Acta Geochimica最新文献

It is well-known that the equilibrium isotope fractionation of mercury (Hg) includes classical mass-dependent fractionations (MDFs) and nuclear volume effect (NVE) induced mass-independent fractionations (MIFs). However, the effect of the NVE on these kinetic processes is not known. The total fractionations (MDFs + NVE-induced MIFs) of several representative Hg-incorporated substances were selected and calculated with ab initio calculations in this work for both equilibrium and kinetic processes. NVE-induced MIFs were calculated with scaled contact electron densities at the nucleus through systematic evaluations of their accuracy and errors using the Gaussian09 and DIRAC19 packages (named the electron density scaling method). Additionally, the NVE-induced kinetic isotope effect (KIE) of Hg isotopes are also calculated with this method for several representative Hg oxidation reactions by chlorine species. Total KIEs for ²⁰²Hg/¹⁹⁸Hg ranging from − 2.27‰ to 0.96‰ are obtained. Three anomalous ²⁰²Hg-enriched KIEs (δ²⁰²Hg/¹⁹⁸Hg = 0.83‰, 0.94‰, and 0.96‰,) caused by the NVE are observed, which are quite different from the classical view (i.e., light isotopes react faster than the heavy ones). The electron density scaling method we developed in this study can provide an easier way to calculate the NVE-induced KIEs for heavy isotopes and serve to better understand the fractionation mechanisms of mercury isotope systems.

The tectonic evolution and crustal accretion process of the North Qilian Orogenic Belt (NQOB) are still under debate because of a lack of integrated constraints, especially the identification of the tectonic transition from arc to initial collision. Here we present results from zircon U–Pb geochronology, whole-rock geochemistry, and Sr–Nd–Pb isotope geochemistry of the Beidaban granites to provide crucial information for geodynamic evolution of NQOB. Zircon U–Pb dating yields an age of 468 ± 10 Ma for the Beidaban granites and most of the Beidaban samples contain amphibole, are potassium-rich, and have A/CNK values ranging from 0.7 to 0.9, illustrating that the Middle Ordovician Beidaban granites are K-rich, metaluminous, calc-alkaline granitoid. The geochemical characteristics indicate that the Beidaban granites are transitional I/S-type granitoids that formed in an arc setting. The isotopic compositions of initial (⁸⁷Sr/⁸⁶Sr)_i values ranging from 0.70545 to 0.71082 (0.70842 on average) and εNd(t) values ranging from − 10.9 to − 6.7 (− 8.8 on average) with two-stage Nd model ages (T_DM2) of 1.74–2.08 Ga suggest that the Beidaban granites originated from Paleoproterozoic crustal materials. In addition, the initial Pb isotopic compositions (²⁰⁶Pb/²⁰⁴Pb = 19.14–20.26; ²⁰⁷Pb/²⁰⁴Pb = 15.71–15.77; ²⁰⁸Pb/²⁰⁴Pb = 37.70–38.26) and geochemical features, such as high Th/Ta (17.43–30.12) and Rb/Nb (6.01–15.49) values, suggest that the Beidaban granite magma source involved recycled crustal components with igneous rocks. Based on these results in combination with previously published geochronological and geochemical data from other early Paleozoic igneous rocks, we suggest that the timing of the tectonic transition from arc to the initial collision to the final closure of the North Qilian Ocean can be constrained to the Middle-Late Ordovician (ca. 468–450 Ma).

The Pulang giant porphyry Cu-Mo polymetallic deposit is located in the Zhongdian area in the center of the Sanjiang Tethys tectonic domain, which was formed by the westward subduction of the Garze-Litang oceanic slab beneath the Zhongza massif. Chalcopyrite-pyrrhotite-pyrite-molybdenite occurs as disseminations, veins, veinlets, and stockworks distributed in the K-silicate alteration zone in the monzonite porphyry, which is superimposed by propylitization. The chemical compositions of biotite and amphibole analyzed by electron probe microanalysis (EPMA) indicate that the ore-forming magma and exsolved fluids experienced a continuous decrease in the oxygen fugacity (fO₂). Primary amphibolite and biotite (type I) crystallized at relatively high temperatures (744–827 °C) and low fO₂ (logfO₂ = − 12.26 to − 11.91) during the magmatic stage. Hydrothermal fluids exsolved from the magma have a relatively lower temperature (621–711 °C) and fO₂ (logfO₂ = − 14.36 to − 13.32) than the original magma. In addition, the presence of a high abundance of pyrrhotite and an insufficiency of primary magnetite and sulfate in the ore (i.e., anhydrite and gypsum) indicate that the deposit may be a reduced porphyry deposit. Magma and fluid fO₂ results, combined with previous research on magmatic fO₂ at the Pulang deposit, indicate that the magma associated with the reduced Pulang ore assemblages was initially generated as a highly oxidized magma that was subsequently reduced by sedimentary rocks of the Tumugou Formation.

The Gysian ophiolite of NW Iran is located at the intersection of the ophiolite belts of SE Turkey, NE Iraq, and Iran, and provides the opportunity to investigate the preserved subduction and obduction history of an important tectonic site that has not previously been studied. The serpentinized peridotites of the Gysian ophiolite contain the assemblage lizardite + chrysotile + spinel/Mg-spinel with relict clinopyroxene (diopside) and very rare relict orthopyroxene and olivine. The compositions of clinopyroxenes and spinels are more consistent with the formation of the inferred protolith harzburgites in a fore-arc or supra-subduction zone instead of an abyssal environment. The Gysian ophiolite is a remnant of the Neo-Tethyan arc-ophiolitic system and records shallow subduction (< 50 km, indicated by the absence of antigorite) in the Late Cretaceous to Paleocene before obduction along thrust faults over the continental margin. We review the spatial trends of the metamorphic grade of the Neo-Tethyan ophiolites in this region and provide detailed information about the petrology and mineral chemistry of the Gysian ophiolite.

Hydraulic fracturing is an effective technology for hydrocarbon extraction from unconventional shale and tight gas reservoirs. A potential risk of hydraulic fracturing is the upward migration of stray gas from the deep subsurface to shallow aquifers. The stray gas can dissolve in groundwater leading to chemical and biological reactions, which could negatively affect groundwater quality and contribute to atmospheric emissions. The knowledge of light hydrocarbon solubility in the aqueous environment is essential for the numerical modelling of flow and transport in the subsurface. Herein, we compiled a database containing 2129 experimental data of methane, ethane, and propane solubility in pure water and various electrolyte solutions over wide ranges of operating temperature and pressure. Two machine learning algorithms, namely regression tree (RT) and boosted regression tree (BRT) tuned with a Bayesian optimization algorithm (BO) were employed to determine the solubility of gases. The predictions were compared with the experimental data as well as four well-established thermodynamic models. Our analysis shows that the BRT-BO is sufficiently accurate, and the predicted values agree well with those obtained from the thermodynamic models. The coefficient of determination (R²) between experimental and predicted values is 0.99 and the mean squared error (MSE) is 9.97 × 10⁻⁸. The leverage statistical approach further confirmed the validity of the model developed.

The Yangla Cu skarn deposit is located in the central part of the Jinshajiang Suture Zone, southwest China, with a total reserve of 150 Mt Cu @ 1.03%. The newly discovered antimony orebodies at the depth of Yangla are strictly controlled by the stratum, structure, and lithology, which are lenticular and vein-like within the marble fracture zone, which can provide a window into multistage mineralization and ore genesis at Yangla. Mineralization can be divided into three types, Cu–Pb–Zn (skarn) pyrite, galena, and sphalerite, Cu (porphyry) chalcopyrite and pyrite, and Sb (hydrothermal) stibnite and pyrite. The mineral assemblages were stibnite + pyrite + calcite + quartz ± minor scheelite in antimony ores. This study presents quantitative measurements of the trace element compositions of pyrite and stibnite from the Yangla antimony ores. Analysis of pyrite with electron probe microanalysis (EPMA) showed enrichment in Co, Ni, Sb, As, and Mo, and deficit in its S and Fe contents when compared to the stoichiometric concentrations of S and Fe in pyrite. The Sb-related pyrite may belong to sedimentary-reworked genesis and may be modified by hydrothermal fluids, thereby presenting a certain difference (i.e., crystal morphology, texture, and chemical composition) compared to the skarn and porphyry Cu-related pyrite in the Yangla Cu skarn deposit. Analysis of stibnite with EPMA and inductively coupled plasma-mass spectrometry showed enrichment in As, Pb, Sn, Pb, Cu, and Zn, and presented much higher Sb contents and slightly lower S contents when compared to the stoichiometric concentrations of Sb and S in stibnite. Statistical analysis of the stibnite trace elements showed correlations for the elemental pairs Cu–Pb, As–Sb, and Sn–Pb, and the coupled substitution equations Sb³⁺ ↔ Cu⁺ + Pb²⁺, Sb³⁺ ↔ As³⁺, and Sn²⁺ ↔ Pb²⁺ may be the major factors governed the incorporating Cu, Pb, As and Sn within the stibnite. Moreover, this study preliminary shows that the antimony mineralization may belong to a carbonate replacement hydrothermal genesis at Yangla.

The petrographic and geochemical attributes of the Oligocene Barail Group of rocks are used to decipher the likely source area(s) or tectonic domains, as this sequence of rocks was deposited in a foreland basin governed by orogenic domain, namely the North-east Arunachal Himalayas. The river system that gave rise to the Brahmaputra River (Yarlung-Tsangpo), which flowed through several tectonic domains of the Himalayan ranges, primarily from Bomi-Chayu, Gangadese Granitoid, Higher Himalayan Leucogranites, and Namche Barwa into the proto Bengal Basin now a part of Assam Arakan Basin and Naga Schuppen Belt, was the main source of the sandstone formation of the Barail Group. The purpose of sandstone petrography, which combines modal analysis with XRF (Major Oxides) and HR-ICP-MS (Trace & Rare Earth Elements) research, is to identify the type of source rock(s), their weathering pattern, and its paleo-environmental circumstances. These sandstones were formed from recycled orogen and include lithic and sublithic arenite variants with advanced texture and chemical maturity. The sediments were felsic (Th/Co: 1.38, Cr/Th: 9.78, La/Lu: 11.58, Th/Sc: 0.99, Eu/Eu*: 0.66, La/Sc: 3.05, La/Co: 4.18), with contributions from intermediate source rocks and low-rank metamorphics deposited in an active continental margin to a continental island arc setting. Climatic conditions impacted the sediments of Barails, characterised by being warm and semi-humid to humid which resulted in moderate to a high degree of chemical weathering, as shown by weathering indices like CIA (79.14), PIA (85.47), CIW (86.9), WIP (32.50), ICV (0.71), and Th/U (6.03), which were further additionally supported by C-Value (1.01), PF (1.20), Sr/Cu (2.04), and Rb/Sr (0.97).

The magnitude and spatial variability of CO₂ surface emissions and processes involving CO₂ released to the atmosphere from the soils are relevant issues in the context of climate change. This work evaluated CO₂ fluxes and ¹³C/¹²C ratio of vegetation, organic matter, and soil gases from no disturbed soils of Chaco Pampean Plain (Argentina) with different soil properties and environmental conditions (PL and PA units). Soil organic decomposition from individual layers was accompanied by δ¹³C of total organic carbon (δ¹³C-TOC) values more enriched to depth. δ¹³C-TOC values in the upper soil profile ~ ca. 0–15 cm were like the plant community of this area (~−33 to −29 ‰) while δ¹³C-TOC varied stronger bellow horizon A, till ~ −24‰. Both δ¹³C-TOC and soil δ¹³C-CO₂ were similar (~ −24 to 26 ‰) at deeper horizons (~ 50–60 cm). Toward the superficial layers, δ¹³C-TOC and δ¹³C-CO₂ showed more differences (till ~ 4 ‰), due influence of the diffusion process. Horizon A layer (~ 0–20 cm) from both PL and PA units contained the most enriched δ¹³C-CO₂ values (~ −15–17 ‰) because atmospheric CO₂ permeated the soil air. A simple two-component mixing model between sources (atmospheric δ¹³C-CO₂ and soil CO₂) confirmed that process. Isotopically, CO₂ fluxes reflected the biodegradation of C3 plants (source), diffusive transport, and CO₂ exchange (atmosphere/soil). Soil moisture content appeared as a determining factor in the diffusion process and the magnitude of CO₂ surface emissions (12–60 g·m⁻²·d⁻¹). That condition was confirmed by CO₂ diffusion coefficients estimated by air-filled porosity parameters and soil radon gradient model.

This research presents the results of a comprehensive study of mineralogical and geochemical features of REE distribution in coals of Central Kazakhstan deposits—Karaganda coal basin and Shubarkol deposit, which have large hard coal reserves and are industrially important for the coal industry of Kazakhstan; the research is based on 205 samples of clayey interlayers and coal seams. It shows basic patterns of distribution and features of concentration for impurity elements, gives an estimate of the impurity elements concentration, including REE, defines conditions and factors of their accumulation, and studies features of their forms in coal and coal-bearing rocks, which allows estimating the mechanisms of their migration and conditions of accumulation. According to the results of geochemical indicators, the article establishes the factors of REE dislocation, reveals the composition of margin rocks that have influenced REE concentration in coal seams, and the presented latest data on mineralogy allowed to establish the ways of their transportation to the paleobasin during the syn- and epigenetic periods of formation of the coal deposits of Central Kazakhstan being researched. It was found that the coals are insignificantly enriched with heavy lanthanides from Ho to Lu. The distribution curves of UCC normalized REE values in the coals are similar and coincide, but they are less than the average value for world coal, and amount to only one-third of the UCC. It was found that the highest concentrations of all REE are characteristic of clayey interlayers and oxidized coals. The La/Yb ratio in this case increases upwards along the section, indicating mainly clastogenic mechanism of REE delivery to the coals. In coal and clay samples, the predominant mineral form of REE is light lanthanide phosphates. Identified particles of REE from minerals and their composition peculiarities suppose autigene nature of their formation. The formation of the bulk of autigene minerals occurred during the maturation of brown coals and their transformation into hard ones.

Clay-type Li deposits are poised to play a pivotal role in addressing the surging global demand for Li. The McDermitt clay-type Li deposit, located in Nevada, is the largest Li deposit in the United States, with Li hosted by a clay-rich sequence of smectite-dominated intervals and illite-dominated intervals, respectively. However, the occurrence of Li and the genesis of Li-bearing minerals within smectite-dominated intervals have not been thoroughly investigated in previous research. Here, we studied the mineralogy, the in-situ Li distribution, and the bonding environments of Li within the smectite intervals using a combination of instrumental techniques including scanning electron microscope, transmission electron microscope, time-of-flight secondary ion mass spectrometry, and nuclear magnetic resonance. Our results indicate that the smectite exhibits low crystallinity characteristics of lacustrine clay authigenesis and is commonly found to fill the interstices among volcanic minerals or envelop them; Li is mainly hosted by Mg-smectite rather than the volcanic minerals. Within the tuffaceous sediment samples, the volcanic glass has undergone a transformation, resulting in its complete disappearance and alteration into clay minerals. Owing to the octahedral sites of Mg-smectite bounded in Li, it is referred to be hectorite. We interpret that the hectorite’s precipitation occurs in a high saline-alkaline water environment, a result of McDermitt tuff dissolution. This conclusion can be supported by the coexistence of spherulitic calcite and hectorite. Overall, this study confirms hectorite as the main Li-bearing mineral and increases the understanding of the genetic model of hectorite formation in intracontinental caldera basins.