Acta Geochimica最新文献

Geodynamic mechanism responsible for the generation of Silurian granitoids and the tectonic evolution of the Qilian orogenic belt remains controversial. In this study, we report the results of zircon U–Pb age, and systematic whole-rock geochemical data for the Haoquangou and Liujiaxia granitoids within the North Qilian orogenic belt and the Qilian Block, respectively, to constrain their petrogenesis, and the Silurian tectonic evolution of the Qilian orogenic belt. Zircon U–Pb ages indicate that the Haoquangou and Liujiaxia intrusions were emplaced at 423 ± 3 Ma and 432 ± 4 Ma, respectively. The Haoquangou granodiorites are calc-alkaline, while the Liujiaxia granites belong to the high-K calc-alkaline series. Both are peraluminous in composition and have relatively depleted Nd isotopic [ε_Nd(t) = (− 3.9 – + 0.6)] characteristics compared with regional basement rocks, implying their derivation from a juvenile lower crust. They show adakitic geochemical characteristics and were generated by partial melting of thickened lower continental crust. Post-collisional extensional regime related to lithospheric delamination was the most likely geodynamic mechanism for the generation of the Haoquangou granodiorite, while the Liujiaxia granites were generated in a compressive setting during continental collision between the Qaidam and Qilian blocks.

Water in Earth’s mantle plays a critical role in both geodynamic and surficial habitability. Water in the upper mantle and transition zone is widely discussed, but less is known about the water in the lower mantle despite it constituting over half of Earth’s mass. Understanding the water storage in Earth’s lower mantle relies on comprehending the water solubility of bridgmanite, which is the most abundant mineral both in the lower mantle and throughout Earth. Nevertheless, due to limited access to the lower mantle, our understanding of water in bridgmanite mainly comes from laboratory experiments and theoretical calculations, and a huge controversy still exists. In this paper, we provide a review of the commonly employed research methods and current findings concerning the solubility of water in bridgmanite. Potential factors, such as pressure, temperature, compositions, etc., that influence the water solubility of bridgmanite will be discussed, along with insights into future research directions.

Geochemistry, zircon U–Pb geochronology, and Hf isotope data for the Early Paleozoic granites in the Baoshan Block reveal the Early Paleozoic tectonic evolution of the Proto-Tethys. The samples are high-K, calc-alkaline, strongly peraluminous rocks with A/CNK values of 1.37–1.46, are enriched in SiO₂, K₂O, and Rb, and are depleted in Nb, P, Ti, Eu, and heavy rare earth elements, which indicates the crystallization fractionation of the granitic magma. Zircon U–Pb dating indicates that they formed in ca. 480 Ma. The Nansa granites have ε_Hf(t) values ranging from − 16.04 to 4.36 with corresponding T^C_DM ages of 2.10–0.81 Ga, which suggests the magmas derived from the partial melting of ancient metasedimentary with minor involvement of mantle-derived components. A synthesis of data for the Early Paleozoic igneous rocks in the Baoshan block and adjacent (Tengchong, Qiangtang, Sibumasu, Himalaya, etc.) blocks indicates that these blocks were all aligned along the proto-Tethyan margin of East Gondwana in the Early Paleozoic. The Early Paleozoic S-type granites from Nansa were generated in a high-temperature and low-pressure (HTLP) extensional tectonic setting, which resulted from Andean-type orogeny instead of the final assembly of Gondwana or crustal extension in a non-arc environment. In certain places, an expanding environment may exist in opposition to the tectonic backdrop of the lithosphere's thickening and shortening, leading the crust to melt and decompress, mantle-derived materials to mix, and a small quantity of peraluminous granite to emerge.

Ragahama Formation comprises a siliciclastic continental deposits followed by marine carbonates, representing prograding alluvial fans from adjacent high hinterlands seaward into lagoons and fringing reef environments. The present work aimed to document the facies development and sedimentology of the Raghama carbonates exposed along the eastern coastal plain of the Red Sea, northwestern Saudi Arabia. Four stratigraphic sections were measured and sampled (D1–D4) and thin sections and major and trace element analyses were prepared and applied for petrographic and geochemical approaches. The carbonates were subdivided into three successive fore-reef, reef-core, and back-reef depositional facies. Sandy stromatolitic boundstone, microbial laminites, dolomitic ooidal grainstone, bioclastic coralline algal wackestone, sandy bioclastic wackestone, and coral boundstones were the reported microfacies types. Petrographic analysis reveals that the studied carbonates were affected by dissolution, dolomitization, and aggrading recrystallization, which affects both the original micrite matrix and grains or acts as fracture and veinlet filling leading to widespread vuggy and moldic porosity. No evidence of physical compaction, suggesting rapid lithification and recrystallization during early diagenesis and prior to substantial burial and intensive flushing by meteoric waters. Most of the original microstructure of corals were leached and destructed. This is indicated by the higher depletion in Sr and Ca levels and increase in Mg, Na, Fe, and Mn levels, especially in section D1, in comparison with the worldwide carbonates.

The Nigerian oil sands represent the largest oil sand deposit in Africa, yet there is little published information on the distribution and potential health and ecological risks of trace elements in the oil resource. In the present study, we investigated the distribution pattern of 18 trace elements (including biophile and chalcophile elements) as well as the estimated risks associated with exposure to these elements. The results of the study indicated that Fe was the most abundant element, with a mean concentration of 22,131 mg/kg while Br had the lowest mean concentration of 48 mg/kg. The high occurrence of Fe and Ti suggested a possible occurrence of ilmenite (FeTiO₃) in the oil sands. Source apportionment using positive matrix factorization showed that the possible sources of detected elements in the oil sands were geogenic, metal production, and crustal. The contamination factor, geo-accumulation index, modified degree of contamination, pollution load index, and Nemerow pollution index indicated that the oil sands are heavily polluted by the elements. Health risk assessment showed that children were relatively more susceptible to the potentially toxic elements in the oil sands principally via ingestion exposure route (HQ > 1E-04). Cancer risks from inhalation are unlikely due to CR < 1E-06 but ingestion and dermal contact pose severe risks (CR > 1E-04). The high concentrations of the elements pose serious threats due to the potential for atmospheric transport, bioaccessibility, and bioavailability.

Elemental information carried by phytoliths plays a crucial indicative role in geochemical research. For instance, it serves as an indicator of the carbon pool effect in phytoliths and aids in the elucidation of silicon sources. However, early extraction methods for soil phytoliths primarily focused on obtaining their morphological and quantitative information, lacking efficient techniques for quantitative elemental analysis. In this study, we aimed to extract Si/Al information from soil phytoliths. Considering the need for complete extraction of phytolith, six extraction methods were developed and further by alkaline dissolution to determine Si/Al. Six methods were compared in terms of enrichment capacity, the weight of extracted phytoliths, and Si/Al differences. The results indicated that the addition of Ammonia-Catechol in the commonly used heavy liquid flotation method effectively improved phytolith extraction capability and the accuracy of Si/Al results. Additionally, the inclusion of an acetic acid step before alkaline dissolution further removed surface-adsorbed impurities and enhanced the analytical quality of Si/Al in phytolith. The comparison of the data in this study with other published data shows that our method is relatively robust. The improved method proposed in this study can provide a new idea for the quantitative analysis of other elements in soil phytoliths.

The wadi dahab delta is in a dry, arid coastal zone within Egypt's south Sinai Peninsula's eastern portion. The primary water source is the Quaternary coastal alluvial aquifer. The groundwater salinity varies from 890 to 8213 mg/L, with a mean value of 3417 mg/L. The dissolved major ions have been used to calculate the seawater mixing index (SWMI) using a linear equation that discriminates the groundwater mostly affected by water–rock interaction (SWMI 1 >) and other samples mixed with Seawater (SWMI < 1). The isotopic composition of groundwater for specifically chosen groundwater samples ranges from −0.645‰ to +5.212‰ for δ¹⁸O and from − 9.582‰ to + 22.778‰ for δ²H, where the seawater represented by a Red Sea water sample (δ¹⁸O + 1.64‰ − δ²H + 9.80‰) and reject brine water are considerably enriched the isotopic groundwater values. The geochemical NETPATH model constrained by the dissolved significant ions, isotopes, and the rock aquifer forming minerals as phases indicate the mixing percent with the seawater ranges from 9% to 97% of seawater from 91% to 3% of original recharge water. According to the SEAWAT 3-D flow models, seawater has penetrated the Northeastern Dahab delta aquifer, with the intrusion zone extending 1500 m inland. The salt dissolution, upwelling of saline water, recharge from the upstream mountain block, and seawater encroachment are the primary aspects contributing to the deterioration of groundwater quality. These findings may have significance for effective groundwater withdrawal management in arid locations worldwide with similar hydrogeological systems.

We studied an Archean mafic dike in the Trans-North China Orogen of the North China Craton, which has a magmatic age of 2701 ± 83 Ma and is currently the oldest mafic dike in the North China Craton. Such an old dike is extremely rare in the world. The presence of mafic dikes indicates that the North China Craton was in a tensional tectonic environment at 2.7 Ga. Geochemical characteristics reveal that this mafic dike belongs to continental tholeiitic basalt. Results from Hf isotope analysis reveal that the mafic dike originates from a depleted mantle. The plate assembly in the North China landmass was realized during the Archean era (2.7 Ga), and a thick and stable continental crust was formed. Therefore, the first cratonization of the North China Craton was completed before 2.7 Ga. The intrusion of the 2.7-Ga-old mafic dike from the deep lithospheric mantle of the continent indicates that the North China Craton has undergone a period of extensional tectonic activity. This event marks a significant extensional event that occurred after the cratonization of the North China Craton.

The (ultra-)mafic mine tailings pond revealed a weathering discrepancy in the tailings profile, which provided a valuable analog to assess the role of carbonate and silicate weathering of the basalt. In this study, drill-cores samples were selected from the Wanniangou V–Ti–Fe mine tailings pond (Sichuan province, China) to investigate the mineralogicand geochemical characteristics in the tailings profile. The results reveal (1) the tailings pond profile consist of upper and lower layers. The upper layer is composed of carbonate weathering (1.4 %), which was formed in the initial stages of tailings exposure and represented a minimal weathering degree. (2) The lower layer was primarily observed at the aquifer zone of the tailings pond, and was consistent with 0.45 % carbonate weathering and 48.4 % silicate weathering. (3) The weathering discrepancy in the tailings profile could be due to the sulfide oxidation and aerobic/flowing aquifer, which facilitate the water-tailings reactions. The tailings profile provides an analog to studying basalt weathering, as it spans both carbonate and silicate weathering. This research reinforces the idea that silicate weathering is predominant in basaltic areas and plays a crucial role in regulating atmospheric CO₂ (carbon dioxide) levels on Earth.

The Proterozoic metasedimentary rocks of the Yaounde Group on the northern edge of the Congo Shield in Central Africa were investigated to understand their provenance and depositional environment. Petrography, geochemistry, and field evidence helped to subdivide the metasediments into paragneiss, mica schist, chlorite schist, and quartzite which were derived from greywacke, shale, quartz arenite, litharenite protoliths. They are immature with some mature samples, moderately weathered and reworked Neo- and Post-Archean metasediments. Rare earth element signatures (Chondrite Eu/Eu* ≤ 1), enrichment of light rare earth elements over the heavy ones, and the La/Sc ratio (> 0.7) are compatible with those of the intermediate and felsic sources from the upper continental crust. These metasediments were deposited in the continental arc setting and have evolved during Proterozoic times according to the Wilson cycle to form the West Gondwana including NE Brazil.