Acta Geochimica最新文献

The Wadi Natash volcanic field (WNVF) in the south of the Eastern Desert of Egypt is a typical example of well-preserved intraplate alkaline magmatism during the Late Cretaceous, i.e., prior to the Oligo-Miocene Red Sea rift. We compiled stratigraphic sections at two sectors; namely East Gabal Nuqra and West Khashm Natash (WKN) where the volcanic flows are intercalated with the Turonian Abu Agag sandstone with occasional paleosols when volcanic activity is intermittent. Peridotite mantle xenoliths are encountered in the first sector whereas flows in the second sector are interrupted by trachyte plugs and ring dykes. On a geochemical basis, the mafic melt originating from the lithospheric mantle beneath the WNVF practiced ~ 5% partial melting of phlogopite-bearing garnet peridotite. Basalts dominate in the two sectors and highly evolved (silicic) rocks are confined to the WKN sector. Rejuvenation of ancient Precambrian fractures following the NW–SE and ENE-WSW trends facilitated the ascend of Late Cretaceous mantle-derived alkaline magma. Structurally, the WNVF developed at the eastern shoulder of the so-called “Kom Ombo-Nuqra-Kharit rift system” that represents a well-defined NW-trending intracontinental rift basin in the southern Eastern Desert. In such a structural setup, the Natash volcanic are confined to half-grabens at the East Gabal Nuqra sector whereas the West Khashm Natash sector is subjected to extensional stresses that propagated eastwards. The WNVF is a typical example of fluvial clastics (Turonian) intercalation with rift-related alkaline volcanic rocks in northeast Africa.

The Tahtai Logomiti area is characterized by metavolcanic and metavolcaniclastic interbedded with clastic and carbonate metasedimentary rocks of Neoproterozoic age. New geological, petrographic, major, and trace elements data were used to evaluate the metamorphism, petrogenesis, and paleo-tectonic setting of the area. The field and petrographic observation indicate that the area has undergone greenschist facies metamorphism. Based on mineralogy and geochemical attributes, these metavolcanic rocks are classified as basalt, basaltic-andesite, andesite, and dacite. The moderate degrees of light rare earth element (LREE) enrichment, flat heavy rare earth element (HREE) pattern, and low Nb/Y ratio, represent shallow mantle sources. In addition to that, the TiO₂/Yb vs. Nb/Yb diagram, high (La/Yb)N ratio (> 3.44), indicates shallow melting and depleted magma sources. However, the high ratios of (Th/Ta) > 3.8, (La/Ta) > 38, and low ratios of (Th/La) < 1, (Nb/La) < 1, and high Pb content would indicate crustal contamination of the magma. The discrimination diagram and trace element ratios (Nb/Y, La/Sc, La/Y, and La/Th) indicate that the metavolcanic rocks have a calc-alkaline affinity. In addition, the Zr-Nb-Y and Th-Hf-Ta plots show that the rocks formed under a volcanic-arc setting. The general petrological and geochemical characteristics of the Tahtai Logomiti metavolcanic rocks suggest that the area is associated with subduction-related arc accretion of the Arabian Nubian Shield.

This study examines the potential impacts of climate change on Lake Biwa, Japan’s largest freshwater lake, with a focus on temperature, wind speed, and precipitation variations. Leveraging data from the IPCC Sixth Assessment Report, including CCP scenarios, projecting a significant temperature rise of 3.3–5.7 °C in the case of very high GHG emission power, the research investigates how these shifts may influence dissolved oxygen levels in Lake Biwa. Through a one-dimensional model incorporating sediment redox reactions, various scenarios where air temperature and wind speed are changed are simulated. It is revealed that a 5 °C increase in air temperature leads to decreasing 1–2 mg/L of dissolved oxygen concentrations from the surface layer to the bottom layer, while a decrease in air temperature tends to elevate 1–3 mg/L of oxygen levels. Moreover, doubling wind speed enhances surface layer oxygen but diminishes it in deeper layers due to increased mixing. Seasonal variations in wind effects are noted, with significant surface layer oxygen increases from 0.4 to 0.8 mg/L during summer to autumn, increases from 0.4 to 0.8 mg/L in autumn to winter due to intensified vertical mixing. This phenomenon impacts the lake’s oxygen cycle year-round. In contrast, precipitation changes show limited impact on oxygen levels, suggesting minor influence compared to other meteorological factors. The study suggests the necessity of comprehensive three-dimensional models that account for lake-specific and geographical factors for accurate predictions of future water conditions. A holistic approach integrating nutrient levels, water temperature, and river inflow is deemed essential for sustainable management of Lake Biwa’s water resources, particularly in addressing precipitation variations.

The high Ba–Sr rocks can provide significant clues about the evolution of the continent lithosphere, but their petrogenesis remains controversial. Identifying the Late Cretaceous high Ba–Sr granodiorites in the SE Lhasa Block could potentially provide valuable insights into the continent evolution of the Qinghai-Tibet Plateau. Zircon U–Pb ages suggest that the granodiorites were emplaced at 87.32 ± 0.43 Ma. Geochemically, the high Ba–Sr granodiorites are characterized by elevated K₂O + Na₂O contents (8.18–8.73 wt%) and K₂O/Na₂O ratios (0.99–1.25, mostly > 1), and belong to high-K calc-alkaline to shoshonitic series. The Yonglaga granodiorites show notably high Sr (653–783 ppm) and Ba (1346–1531 ppm) contents, plus high Sr/Y (30.92–38.18) and (La/Yb)_N (27.7–34.7) ratios, but low Y (20.0–22.8 ppm) and Yb (1.92–2.19 ppm) contents with absence of negative Eu anomalies (δEu = 0.83–0.88), all similar to typical high Ba–Sr granitoids. The variable zircon εHf(t) values of − 4.58 to + 12.97, elevated initial ⁸⁷Sr/⁸⁶Sr isotopic ratios of 0.707254 to 0.707322 and low εNd(t) values of − 2.8 to − 3.6 with decoupling from the Hf system suggest that a metasomatized mantle source included significant recycled ancient materials. The occurrence of such high Ba–Sr intrusions indicates previous contributions of metasomatized mantle-derived juvenile material to the continents, which imply the growth of continental crust during the Late Cretaceous in the SE Lhasa. Together with regional data, we infer that the underplated mafic magma provides a significant amount of heat, which leads to partial melting of the juvenile crust. The melting of the metasomatized mantle could produce a juvenile mafic lower crust, from which the high Ba–Sr granitoids were derived from reworking of previous mafic crust during the Late Cretaceous (ca. 100–80 Ma) in the SE Lhasa.

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.