane inclusions. This suggests that the FIA was formed through the cracking of precursor oil-H₂O immiscible inclusions. Importantly, the FIA track was confined to the interior of the host mineral quartz, indicating that the thermal cracking of oil occurred within a closed system inside the quartz. The composition of the methane inclusions primarily consisted of CH₄, with trace amounts of CO₂, H₂S, and aromatic hydrocarbons. The absence of pyrobitumen in methane inclusions refutes the notion that methane in highly mature gas pools is directly generated by the fracture of carbon–carbon bonds through C₂–C₄ hydrocarbons. The presence of aromatic hydrocarbons in the methane inclusions suggests that methane in highly mature gas pools may be generated through the demethylation of aromatic ring systems.

The paper reports new geological, geochemical, and U–Pb zircon ages on the northern flank of the Kukasozero structure. It has been established that biotite and amphibole–biotite gneisses of the northeastern framing of Lake Kukas were formed after calc-alkaline andesites and dacites and are complete analogues of greenstone rocks of the adjacent Neoarchean Chelozero structure. The studied volcanic sequence was formed in the Neoarchean at the stage of 2775–2715 Ma, under conditions close to the modern island-arc settings. Sm–Nd isotope-geochemical data indicate an increase of subduction contribution at the final stages of volcanic activity. The central and western parts of the northern shore of Lake Kukas are composed of alternating schists of different mineral composition. Amphibole and biotite–amphibole schists correspond to the calc-alkaline intermediate volcanics formed in island-arc settings. They are comagmatic to the gabbro-diorite bodies with an age of 2739 ± 6 Ma cutting across these sections. The origin of two-mica (muscovite–biotite) schists is controversial: one group is petrogeochemically close to graywackes, while the other, most likely, was produced by metasomatic reworking of greenstone rocks. It has been suggested that the studied two-mica schists previously attributed to the Paleoproterozoic rocks may be Neoarchean (one of the segments of the Chelozero greenstone structure). They were reworked by strong tectonic-metamorphic processes at the Paleoproterozoic collisional stage, when the island arc was accreted to the Karelian craton margin.

This paper presents results of geochemical studies of source-rock, reservoir-rock, and oil samples from oilfields in the Udmurt Republic. The molecular and isotopic composition of the oils and bitumens shows their strong genetic relations to a single source of liquid hydrocarbons: the Domanik (Semiluki) source rocks (of Late Devonian to Early Tournaisian age, D₃f₂). At the same time, features of the molecular and isotopic composition of the hydrocarbons suggest the participation of several interrelated kitchens of same genetic type for the different parts of this territory. This information will be useful for verifying basin modeling results and planning geological explorations at the territory.

The Laptev Sea belongs to the polar climate region, characterized by minimal influence on geochemical weathering processes. The dominant role in the formation of sedimentary rocks in high latitudes is played by mechanical sorting of sedimentary material. Using 15 sediment cores located along 120°‒130° E from the Lena Delta to the continental slope and in the Sadko Trough as an example, a close relationship between the grain-size and geochemical composition of marine sediments was revealed. The results of the study showed that shelf sedimentation occurs in at least two stages: (i) accumulation of sediments in coastal area, the area directly adjacent to the sources of sediments, and (ii) the subsequent mechanical sorting of sedimentary material on the shelf itself. At the first stage, the finest clay fraction is partially separated from the primary terrestrial sedimentary material. At the second stage, as one move away from the coastal area, the sand and silt fractions are separated. This sedimentation pattern is determined by the combination of the Lena Delta morphology and the Siberian Coastal Current. In conditions of an unstable hydrodynamic regime of the shallow shelf, predominantly sandy sediments (SiO₂/Al₂O₃ > 4.5) accumulate at a rate of <0.5 g cm^‒2 yr^‒1. These sediments differ from the sediments of the terrestrial area by a higher content of Si and a lower content of Ti, Mg, Fe and Ni. In conditions of a calm hydrodynamic environment of the Sadko Trough, predominantly silty sediments (SiO₂/Al₂O₃ < 4.5) accumulate at a rate of >0.5 g cm^‒2 yr^‒1, characterized by a higher content of Ti, Mg, Fe and Ni and a lower content of Si. The contents of K, Rb, Cr and Ca in shelf sediments do not show a clear correlation with grain-size distribution and are thus not informative for reconstructing sedimentation patterns.

Helium is an indispensable and scarce strategic resource. Both research and exploration targeting geothermal-associated helium resources in geothermal fields on both sides of the Cangdong Fault, China remain entirely lacking. To investigate the helium origin, source rocks, phase partitioning, as well as identify helium accumulations, the research team collected geothermal gases and water samples from 25 sampling sites across different geothermal reservoirs in the study area. The collected samples were subjected to comprehensive geochemical analyses including: helium isotope composition, major gas composition, CH₄ and CO₂ carbon isotope, and hydrochemical parameters. The test results show that the helium in the study area is mainly of crustal origin; the average proportion of mantle-derived helium of sandstone geothermal reservoirs, bedrock geothermal reservoirs, and mixed geothermal reservoirs are: 2.78, 4.63, and 5.89%, respectively; N₂ accounts for the largest proportion of geothermal gases composition; CH₄ is coal-type or oil-type. Quantitative calculations revealed that granite and bauxite constitute the predominant helium source rocks, with free gas phase helium being hosted within geothermal reservoirs. The detection of helium-bearing horizons within bedrock geothermal reservoirs and adjacent regions surrounding sampling site D81 constitutes a critical factor for successful helium exploration.