Guangzhou Institute of Geochemistry
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Pyrolysis experiments of model compounds to explore carbon isotope fractionation in propane from natural gas
No full textInvestigating the mechanisms that determine the bulk and position -specific carbon isotopic distributions of propane in natural gas will help elucidate its formation and evolution. We used octadecane and squalane as model compounds that give simple precursors for gas production in gold -tube isothermal pyrolysis experiments to study the variations in intermolecular and intramolecular distributions of isotopes in the generated gaseous hydrocarbons. The delta 13C values of the products and inverses of their carbon numbers (1/n where n = C1-C5) showed a negative linear relationship versus maturity, indicating that they formed by homolytic cleavage of C-C bonds and that the precursors showed homogeneous distributions of carbon isotopes. However, the significant difference in the kinetic isotopic effects (KIEs) of the gas generated by cracking of two model compounds under the same experimental conditions is not readily explained by single homolytic bond cleavage. The results indicate that besides the KIE of C-C bond cleavage, the bulk and position -specific carbon isotopic compositions of propane are related to the chemical and isotopic structures of the precursors and the propane transformation ratio. Based on the sites of C-C bond cleavage, two isotopic fractionation patterns (i.e., normal propyl and isopropyl models) may explain the bulk and position -specific carbon isotopic distributions of the generated propane. According to the propyl model, propane originates from (normal) propyl structures (CH3CH2CH2*) in the precursor via C-C bond cleavage at a terminal site of a propyl group, while the isopropyl model involves propane derived from isopropyl structures (CH3CH*CH3) in the precursor, with C-C bond cleavage at the central site. Simulations show that these distributions for propane cracked from octadecane closely follow the propyl model, whereas propane generated from squalane showed mixed contributions from both models, and its bulk and position -specific carbon isotopic distributions depend on the proportions of propyl and isopropyl structures in the precursors. Variations of propane's position -specific carbon isotopic distributions during the main stage of propane generation indicate that free radical reactions are the main pathway for thermogenic propane formation, resulting in similar KIEs for propane terminal and central carbons. Therefore, the position -specific carbon isotopic distribution of propane can provide evidence of its formation mechanism and shows potential for revealing the origin and evolution of natural gas
Improving the flame retardancy and smoke suppression of ethylene vinyl acetate composites by introducing the hybrid material of Zn/TiO2@AKaol
No full textEthylene vinyl acetate copolymer (EVA) is widely applied in many important fields, but its broader utility is impeded by its flammability. To address this problem, the pre-acidified kaolinite (AKaol) by sulfuric acid was further dealt with TiO2 loading and Zn2+ doping to obtain Zn/TiO2@AKaol to improve the flame retardancy and smoke suppression of EVA composites containing intumescent flame retardants (IFR) typically. The results showed that the addition of 1 wt% Zn/TiO2@AKaol improved the flame retardancy and reduced the smoke release of EVA/IFR composite. The limiting oxygen index (LOI) value of the EVA/IFR composite containing Zn/ TiO2@AKaol increased to 32.7 % from 27.3 % of EVA/IFR composite. In the cone calorimeter test (CCT), the peak heat release rate (pHRR) and total heat release (THR) values were reduced to 297.5 kW/m2 and 98.8 MJ/ m2 from the 409.9 kW/m2 and 109.3 MJ/m2 of EVA/IFR composite, respectively. Meanwhile, the values of total smoke production (TSP) in the CCT and smoke density tests in building materials were reduced to 13.5 m2 and 33.39 % of EVA/IFR/Zn/TiO2@AKaol composite from 18.4 m2 and 63.91 % of EVA/IFR composite, respectively. Based on the mechanism analysis, it was proposed that Zn/TiO2@AKaol not only promoted the production of P & sdot; in advance, but also increased a cross-linked structure char in the condensed phase to achieve flame retardancy and smoke suppression of EVA composites
Differences in Secondary Organic Aerosol Formation from α-Pinene Photooxidation in a Chamber with Purified Air and Ambient Air as Matrices: Preliminary Results
No full textalpha-Pinene is a biogenic volatile organic compound (BVOC) that significantly contributes to secondary organic aerosols (SOA) in the atmosphere due to its high emission rate, reactivity, and SOA yield. However, the SOA yield measured in chamber studies from alpha-pinene photooxidation is limited in a purified air matrix. Assessing SOA formation from alpha-pinene photooxidation in real urban ambient air based on studies conducted in purified air matrices may be subject to uncertainties. In this study, alpha-pinene photooxidation and SOA yield were investigated in a smog chamber in the presence of NO and SO2 under purified air and ambient air matrices. With the accumulation of ozone (O-3) during the photooxidation, an increasing part of alpha-pinene was consumed by O-3 and finally nearly half of the alpha-pinene was oxidized by O-3, facilitating the production of highly oxidized organic molecules and thereby SOA formation. Although the ambient air we introduced as matrix air was largely clean, with initial organic aerosol mass concentrations of similar to 1.5 mu g m-3, the alpha-pinene SOA yield in the ambient air matrix was 42.3 +/- 5.3%, still higher than that of 32.4 +/- 0.4% in the purified air matrix. The chemical characterization of SOA by the high-resolution time-of-flight aerosol mass spectrometer (HR-ToF-AMS) revealed that CxHy accounted for 53.7 +/- 1.1% of the total signal in the ambient air matrix experiments, higher than 48.1 +/- 0.3% in the purified air, while CxHyO and CxHyO>1 together constituted 45.0 +/- 0.9% in the ambient air matrix, lower than 50.1 +/- 1.0% in the purified air. The O:C ratio in the ambient air matrix experiments was 0.41 +/- 0.01, lower than 0.46 +/- 0.01 in the purified air. The higher SOA yield of alpha-pinene in the ambient air matrix compared to that in the purified air matrix was partly due to the presence of initial aerosols in the ambient air, which facilitated the low volatile organic compounds produced from photochemical oxidation to enter the aerosol phase through gas-particle partitioning. The in-situ aerosol acidity calculated by the ISORROPIA-II model in the ambient air matrix experiments was approximately six times higher than that in purified air, and the higher SOA yield in the ambient air matrix experiments might also be attributed to acid-catalyzed SOA formation
Secondary inorganic aerosols and aerosol acidity at different PM2.5 pollution levels during winter haze episodes in the Sichuan Basin, China
No full textWintertime fine particle (PM2.5) pollution remains to be perplexing air quality problems in many parts of China. In this study, PM2.5 compositions and aerosol acidity at different pollution levels at an urban cite in the southwest China's Sichuan Basin were investigated during a sustained winter haze episode. Organic matter was the most abundant component of PM2.5, followed by nitrate, sulfate and ammonium. Shares of organic aerosol in PM2.5 mass decreased with the elevated PM(2.5 )levels, while the enhancements of sulfate and secondary organic aerosol were much less than that of nitrate and ammonium during heavy pollution with increased ratios of nitrate to sulfate, implying a significant role of nitrate in the haze formation. Results also suggest the nighttime chemistry might contribute substantially to the formation of nitrate under severe pollutions. The daily average aerosol pH showed a decreasing trend with the elevated levels of PM2.5, and this increased aerosl acidity was mainly due to the fast rising secondary inorganic aerosol (SIA) concentration, with the increase in hydronium ion concentration in air (Hair+) surpassing the dilution effect of elevated aerosol liquid water content (LWC). Thermodynamic model calculations revealed that the air environment was NH3-rich with total NHx (NH3 + NH4+) greater than required NHx, and the aerosol pH exponentially declined with the decreasing excess NHx (p < 0.01). This study demonstrated that under air stagnation and NH3-rich environment during winter, the raised relative humidity (RH) would lead to an increase in LWC and thereby facilitate the aqueous chemistry processes with the neutralization capacity of NH3 to form sulfate and nitrate, which would further increase the LWC and lower the pH. This self -amplifying SIA formation might be crucial to the severe PM2.5 pollution and haze events during winter, and therefore cutting both NOx and NH3 emissions would benefit stopping the self -amplification
Linking Pacific Plate formation and Early Cretaceous metallogenic response on the circum-Pacific continental margins
No full textHydrothermal mineralization along the circum-Pacific continental margin is genetically linked to the motions of the Pacific Plate. Compiled geochronological results of ore deposits, coupled with the newly reconstructed geometry of the late Early Cretaceous subduction zones in East China, North America, and the Central Andes using GPlates and I2VIS software, were applied to investigate the relationships between mineralization and Pacific Plate formation. The -120-m.y.- old orogenic Au provinces in East China and North America are related to transpression caused by high -rate oblique subduction with intermediate-high dip angles of the Izanagi and Farallon plates, respectively. In contrast, the -105-m.y.-old porphyry-epithermal belt in Southeast China was produced by oblique subduction of the Izanagi Plate with low-intermediate subduction rates and intermediate-high dip angles. In the Central Andes, the oblique subduction of the Farallon Plate with low-intermediate rates and low dip angle accounted for iron oxide-copper-gold ore (IOCG) deposit mineralization in South Peru at ca. 110 Ma; whereas the high rate and low dip -angle subduction of the paleo-Phoenix Plate, which caused mild compression, was responsible for Fe, porphyry Cu, and IOCG mineralization in North Chile at ca. 110 Ma. The late Early Cretaceous metallogenic response in the circum-Pacific region coincides with superplume events that triggered the significant growth of the modern Pacific Plate. The forward simulations reveal that different subsequent styles of subduction and associated magmatism are likely responsible for the distinct mineralization types present in the region, including orogenic Au, porphyry-epithermal, Fe, and IOCG deposits. The precise dynamics of the subduction zone determined by this study led to the improved metallogenesis models in the Pacific margin
Microbial-mediated oxidative dissolution of orpiment and realgar in circumneutral aquatic environments
No full textArsenic (As) is a toxic metalloid that causes severe environmental contamination worldwide. Upon exposure to aqueous phases, the As-bearing minerals, such as orpiment (As2S3) and realgar (As4S4), undergo oxidative dissolution, in which biotic and abiotic activities both contributed significant roles. Consequently, the dissolved As and S are rapidly discharged through water transportation to broader regions and contaminate surrounding areas, especially in aquatic environments. Despite both orpiment and realgar are frequently encountered in carbonate-hosted neutral environments, the microbial-mediated oxidative dissolution of these minerals, however, have been primarily investigated under acidic conditions. Therefore, the current study aimed to elucidate microbial-mediated oxidative dissolution under neutral aquatic conditions. The current study demonstrated that the dissolution of orpiment and realgar is synergistically regulated by abiotic (i.e., specific surface area (SSA) of the mineral) and biotic (i.e., microbial oxidation) factors. The initial dissolution of As(III) and S2- from minerals is abiotically impacted by SSA, while the microbial oxidation of As(III) and S2- accelerated the overall dissolution rates of orpiment and realgar. In As-contaminated environments, members of Thiobacillus and Rhizobium were identified as the major populations that mediated oxidative dissolution of orpiment and realgar by DNAstable isotope probing. This study provides novel insights regarding the microbial-mediated oxidative dissolution process of orpiment and realgar under neutral conditions
First identification of Early Cretaceous mafic dikes in the Baingoin area, central Tibet: Implications for crust-mantle interactions and magmatic flare-up
No full textMafic dikes are generally emplaced in extensional tectonic settings and provide key information regarding deep mantle processes and sources. The Bangong-Nujiang suture zone was formed by the collision of the Qiangtang and Lhasa terranes and experienced intense magmatism during the Early Cretaceous. However, the deep mantle processes and mechanisms involved in this magmatic flare-up (ca. 115 Ma) in the collisional belt remain controversial because of the lack of evidence for coeval mafic magmatism. Here, we present detailed petrological, geochronological, geochemical, and Sr-NdHf-O isotope data for the newly discovered hypersthene-bearing mafic dikes in the Baingoin area in the middle -eastern parts of the Bangong-Nujiang suture zone, central Tibet. Secondary ion mass spectroscopy (SIMS) zircon U-Pb dating showed that the mafic dikes were emplaced during 120-115 Ma. These mafic rocks are characterized by variable MgO contents (2.7-5.2 wt%) and Mg# values (38.5-52.8), slight enrichment in light rare earth elements (REEs; [La/ Yb]N = 7.5-8.1), relatively flat heavy REE patterns ([Gd/Yb]N = 1.75-1.84), and negative Eu, Ta, Nb, and Ti anomalies. The dikes also have relatively low initial 87Sr/86Sr ratios (0.7060-0.7062) and negative eNd(t) (-2.2 to -1.6) and positive eHf(t) (+2.5 to +3.6) values, and variable zircon eHf(t) (-2.2 to +7.2) and slightly elevated zircon delta 18O (5.6%0-7.0%0) values. These geochemical characteristics indicate that the mafic dikes were derived from an enriched lithospheric mantle source. However, compared with coeval magmatic rocks, the mafic dikes have relatively high eNd(t) and eHf(t) values, indicating that they contain a depleted mantle component. The mafic dikes contain clinopyroxene and orthopyroxene (i.e., hypersthene), indicative of derivation from a high -temperature magma source. Clinopyroxene-melt thermobarometry yielded a temperature range of 1167- 1213 degrees C, further supporting the involvement of a high -temperature asthenospheric component. Therefore, we suggest that the parental magmas of the Nakoulai mafic dikes were probably generated by the interaction between the asthenospheric mantle and overlying metasomatized lithospheric mantle. Combined with data from nearby Cretaceous magmatic rocks and sedimentary rocks, we suggest that the mafic dikes were generated in a postcollisional setting caused by upwelling of asthenospheric mantle owing to slab breakoff beneath the Bangong-Nujiang suture zone. Slab breakoff played a key role in the crust -mantle interactions and the onset of the magmatic flare-up in the middle -eastern parts of the Bangong-Nujiang suture zone
Horizontally forced initiation of the Izu-Bonin-Mariana subduction zone
No full textThe sparsity of a direct record for the moment of subduction zone initiation has led to various models describing the infancy and evolution of modern oceanic subduction systems. Recently, with increases in available samples and geochemical data for subduction zone initiation-to-mature-arc lavas, better constraints on subduction evolution are possible. Here, by systemically modeling the time-space pattern and geochemical characters of forearc magmas with forward numerical modeling, we attempt to search for a best-fit geodynamic scenario where Izu-Bonin-Mariana-type subduction tends to develop. Our modeling and geochemical constraints have identified a necessary and possibly transitory pre-subduction zone initiation trenchward contraction consistent with observed Izu-Bonin-Mariana forearc magma geochemistry. Our results also reveal a typical maturation process for Izu-Bonin-Mariana-type oceanic subductions, controlled by the pace of the upper plate's rifting and solidification.
A three-stage process of contraction, rifting and final maturation best explains subduction zone initiation in Izu-Bonin-Mariana-type systems, according to numerical and geochemical modelling informed by existing geochemical and petrological data
Quartz Crystallinity Characteristics and Their Effects on Shale Gas Reservoir Performance: A Case Study of the Deep Longmaxi Formation Shale in the Sichuan Basin, China
No full textQuartz, as a pivotal constituent of shale, has garnered considerable attention in research. Nevertheless, prior studies on quartz in shale have concentrated on its influence on organic matter accumulation, pore development, and physical properties. The crystal morphology of quartz and its implications for shale reservoir performance have been largely overlooked. This study utilizes the Longmaxi (LMX) shale in the Sichuan Basin, China, as an example to conduct this work. We used X-ray powder diffraction, polarizing microscopy, scanning electron microscopy, and cathodoluminescence testing, characterizing the crystallinity and characteristics of quartz, revealing the influence of quartz crystallinity on shale reservoirs. The results indicate that the quartz crystallinity index (QCI) of the LMX shale ranges from 2.81 to 8.09, and a significant correlation between the QCI and the sources of quartz is observed. Shale samples with lower QCI values tend to exhibit a higher content of biogenic quartz, whereas the content of clay-transformed quartz and/or terrigenous detrital quartz increases. Furthermore, the pore structure parameters of shale exhibit synergistic variations with QCI, especially for shale with low crystallinity, despite showcasing differences for pores with different pore sizes. Notably, QCI exhibits the strongest negative correlation with micropores, followed by mesopores, while this correlation is less apparent in macropores. The influence of QCI on pore development is primarily ascribed to its synergistic interaction with organic matter enrichment and the constraints imposed on the development and preservation of organic-matter-hosted pores. Under the well-preserved geological conditions of the LMX shale reservoirs, a definite correlation exists between the QCI value and shale gas content/production. This correlation signifies that QCI could serve as a potential indicator for assessing shale reservoir quality, complementing conventional parameters, such as the contents of total organic carbon and quartz
Quantitative reconstruction of a single super rainstorm using daily resolved 5 18 O of land snail shells
No full textA "once-in-a-millennium" super rainstorm battered Zhengzhou, central China, from 07/17/2021 to 07/22/2021 (named "7.20" Zhengzhou rainstorm). It killed 398 people and caused billions of dollars in damage. A pressing question is whether rainstorms of this intensity can be effectively documented by geological archives to understand better their historical variabilities beyond the range of meteorological data. Here, four land snail shells were collected from Zhengzhou, and weekly to daily resolved snail shell 518O 18 O records from June to September of 2021 were obtained by gas-source mass spectrometry and secondary ion mass spectrometry. The daily resolved records show a dramatic negative shift between 06/18/2021 and 09/18/2021, which has been attributed to the "7.20" Zhengzhou rainstorm. Moreover, the measured amplitude of this shift is consistent with the theoretical value estimated from the flux balance model and instrumental data for the "7.20" Zhengzhou rainstorm. Our results suggest that the ultra-high resolution 518O 18 O of land snail shells have the potential to reconstruct local synoptic scale rainstorms quantitatively, and thus fossil snail shells in sedimentary strata can be valuable material for investigating the historical variability of local rainstorms under different climate backgrounds. (c) 2024 Science China Press. Published by Elsevier B.V. and Science China Press. All rights are reserved, including those for text and data mining, AI training, and similar technologies