Acta Geochimica最新文献

Isotope effects are pivotal in understanding silicate melt evaporation and planetary accretion processes. Based on the Hertz–Knudsen equation, the current theory often fails to predict observed isotope fractionations of laboratory experiments due to its oversimplified assumptions. Here, we point out that the Hertz-Knudsen-equation-based theory is incomplete for silicate melt evaporation cases and can only be used for situations where the vaporized species is identical to the one in the melt. We propose a new model designed for silicate melt evaporation under vacuum. Our model considers multiple steps including mass transfer, chemical reaction, and nucleation. Our derivations reveal a kinetic isotopic fractionation factor (KIFF or α) α_{our model} = [m(¹species)/m(²species)]^0.5, where m(species) is the mass of the reactant of reaction/nucleation-limiting step or species of diffusion-limiting step and superscript 1 and 2 represent light and heavy isotopes, respectively. This model can effectively reproduce most reported KIFFs of laboratory experiments for various elements, i.e., Mg, Si, K, Rb, Fe, Ca, and Ti. And, the KIFF-mixing model referring that an overall rate of evaporation can be determined by two steps jointly can account for the effects of low P_H2 pressure, composition, and temperature. In addition, we find that chemical reactions, diffusion, and nucleation can control the overall rate of evaporation of silicate melts by using the fitting slope in ln(− lnf) versus ln(t). Notably, our model allows for the theoretical calculations of parameters like activation energy (E_a), providing a novel approach to studying compositional and environmental effects on evaporation processes, and shedding light on the formation and evolution of the proto-solar and Earth-Moon systems.

Theoretical studies of the diffusional isotope effect in solids are still stuck in the 1960s and 1970s. With the development of high spatial resolution mass spectrometers, isotopic data of mineral grains are rapidly accumulated. To dig up information from these data, molecular-level theoretical models are urgently needed. Based on the microscopic definition of the diffusion coefficient (D), a new theoretical framework for calculating the diffusional isotope effect (DIE_(v)) (in terms of D^*/D) for vacancy-mediated impurity diffusion in solids is provided based on statistical mechanics formalism. The newly derived equation shows that the DIE_(v) can be easily calculated as long as the vibration frequencies of isotope-substituted solids are obtained. The calculated DIE_(v) values of ¹⁹⁹Au/¹⁹⁵Au and ⁶⁰Co/⁵⁷Co during diffusion in Cu and Au metals are all within 1% of errors compared to the experimental data, which shows that this theoretical model is reasonable and precise.

This study comprises the relationship between organic matter (OM) and gold occurrence using two distinctive ore deposits of the Bakyrchik gold-sulfide deposit (Kazakhstan) and Western Mecsek uranium ore deposit (Hungary). The two ore deposits are identified as organic-rich sedimentary formations linked to the Variscan gold cycle globally. Characterizing OM is essential because it can act as a carrier for gold, influencing its distribution and behavior within the deposit. Understanding the nature and distribution of OM can provide insights into the processes of gold deposition and help optimize exploration and extraction strategies in mining operations. The primary objective is to characterize OM by identifying its elemental composition, thermal maturity, functional groups, and soluble fractions; and extract gold from OM using a two-step sequential extraction method (hydrogen peroxide and aqua regia) combined with geochemical techniques. Analytical and experimental results from samples of both ore deposits indicate the presence of finely disseminated solid bitumen and reworked vitrinite, originating from thermally matured (RmcRo%—3.76 in Bakyrchik; Ro%—2.25 in W-Mecsek) terrigenous high plants. Both deposits exhibit extremely low extractable bitumen yield and TOC (0.34% in Bakyrchik; 0.25 wt% in W-Mecsek), characterized by an aromatic carboxylic acid organic structure and a composition rich in sulfur-containing (1.17% in Bakyrchik; 5.81% in W-Mecsek) aromatic hydrocarbons. Gold occurrence and enrichment within OM were confirmed through the sequential extraction method employing ICP-OES and LA-ICP-MS techniques. The sequentially extracted gold content from OM reached up to 3 ppm in Bakyrchik and up to 3.28 ppm in Western Mecsek, accompanied by Ag (ranging from 0.01 to 0.32 ppm). Higher concentrations of Au (4 ppm) and Ag (27 ppm) were extracted from residue materials, which are likely associated with sulfide minerals. The presence of gold in OM was further validated using LA-ICP-MS. Gold bonding within OM structure, gold is preserved in the form of lattice gold or structurally bonded metal most likely within the aromatic hydrocarbon fractions of the OM in both the W-Mecsek and Bakyrchik deposits. These findings underscore the profound potential of ongoing exploration endeavors, offering pivotal revelations regarding the extraction and practical application of Au and Ag derived from OM within the geochemical framework of both ore deposits.

Perchlorate and chlorate are present in various extraterrestrial celestial bodies throughout the solar system, such as Mars, the moon, and asteroids. To date, the origin mechanisms of perchlorate and chlorate on the Martian surface have been well-established; however, relatively little attention has been cast to airless bodies. Here, we experimentally investigated the potential oxidation mechanisms of chloride to chlorate and perchlorate, such as ultraviolet irradiation under H₂O- and O₂-free conditions and mechanical pulverization processes. Individual minerals, olivine, pyroxene, ilmenite, magnetite, TiO₂ and anhydrous ferric sulfate, and lunar regolith simulants (low Ti, CLRS-1; high-Ti, CLRS-2) and their metallic iron (Fe⁰) bearing counterparts were examined. We found that pulverization of dry matrix material-halite mixtures, even in the presence of O₂, does not necessarily lead to perchlorate and chlorate formation without involving water. Under photocatalytic and H₂O- and O₂-free conditions, olivine and pyroxene can produce oxychlorine (ClO_x⁻) species, although the yields were orders of magnitude lower than those under Martian-relevant conditions. Nanophase-Fe⁰ particles in the lunar regolith and the common photocatalyst TiO₂ can facilitate the ClO_x⁻ formation, but their yields were lower than those with olivine. The oxides ilmenite and magnetite did not efficiently contribute to ClO_x⁻ production. Our results highlight the critical role of H₂O in the oxidation chloride to chlorate and perchlorate, and provide essential insights into the environmental influence on the formation of oxychlorine species on different celestial bodies.

Research on the origin of carbonates in Changdu Basin holds significant importance for understanding the regional potash formation model. Based on a comprehensive review of previous studies, field geological surveys, and laboratory investigations, this study analyzes the origin and properties of carbonates within the context of regional potash formation. Petrographic studies show that magnesite deposits, with the characteristics of sedimentary origin. The results of elemental geochemical analysis show that the carbonates in this area were formed in the sedimentary environment via evaporation followed by concentration, and the formation of magnesite was possibly caused by the substitution of calcium in the dolomite with magnesium-rich brine. The δ¹³C values of carbonats in the study area are between 5.9‰ and 9.1‰. The δ¹⁸O values of magnesite samples range from − 7.3‰ to − 1.3‰, and the δ¹⁸O values of dolomites range from − 10.3‰ to − 8.4‰. All the calculated Z values of oxygen isotopes of carbonates greater than 120. A comprehensive analysis of carbon and oxygen isotopes indicates that the magnesite was formed in a highly concentrated Marine sedimentary environment and does not show any relation with the metasomatism of hydrothermal fluids. The results on the correlation of magnesite with seawater and its sedimentary origin provide key information for explaining the migration direction of brine between the Changdu and Lanping–Simao Basins. The residual metamorphic seawater in the Changdu Basin migrated to the Lanping–Simao Basin, where potash underwent deposition. Whereas, magnesite and dolomite in the early stage of potash formation were left in the Changdu Basin.

Chemical (REE and major elements) and isotope (δ¹³C, δ¹⁸O) composition of carbonate manganese ores and manganese-bearing carbonates of the Usa deposit (Siberia, Russia) were studied. Received data on the composition of REE exhibit both the distinct negative (Ce/Ce*_PAAS < 1) and positive (Ce/Ce*_PAAS˃1) cerium anomalies and the positive Eu-anomaly (Eu/Eu*_PAAS˃1). Negative Eu-anomalies are not observed. The contents of Mn, Fe, REE, and Ce-anomalies show a positive correlation with each other. Ce-anomalies and the amount of manganese and REE in relation to the carbon isotope composition (δ¹³C) show a negative relationship and indicate that oxidized carbon of organic matter played an important role in the concentration of manganese and REE in manganese ores. The chemical and isotope composition of examined rocks indicates on secondary formation of Mn-ores. Two major phases and sources are distinguished in the ore-forming process characterized by different chemical (REE and ore elements) and isotope composition: (i) high-grade manganese ores (with high contents of REE and light carbon isotope composition) and (ii) low-grade manganese ores (with low contents of REE and heavy carbon isotope composition).

Rice (Oryza sativa L.) paddies are increasingly threatened by cadmium (Cd) pollution, and potentially serve as CH₄ emitters to the atmosphere. Remediation agents widely mitigate Cd pollution in paddy soil, however, we know little about their regulations on CH₄ emission. Here, via adding biochar (B), sulfhydryl-modified palygorskite (SMP), and selenium foliar fertilizer (SFF), we conducted a pot experiment to investigate soil and rice Cd contents together with in-situ CH₄ fluxes. Compared to CK, the addition of SMP, SFF, and B-SMP reduced Cd in brown rice by 25% to 50%, 25%, and 50% to 75%, respectively. Agents 7% B, 7% B-0.01% SMP, and SFF reduced CH₄ emissions by 8.46%, 5.30%, and 4.11%, respectively. CH₄ emission increased gradually along the growing season, with the cumulative CH₄ fluxes ranging between 338.82 and 619.13 kg hm⁻². Our results highlight that mixed 7% B-0.01% SMP and SFF showed collaborative effects on Cd remediation and CH₄ emission. This study reveals the feasibility of reducing Cd pollution and CH₄ emission in karst rice paddies, which hopes to supplement the knowledge of collaborative controls on soil remediation and carbon emission.

Copper possesses very strong chacophile properties, but under the conditions found in meteorites, its behavior is like that of siderophile elements. The Suizhou meteorite is a highly shocked L6 chondrite. Troilite and taenite are considered the main primary carrier of copper in this meteorite, and the post-shock thermal episode is considered the main reason that elemental Cu migrates from its original host phase and forms metallic grains. The Suizhou meteorite contains a few very thin shock melt veins. The occurrence and behavior of metallic copper in this meteorite were studied by optical microscopic examination, electron microprobe analyses, and high-resolution X-ray elemental intensity mapping. Our results show that metallic copper is abundant in the Suizhou chondritic rock. Metallic copper grains adjacent to small troilite grains inside FeNi metal are the most common occurrence, and those at the FeNi metal–troilite interface are the second most common case. The metallic copper grains occurring at the interface of FeNi metal/troililte and silicate are rather rare. Metallic copper grains are not observed within the Suizhou shock veins, Instead, Cu in elemental form is transferred through shock metamorphism into FeNi metal + troilite intergrowths. Four different occurrence types of Cu in the FeNi metal + troilite intergrowths have been identified: the concentrations of Cu in the FeNi + FeS intergrowths for four occurrence types are rather close, we estimate it might be lower than 1 wt%.

Space weathering is a primary factor in altering the composition and spectral characteristics of surface materials on airless planets. However, current research on space weathering focuses mainly on the Moon and certain types of asteroids. In particular, the impacts of meteoroids and micrometeoroids, radiation from solar wind/solar flares/cosmic rays, and thermal fatigue due to temperature variations are being studied. Space weathering produces various transformation products such as melted glass, amorphous layers, iron particles, vesicles, and solar wind water. These in turn lead to soil maturation, changes in visible and near-infrared reflectance spectra (weakening of characteristic absorption peaks, decreased reflectance, increased near-infrared slope), and alterations in magnetism (related to small iron particles), collectively termed the “lunar model” of space weathering transformation. Compared to the Moon and asteroids, Mercury has unique spatial environmental characteristics, including more intense meteoroid impacts and solar thermal radiation, as well as a weaker particle radiation environment due to the global distribution of its magnetic field. Therefore, the lunar model of space weathering may not apply to Mercury. Previous studies have extensively explored the effects of micrometeoroid impacts. Hence, this work focuses on the effects of solar-wind particle radiation in global magnetic-field distribution and on the weathering transformation of surface materials on Mercury under prolonged intense solar irradiation. Through the utilization of high-valence state, heavy ion implantation, and vacuum heating simulation experiments, this paper primarily investigates the weathering transformation characteristics of the major mineral components such as anorthite, pyroxene, and olivine on Mercury's surface and compares them to the weathering transformation model of the Moon. The experimental results indicate that ion implantation at room temperature is insufficient to generate np-Fe⁰ directly but can facilitate its formation, while prolonged exposure to solar thermal radiation on Mercury's surface can lead directly to the formation of np-Fe⁰. Therefore, intense solar thermal radiation is a crucial component of the unique space weathering transformation process on Mercury's surface.

Coal seams can enrich a variety of harmful trace elements under specific geological conditions. The spatial distribution of harmful trace elements in coal is extremely uneven, and the distribution characteristics of each element content are different. The harmful elements released in the process of coal mining and utilization will cause serious harm to the environment and the human body. It is of great resource significance to study the geochemistry of coal that affects the enrichment and distribution characteristics of harmful trace elements. Based on the domestic and foreign literature on coal geochemistry in Guizhou published by previous investigators, this study counted 1097 sample data from 23 major coal-producing counties in Guizhou Province, systematically summarized the relevant research results of harmful trace elements in the coal of Guizhou, and revealed the overall distribution and enrichment characteristics of harmful trace elements in the coal of Guizhou. The results show that the average contents of Cd, Pb, Se, Cu, Mo, U, V, As, Hg, and Cr in coal of Guizhou are higher than those in Chinese coal and world coal. A variety of harmful trace elements in the coal of Guizhou have high background values, especially in Liupanshui, Xingyi and Qianbei coalfield. The enrichment of various harmful trace elements in the Late Permian coal in Guizhou is mainly related to the combined action of various geological and geochemical factors. The supply of terrigenous debris and sedimentary environment may be the basic background of the enrichment of harmful elements in western Guizhou, while low-temperature hydrothermal activity and volcanic ash deposition may be the main reasons for the enrichment of harmful elements in southwestern Guizhou.