Acta Geochimica最新文献

Stable Sr isotopic composition (δ^88/86Sr) can be used to study magmatic processes, but their fractionation mechanism during magmatic evolution remains unclear. To understand the fractionation behaviors of the stable Sr isotopes during magmatism, we report the δ^88/86Sr values of the Huili granitic pluton, which was subjected to intensive crystal-melt separation. The Huili pluton consists of K-feldspar granite and more evolved albite granite, and the albite granite exhibits significantly higher δ^88/86Sr values (+ 0.36‰ to + 0.52‰) than that of K-feldspar granite (+ 0.11‰ to + 0.25‰). K-feldspar, which contributes most of the Sr budget of the K-feldspar granite, has slightly lower δ^88/86Sr values (− 0.01‰ to + 0.17‰) than the whole rock. The δ^88/86Sr variation of the Huili granites can be explained by separation of melt from K-feldspar-dominated crystals, because crystallization of K-feldspar can result in heavy Sr isotopic composition of the extracted interstitial melt. Stable Sr and Ba isotopic ratios in the Huili granites are highly coupled toward the heavy direction, reflecting their similar element partitioning and isotope fractionation behaviors between the crystalline K-feldspar and melt. This study indicates that melt extraction plays a key role in granitic magma evolution.

The lack of a comprehensive whole-rock geochemical and mineralogical dataset for the Archean granitoids of the Aravalli-Banded Gneissic Complex (BGC), northwest India, results in significant challenges for their correct characterization and assessment of their antiquity. The new field, mineralogical and geochemical data classify the Jaisamand granitoids into sanukitoids, TTGs, and transitional TTGs, which are most likely coeval in nature. The obtained results, in conjuncture with the previously published geochemical and geochronological results of the Aravalli–BGC granitoids, unveil the Neoarchean affinity of the Jaisamand pluton. The TTGs were generated by the melting of a subducting slab (metabasite) at shallow (high-HREE-Y TTGs) to moderate depths (medium-HREE-Y TTGs) above the garnet-in line but still within the plagioclase stability field, with garnet-poor residue. The ascending TTG melts were transformed into sanukitoids through differential interaction with the overlying mantle wedge peridotite. The TTG melts, generated at different pressures, interacted with older TTGs at lower and middle crustal levels to form the transitional TTGs. The coexistence of high-HREE-Y and medium-HREE-Y TTGs and sanukitoids suggests a subduction-related setting for the Jaisamand granitoids. The heat required for simultaneous melting at shallow and deeper depths during the Neoarchean was provided by the upwelling asthenosphere due to slab break-off. The study also revealed the occurrence of altered granitoids in the Jaisamand pluton, showing evidence of albitization and silicification. These rocks do not represent the pristine mineralogy and should be carefully examined to avoid misleading interpretations, particularly for the Archean granitoids.

There is significant debate concerning the tectonic characteristics and evolutionary understanding of the South China Block (SCB) during the Early Mesozoic. One of the key points of contention is the tectonic–magmatic activity during the Triassic and its dynamic mechanisms. However, research on the detailed chronology and tectonic settings of granite plutons in key regions remains insufficient, limiting the understanding of the tectonic–magmatic dynamic mechanisms in the interior of SCB during the Triassic. In this contribution, we present whole-rock major and trace elemental data, Sr–Nd isotope data, LA-ICP-MS zircon U–Pb age dating, and Lu–Hf isotope data for granites of Dashenshan pluton in the Xiangzhong, northwest part of SCB. The results indicate that the Dashenshan granite has an emplacement age of 208.4–212.5 Ma, characterized by high SiO₂, Na₂O, and K₂O contents and low MgO and CaO. The Dashenshan granite is enriched in light rare-earth elements with a significant negative Eu anomaly (average δEu = 0.42). It is also enriched in Rb, K, and Th and shows pronounced depletion in Nb, Ta, and Ti, classifying it as peraluminous calc-alkaline granite, specifically of the I-type. The zircon ε_Hf(t) values range from − 8.39 to − 4.4, with an average of − 5.82, and the Sr–Nd isotopes are relatively enriched [ε_Nd(t) = − 9.31 to − 6.8]. Combining these geochemical characteristics, it is revealed that the Dashenshan granite was derived from the partial melting of middle to upper crustal metamorphic basement materials under medium- to low-temperature conditions, with possible minor contributions from mantle-derived materials. Furthermore, it underwent fractional crystallization, including plagioclase differentiation. By integrating the geochemical features and spatial distribution of Triassic granites in SCB, this study suggests that the regional tectonic evolution of SCB during the Triassic was primarily controlled by the collision of the SCB with the Indochina Block and the North China Block. In Xiangzhong, the tectonic setting transitioned from syn-collisional compression to post-collisional extension during the Late Triassic. The Dashenshan pluton formed in a post-collisional extensional setting, resulting from the decompression melting of middle-to-upper crustal rocks. The upwelling of the asthenosphere and upward heat transfer likely played a significant role in the formation of the Dashenshan granitic magma.

Permanently shadowed regions (PSRs) on the Moon are potential reservoirs for water ice, making them hot spots for future lunar exploration. The water ice in PSRs would cause distinctive changes in space weathering there, in particular reduction-oxidation processes that differ from those in illuminated regions. To determine the characteristics of products formed during space weathering in PSRs, the lunar meteorite NWA 10203 with artificially added water was irradiated with a nanosecond laser to simulate a micrometeorite bombardment of lunar soil containing water ice. The TEM results of the water-incorporated sample showed distinct amorphous rims that exhibited irregular thickness, poor stratification, the appearance of bubbles, and a reduced number of npFe⁰. Additionally, EELS analysis showed the presence of ferric iron at the rim of the nanophase metallic iron particles (npFe⁰) in the amorphous rim with the involvement of water. The results suggest that water ice is another possible factor contributing to oxidation during micrometeorite bombardment on the lunar surface. In addition, it offers a reference for a new space weathering model that incorporates water in PSRs, which could be widespread on asteroids with volatiles.

The concentrations of polycyclic aromatic hydrocarbons (PAHs), their sources and toxic equivalent (TEQ) in soil, suspended matter and bottom sediments in the Fatala River Basin ecosystem were obtained for the first time to our knowledge. Determination of 14 PAHs (ΣPAHs) was carried out using high-performance liquid chromatography. The ΣPAH content in soil ranged from 13 to 50,920 (Me = 820) ng/g. The composition of PAHs (high proportion of low-molecular-weight compounds and values of individual PAHs ratios) reflected the significant oil pollution of soil. Contaminated soil was localated in the central part of the Fatala River Basin. The median benzo(a)pyrene toxic equivalent of soil at the study site was 1.08 (range, 0.05 to 53.16) ngTEQ/g, showing generally low soil toxicity. The ΣPAH content in suspended matter was in the range of 33 to 1316 (Me = 309) ng/L. The ΣPAH content in bottom sediments ranged from 36 to 6943 (Me = 478) ng/L, corresponding to clean and moderately contaminated sediments. The ΣPAH content in bottom sediments depended on the anthropogenic impact on the Fatala River Basin territory and the bottom sediment features. Bottom sediments and suspended matter had a low toxic equivalent.

Carbonic acid produced by the dissolution of atmospheric and soil CO₂ in water is usually the most dominant catalyst for chemical weathering, but a sulfuric acid-driven phenomenon, different from usual, was found in the orogenic belt watersheds dominated by silicate bedrock. This study, rooted in comprehensive field investigations in the Manas River Basin (MRB) north of the Tianshan Mountains, delves into the mechanisms and impacts of sulfuric and carbonic acid as catalysts driving different types of chemical weathering in the Central Asian Orogenic Belt. Quantitative analyses elucidate that carbonate weathering constitutes 52.4% of the total chemical weathering, while silicate and evaporite account for 18.6% and 25.3%, respectively, with anthropogenic activities and atmospheric precipitation having little effect. The estimated total chemical weathering rate in MRB is approximately 0.075 × 10⁶ mol/km²/year. Quantitative findings further suggest that, preceding carbonate precipitation (< 10⁴ year), chemical weathering can absorb CO₂. Subsequently, and following carbonate precipitation (10⁴–10⁷ year), it will release CO₂. The release significantly surpasses the global average CO₂ consumption, contributing to a noteworthy climate impact. This study underscores the distinctive weathering mechanisms, wherein sulfuric acid emerges as the predominant catalyst. The quantity of sulfuric acid as a catalyst is approximately three times that of carbonic acid. Sulfuric acid-driven carbonate rock weathering (SCW) is identified as the sole chemical weathering type with a net CO₂ release effect. SCW CO₂ release flux (5176 mol/km²/year) is roughly 2.5 times the CO₂ absorption by Ca–Mg silicate weathering, highlighting the pivotal role of chemical weathering in sourcing atmospheric CO₂ over the timescales of carbonate precipitation and sulfate reduction. Lastly, this study posits that catalyst and transport limitations are the most plausible critical factors in MRB. The interplay between sulfuric acid and dissolved CO₂ competitively shapes the types and rates of chemical weathering reactions.

Determining the evolutionary history of the Permian-Triassic Kunlun-Qaidam Continental Arc is essential to understanding the subduction and closure processes of the South Kunlun Ocean. In this paper, we utilize (La/Yb)_N ratios collected from a filtered geochemical dataset on Permian to Triassic calc-alkaline rocks (55 wt%–68 wt% SiO₂) and plutonic rocks within the Kunlun-Qaidam Continental Arc to reconstruct the spatiotemporal variation of the relative crustal thickness. Combined with known geologic observations, we discuss the subduction-accretionary tectonics of the South Kunlun Ocean and the topographic evolution of the Kunlun-Qaidam Continental Arc. Two episodes of crustal thickening and thinning were revealed. The reconstructed thickness reveals two crustal thickening and thinning events for the Kunlun-Qaidam Continental Arc from ca. 270 to 210 Ma. The southern sector of the Kunlun-Qaidam Continental Arc is about 7 km thicker than the northern portion, with a maximum thickness of about 55 km at ca. 270 and 230 Ma. The ca. 270 and 230 Ma crustal thickening events coincide with renewed northward subduction of the South Kunlun Ocean plate and ocean closure, respectively, whereas the ca. 270‒240 Ma and ca. 230‒210 Ma crustal thinning events may reflect slab break-off of the oceanic plate and lithospheric collapse during the post-collision extension, respectively.

The lithospheric thinning and huge gold mineralization in the Jiaodong Peninsula is intensively studied, aiming to better understand the geodynamic setting of the magmatic petrogenesis and the relationship between magmatism and large-scale mineralization. Thus, we conducted detailed research on the Weideshan intrusions in the Jiaodong region, including field investigations, geochemical, geochronological and Hf isotope analysis, to reveal the tectonic implications for the destruction of the eastern North China Craton (NCC). The Weideshan intrusions consist of quartz monzodiorite, quartz monzonite and monzogranite. The SHRIMP zircon U-Pb dating results show that the Weideshan intrusions are emplaced at 115–112 Ma, namely, in the late Early Cretaceous period. Rocks of Weideshan intrusions are high-K calc-alkaline series and metaluminous granites. The trace elements are characterized by enrichment of Rb, Ba, Sr and LREE, with unobvious Eu anomalies and depletion of Nb, Ta, Zr, Hf and Ti. The contents of Ba and Sr are (913.00–1562.00)/1199.29 μg/g and (373.00–793.00)/536.71 μg/g, respectively, showing the features of high Ba–Sr granites (HBS). Development of numerous dark enclaves and negative εHf(t) values (− 17.93 to − 12.19) indicate that the Weideshan granites originate from the mixture of crustal-derived felsic magma from partial melting of the Paleoproterozoic crust and alkali-rich magma from the enriched mantle. The generation of the Weideshan granites was closely related to the asthenospheric upwelling during the lithosphere thinning of the NCC in the late Mesozoic.

The Mahabad rhyolitic complex, mostly composed of rhyolite but also including granite and granodiorite, is exposed in NW Iran as a part of the Central Iran Block. Porphyritic, hyalo-porphyritic and spheroidal are the main textures of the studied samples of rhyolite. U-Pb zircon chronology on three samples of Mahabad rhyolitic complex yielded Cambrian to Ediacaran ages of 537.6 ± 6.6 Ma, 547.4 ± 6.5 Ma and 556.2 ± 7.1 Ma. Based on geochemical analyses, the original magma was high potassium calc-alkaline to shoshonitic. The rocks are enriched in LREEs relative to HREEs. Trace element patterns of Mahabad rhyolite normalized to chondrites show negative anomalies of high-field-strength elements (Ti, Nb, Ta, Hf, Yb, Y and Zr) and high LREEs and large ion lithophile element contents (Rb, K, Th and Ba). ²⁰⁸Pb/²⁰⁴Pb (36.7219–39.0367), ²⁰⁷Pb/²⁰⁴Pb (15.4963–15.7669) and ²⁰⁶Pb/²⁰⁴Pb (16.9405–19.9567) ratios indicate an EM-II enriched mantle source for the rhyolite magma. Large variation of εHf(t) from −5.2 to + 4.5 points to a mantle source with crustal material contribution in the magma genesis. The rhyolitic magma erupted in an active continental margin. The formation of calc-alkaline high potassium magma was probably related to metasomatism of the mantle because of the north to south subduction of Proto-Tethys oceanic crust beneath the northern margin of Gondwana continental crust.