Acta Geochimica最新文献

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.

The Huozhou complex in the Trans-North China Orogen exhibits two events of mafic magmatism (separated by ca. 700 Ma): Neoproterozoic (920 ± 15 Ma) Shimenyu diabase and Late Triassic (217 ± 2.5 Ma) Xingtangsi diabase. Investigations have focused on systematic petrology, zircon U-Pb dating, Lu-Hf isotopes, and lithogeochemistry. The research findings indicate that the Late Triassic Xingtangsi diabase of the Huozhou complex can be classified as a transitional type between intermediate and mafic rocks based on their SiO₂ content. This classification is supported by an average SiO₂ content of 53.94%, ranging from 53.33% to 54.28%. In the Zr/TiO₂ vs. Ce diagram, all samples lie within the range of basalt. The zircons from the Late Triassic Xingtangsi diabase have low ε_Hf(t) values ranging from –12.7 to –8.7, with an average of –11.1. Additionally, the single-stage model age T_DM1 is estimated to be between 1207 and 1701 Ma. These findings suggest that the magma responsible for the dyke originated from either partial melting or an enriched mantle source inside the Meso-Proterozoic lithospheric mantle. The elevated concentrations of Th (thorium) and LREEs (light rare earth elements), as well as the Th/Yb and Th/Nb ratios, suggest the potential incorporation of subducted sediments within the magma source region. The rock displays negative Nb, Ta, Zr, Hf, and Ti anomalies. These geochemical attributes align with the distinctive traits observed in volcanic rocks found within island arcs. The formation of the Late Triassic Xingtangsi diabase is likely associated with the geological context of an arc setting, which arises from the collision between the Yangtze plate and the North China Craton.

The Longshan orogenic belt is located in the southwestern margin of Ordos Basin at the junction zone between the Western Qinling and Northern Qilian orogenic belt. Voluminous Early Paleozoic magmatism in this area is of key significance for determining the Early Paleozoic tectonic evolution and deep crust-mantle structure. Previous studies mainly focused on the Paleozoic granites; the coeval mafic rocks in this area are still poorly understood. A set of Late Silurian intraplate tholeiitic basalts has been discovered in Longshan area, providing key evidence for the mantle source and deep geodynamic background in this area. The Late Silurian Angou basalt has similar geochemical features as intraplate tholeiitic basalt, with high Na₂O/K₂O ratios (5.22–8.25), enriched in large ion lithophile elements and LREE. In combination with their relatively evolved Sr-Nd isotopic composition [⁸⁷Sr/⁸⁶Sr (i) = 0.7128–0.7140; ε_Nd (t) =  − 5.55 to − 3.40], it is suggested that it originated from decompression melting of metasomatized enriched mantle in extensional setting. These results indicate that the mantle source in the junction zone of the West Qinling-North Qilian orogenic belt evolved from depleted to enriched with the continuation of Proto-Tethys subduction from the Cambrian to the Silurian. These results are of great significance to understanding the genesis of contemporaneous granite and the crust-mantle interaction in the junction zone between the Western Qinling and Northern Qilian orogenic belt.

Uncertainties about natural gas source and hydrocarbon accumulation seriously restrict oil and gas exploration in the Lianggaoshan Formation (J₂l) in the Eastern Sichuan Basin, which has demonstrated great exploration potential in recent years. This study determines the origin of natural gas and the hydrocarbon accumulation model of J₂l in the Eastern Sichuan Basin. A new sample pretreatment method named gas purge-microsyringe extraction was employed and confirmed to be a practical and effective method for preparing condensate oil and collecting source rock extract samples. The source rocks of J₂l exhibited moderate to good qualities, characterized by high TOC values, dominance of type II₁ and II₂ kerogens, and high thermal maturities. Biomarker and aromatic characteristics revealed that the source rocks of J₂l were deposited in brackish water with weak anoxic conditions. The natural gas in J₂l was an organic thermogenic gas generated from the secondary cracking of crude oil, indicating that this natural gas was mainly derived from the source rocks of J₂l. The condensate oil-source rock correlation further confirmed the accuracy of the gas-source correlation results. Based on burial, thermal and hydrocarbon-generating histories, two hydrocarbon charging periods (141–133 Ma and 119–112 Ma) and four hydrocarbon accumulation periods of J₂l were determined. Combined with structural evolution, depositional histories and reservoir conditions, a simple gas reservoir accumulation model of J₂l was developed, which was identified as a “self-generating and self-storing” gas reservoir.

Over the past few decades, Middle Jurassic sediments in the Tanzanian Coastal Basin have gained attention on a regional palaeogeographical scale. These sediments, including thick black shales, were deposited following a widespread marine transgression initiated by the breakup of Gondwana supercontinent. Previous studies indicate that these shales possess good to excellent source potential, making them promising regional source rocks. However, no detailed geochemical studies have assessed them in the Mandawa Basin. In this study, geochemical assessment was caried out on cutting samples from two wells to: (i) constrain organic matter richness, type, quality, and: (ii) reconstruct thermal evolution and depositional conditions of the Bajocian-Bathonian black shales in the Mandawa Basin. Organic matter richness was measured using Total Organic Carbon (TOC) analysis, while the quality and thermal maturity of the organic matter were assessed through programmed pyrolysis. Paleo-redox conditions were determined from Th/U ratios derived from Spectra Gamma Ray Logs data. Geochemical data reveal that organic matter content (TOC) is generally low and varies spatially. The analysed TOC content fluctuates along stratigraphy with values ranging from 0.13 to 3.59 wt% with an average of 0.92 wt%. Whereas, Kerogen yield (S2) and Hydrogen Index (HI) are generally low; S2 and HI ranges from 0 to 1.95 (mg HC/g) with an average of 0.29 (mg HC/g), and 0‒92 (mg HC/g TOC) with an average of 24 (mg HC/g TOC), respectively. Organic matter is mainly composed of types III (gas prone) to IV (inert) that have been subjected to wide range of thermal alteration ranging from marginally mature to over mature. Comparison of TOC, Th/U and sediment composition data (clastics and carbonates) derived from cuttings samples suggests episodic deposition of organic matter under sub-oxic conditions. These were mainly controlled by fluctuation in sea level, tectonics and clastic dilution.

Enhanced weathering (EW) of ultramafic rocks from the Muslim Bagh Ophiolite, Pakistan, has been studied in laboratory experiments to explore carbon sequestration as a climate change mitigation strategy for coastal and open sea environments. The research focused on a cost-effective ex situ experiment to examine the effects of EW reaction pathways arising from the interactions among rock powder, seawater and CO₂. The experimental filtrates from different milled peridotite samples exhibit a decrease in the Mg/Ca ratio as the specific surface area increases, which accelerates reaction rates. This suggests that the leached Mg from the original rock may have been consumed in the formation of brucite, serpentine and carbonates during EW. Similar reaction pathways are also responsible for the chemical alterations observed in amphibolite, albeit to varying degrees. On the other hand, the experimental residues showed an increase in loss on ignition compared to the original rock, indicating that EW has facilitated the incorporation of H₂O and CO₂ into secondary mineral structures through various reaction pathways, leading to the formation of brucite, serpentine and carbonates. Thermal gravimetric analysis of the experimental residues confirms the presence of these minerals based on their decomposition temperatures. Additionally, XRD analysis identified a range of carbonates in the residues of both peridotite and amphibolite samples, validating the occurrence of carbonation reactions. SEM images reveal textural changes in both samples, supporting the formation of secondary minerals through EW, consistent with observations from the petrographic study of untreated samples. Control experiments on CO₂ absorption in seawater showed a decrease in pH, highlighting ocean acidification from increased CO₂ emissions. However, when rock powder was added to the seawater-CO₂ mixture, the pH increased. This suggests that the EW of ultramafic rock powders can sequester CO₂ while raising seawater pH through the formation of secondary minerals. This research could serve as an analog for EW applications, considering the worldwide abundance of ultramafic rocks and the availability of coastal and open ocean environments. However, further research is required to understand the behavior of other elements and their impacts on ocean chemistry in EW processes before applying CO₂ sequestration strategies.