Guangzhou Institute of Geochemistry
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The significance of recycled oceanic mantle lithosphere beneath the Arctic Gakkel Ridge
No full textSubduction of oceanic crust has long been considered a major cause of mantle heterogeneity. The oceanic lithospheric mantle, the largest volume of recycled plates, however, is often inferred but rarely observed. Here we report a collection of evidence suggesting that the Gakkel Ridge sampled recycled refractory ocean litho-sphere. (1) A gradient in composition in the western volcanic zone (WVZ) approaching the sparsely magmatic zone (SMZ) suggests lower extents of melting of more depleted sources. (2) The SMZ itself contains substantial portions of sea floor formed by mantle emplaced at the surface. (3) Sparse enriched basalts from the SMZ are consistent with melting of refractory but metasomatized lithosphere. (4) Unique high-Ti basalts at the WVZ-SMZ boundary originated by deep melting of oxide-gabbro-bearing ocean lithosphere mantle. The volume of recycled ocean lithospheric mantle is enormous- it equals roughly 25 % of the entire mantle per gigayear. However, it is almost never observed as an isolated source. If the crustal portion of the lithosphere is isolated to form discrete sources in the mantle, ten to fifteen times as much depleted mantle lithospheric sources would be formed. The rarity of this source material thus suggests that ordinarily recycled ocean crust and mantle lithosphere become well mixed during mantle convection. The enclosed basin and ultra-slow spreading rate of the Gakkel Ridge may have provided inhibited mixing conditions where recycled oceanic mantle lithosphere was able to be preserved
Melting and melt segregation processes controlling granitic melt composition
No full textSeveral important processes in the petrogenesis of granite are still debated due to a poor understanding of complex interactions between minerals during the melting and melt segregation processes. To promote an improved understanding of the mineral-melt relationships, we present a systematic petrographic and geochemical analysis for melanosome and leucosome samples from the Triassic Jindong migmatite, South China. Petrographic observations and zircon U-Pb geochronology indicate that the Jindong migmatite was formed through water-fluxed melting of the Early Paleozoic gneissic granite (437 +/- 2 Ma) during the Triassic (238 +/- 1 Ma), with the production of melt dominated by the breakdown of K-feldspar, plagioclase, and quartz. The Jindong leucosomes may be divided into lenticular and net-structured types. Muscovite, plagioclase, and K-feldspar in the net-structured leucosome show higher Rb and much lower Ba and Sr contents than those in the lenticular leucosome. This may be attributed to the elevation of Rb and decreasing Ba and Sr abundances in melts during the segregation process due to early fractional crystallization of K-feldspar and plagioclase. These leucosomes show negative correlation between epsilon(Nd)(t) and P2O5, reflecting increasing dissolution of low-epsilon(Nd)(t) apatite during the melting process. The continuous dissolution of apatite caused saturation of monazite and xenotime in melt, resulting in the growth of monazite and xenotime around apatite in the melanosome. This process led to a sharp decrease of Th, Y, and REE with increasing P2O5 in the leucosome samples. This complex interplay of accessory mineral reactions in the source impacts REE geochemistry and Nd isotope ratios of granites. As the granites worldwide exhibit similar compositional and isotopic patterns to the Jindong leucosomes, we suggest that both the melting and melt segregation processes strongly control the granitic melt compositions
Highly efficient recovery of Zn2+/Cu2+from water by using hydrotalcite as crystal seeds
No full textThe efficient and waste-free recovery of heavy metals is critical for heavy metal wastewater treatment. In this work, we explored how heavy metals can be recovered as valuable chemicals in the presence of crystal seeds. Hydrotalcite (one kind of layered double hydroxides (LDHs)) was used as crystal seeds to recover Zn2+ in the presence of Al3+ from water (i.e., seed-Zn2+-Al3+ system), which was compared with the monometallic heterogeneous system (seed-Zn2+) and direct coprecipitation (Zn2+-Al3+) system. Our results demonstrated that the seed-Zn2+-Al3+ system possessed a recovery rate of 2.6-2.8 times and a recovery kinetic rate of 2.7-5.9 times higher than those of the other two systems. Differing from the latter two systems, hydrotalcite seeds could induce Zn2+ and Al3+ to form ZnAl-LDH in seed-Zn2+-Al3+. Interestingly, the ZnAl-LDH presents a compositional divalent/trivalent cation molar ratio of ca. 3, which is comparable with the value in the hydrotalcite. It was demonstrated that the hydrotalcite seeds could act as a template to significantly induce the formation of ZnAlLDH complying with the seed's structure and compositional ratio. Similar induction effect of seeds as the Zn2+ system was further verified in Cu2+ systems. This work provides a novel strategy for efficient recovery of heavy metals with product selectivity
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
Magmatic processes within the plumbing system of the ultraslow-spreading southwest Indian ridge: constraints from olivine, plagioclase and melt inclusions
No full textProcesses taking place within the magma plumbing system can exert an important control on the composition of mid-ocean ridge basalts (MORB). Plagioclase ultraphyric basalts (PUBs) found at magma-poor mid-ocean ridges exhibit diverse disequilibrium characteristics, which can provide vital insights for distinguishing the complex effects of melt transport from those of source heterogeneity on the compositions of MORBs. Here, we present new insights into magmatic processes using integrated petrologic and geochemical studies of the PUBs from two zones ( 50(degrees) and 64(degrees)E longitude) along the ultraslow-spreading southwest Indian ridge (SWIR). The studied PUBs have complex mineral morphologies, including skeletal and acicular crystals, glomerocrysts with open and closed structure, reverse and normally zoned crystals and external and internal resorption even in single samples. Both low- and high-Fo olivine and An plagioclase crystals are in disequilibrium with their matrix glasses. Some plagioclase phenocrysts have repeated oscillatory zoning (An(77-86)) going from their core to rim and an abrupt decrease in An content toward the rim. Disequilibrium Sr isotopic compositions are present at several scales: between cores and rims of plagioclase crystals, between different plagioclase crystals and between plagioclase and their host lavas. Inferred pressures of magma storage range from 0.3 to 11.3 kbar. The textural and compositional diversity of crystals together with the variability in melt compositions reflect the combined influences of source heterogeneity and magmatic processes (e.g. crystallization, assimilation and magma mixing processes) taking place within crystal mushes. Our data combined with previous studies suggest that the magmatic processes within the SWIR magma plumbing system involve formation, disaggregation and juxtaposition of crystal-rich mush zones
Carbon cycling during the India-Asia collision revealed by δ<SUP>26</SUP>Mg-δ<SUP>66</SUP>Zn-δ<SUP>98</SUP>Mo evidence from ultrapotassic volcanoes in NW Tibet
No full textIndia-Asia continental collision-induced volcanic gas emissions are thought to have played an important role in driving Cenozoic atmospheric CO2 variations, yet the details of how the deep carbon cycle may influence volcanic CO2 degassing are not understood. We present a novel study employing Mg-Zn-Mo isotopic compositions of Cenozoic ultrapotassic lavas from NW Tibet. The negative Mg-Zn isotope correlation (626Mg = -0.39%o to -0.19%o; 666Zn = +0.27%o to +0.36%o), bolstered by petrographic analysis of mantle-derived xenoliths from these lavas, demonstrates that the ultrapotassic magmas originated from a lithospheric mantle source that had been enriched by recycled carbonate-bearing sediments rich in calcite and dolomite. Very low 698Mo values (-0.78%o to 0%o) relative to the average continental crust (698Mo = +0.10%o to +0.35%o) further indicate that the sedimentary components were derived from the subducted Indian continental crust after its dehydration. Monte Carlo modeling estimates that the input flux of carbon (elemental C) from such sediments into the lithospheric mantle is similar to 5.6 Mt/yr, with a predicted CO2 emission rate of similar to 15.5 Mt/yr. We suggest that the still ongoing subduction of the Indian tectonic plate has played a crucial role in introducing substantial quantities of carbonate-rich sediments into the Tibetan lithospheric mantle, leading to the sequestration of large amounts of CO2 via carbonatite metasomatism. Hence, partial melting of such a carbon-rich mantle reservoir in an orogenic setting provides the positive feedback mechanism that can explain the high flux of volcanic CO2 during IndiaAsia collision. These findings not only highlight the importance of continental subduction, sediment recycling, and mantle metasomatism by carbon-rich melts/fluids in the generation of Tibetan ultrapotassic volcanism, but they also show how the deep carbon cycle influences volcanic CO2 degassing
Annual variations in characteristics and sources analysis of VOCs during the ozone season in the Taiyuan Basin, China, from 2020 to 2022
No full textAs an important precursor of ozone (O3), volatile organic compounds (VOCs) have garnered significant attention in recent years. In this study, VOCs were monitored by a real-time online instrument for three years (from April to September 2020-2022) in Jinzhong, Taiyuan Basin, and comprehensively reported the components characteristics, sources, and ozone formation potential (OFP). The interannual variation in VOCs concentration increased from 11.2 +/- 8.2 ppbv in 2020 to 12.9 +/- 9.2 ppbv in 2021 and 13.3 +/- 8.9 ppbv in 2022. Alkanes were the major VOC groups, accounting for 55.8-64.6% of the total. However, alkenes were the primary contributors to OFP, accounting for 64.3-74.2%. After meteorological normalization, the concentrations of alkanes, alkynes, and aromatics were slightly higher than the observed concentrations, indicating that meteorological conditions favored the dispersion of pollutants. Based on positive matrix factorization (PMF) model, coking sources (28.4-30.7%), LPG/NG usage (17.9-30.5%), and vehicle exhaust (17.5-23.2%) were the major sources of VOCs during the three year observation period. The contributions of solvent usage, biogenic sources, and combustion sources increased with each year. Coking sources (47.5-52.7%) and vehicular emissions (23.2-32.3%), particularly ethylene, were major contributors to OFP. The analysis of potential source regions for VOCs concentration pointed to the southwest region (Qingxu, Wenshui, Xiaoyi, and Jiexiu) as a key emitter of VOCs. Therefore, the study recommends effective mitigation of ozone issues in Jinzhong and the downwind areas of the Taiyuan Basin by controlling coking sources and vehicular emissions, particularly targeting ethylene emissions
Experimental simulation of thermal evolution of nitrogen content and isotopes in source rocks: Implications for nitrogen cycling characterization and oil-source correlation
No full textSedimentary nitrogen isotopes have been widely used to reconstruct the nitrogen cycling and redox characteristics of ancient oceans, but they also have important value in the study of oil genesis, such as oil classification. However, the thermal evolution of nitrogen isotopes in the source rock and associated oil is currently unclear. In this study, the thermal evolution (from the oil generation to the dry gas stage) of three types of low-maturity source rock was simulated. And the content and isotope composition of total organic carbon (TOC) and total nitrogen (TN) of the pyrolyzed source rock, expelled oil and retained oil were measured. In combination with weight-quantitative analyses, the following conclusions can be drawn: (1) During the main oil and gas generation stage, the pyrolyzed source rocks lost 15.1-28.3% of TN, which is much lower than the loss of TOC (26.2-46.1%). This difference may be related to the fact that the TOC content of the generated oil and gas is two orders of magnitude higher than the TN. In addition, TOC loss occurred mainly within the oil generationexpulsion stage, whereas TN loss occurred mainly in the dry gas stage and higher maturity stage. (2) The nitrogen isotope compositions of three types of source rocks and associated kerogens became heavier by 0.5-1.5%o as thermal maturity increased, which is similar to the change of organic carbon isotope compositions. The delta 15N values of pyrolyzed source rocks generally have poor relationships with either the amount of expelled oil and gas or the TN loss, which is different from the delta 13Corg values. The delta 15N fluctuations may be attributed to three processes: oil generation-expulsion, N2 gas generation-expulsion, and inorganic N (like NH4+) expulsion. The bulk nitrogen isotope composition of source rocks is only slightly affected by thermal maturity and can be used to indicate the nitrogen biogeochemical cycle and water redox conditions during the deposition. (3) Similar to delta 13Corg values, delta 15N values of both expelled oil and retained oil increased gradually with enhanced thermal maturity, implying a control of kinetic isotope effect. Among the three types of source rock, two of them having relatively light nitrogen isotope compositions (delta 15N value of -2.5%o) generated and expelled oils with similar nitrogen isotope compositions, while the third type, which has heavy nitrogen isotope composition (delta 15N value of -13.0%o), generated and expelled oils with much lighter nitrogen isotope compositions (by 3-6%o). This finding shows that there may be significant nitrogen isotope fractionation during the generation and primary migration of oil from source rocks. The nitrogen isotopes should be used in oil-source correlation with caution, even though related mechanisms need to be further studied. Nevertheless, using a combination of organic carbon and nitrogen isotopes, the oil generated and expelled by the three types of source rocks could be distinguished. Therefore, the use of nitrogen isotopes for oil-source correlation under geological conditions should be combined with other isotope and molecular indicators
Comprehensive exploration of the anaerobic biotransformation of polychlorinated biphenyls in<i> Dehalococcoides</i><i> mccartyi</i> CG1: Kinetics, enantioselectivity, and isotope fractionation
No full textAnaerobic microbial transformation is a key pathway in the natural attenuation of polychlorinated biphenyls (PCBs). Much less is known about the transformation behaviors induced by pure organohalide-respiring bacteria, especially kinetic isotope effects. Therefore, the kinetics, pathways, enantioselectivity, and carbon and chlorine isotope fractionation of PCBs transformation by Dehalococcoides mccartyi CG1 were comprehensively explored. The results indicated that the PCBs were mainly dechlorinated via removing their double-flanked meta-chlorine, with their first-order kinetic constants following the order of PCB132 > PCB174 > PCB85 > PCB183 > PCB138. However, PCBs occurred great loss of stoichiometric mass balance during microbial transformation, suggesting the generation of other non-dehalogenation products and/or stable intermediates. The preferential transformation of (-)-atropisomers and generation of (+)-atropisomers were observed during PCB132 and PCB174 biotransformation with the enantiomeric enrichment factors of -0.8609 +/- 0.1077 and -0.4503 +/- 0.1334 (first half incubation times)/-0.1888 +/- 0.1354 (second half incubation times), respectively, whereas no enantioselectivity occurred during PCB183 biotransformation. More importantly, although there was no carbon and chlorine isotope fractionation occurring for studied substrates, the delta C-13 values of dechlorination products, including PCB47 (-28.15 +/- 0.35 parts per thousand similar to -27.77 +/- 0.20 parts per thousand), PCB91 (-36.36 +/- 0.09 parts per thousand similar to -34.71 +/- 0.49 parts per thousand), and PCB149 (-28.08 +/- 0.26 parts per thousand similar to -26.83 +/- 0.10 parts per thousand), were all significantly different from those of their corresponding substrates (PCB85: -30.81 +/- 0.02 parts per thousand similar to -30.22 +/- 0.21 parts per thousand, PCB132: -33.57 +/- 0.15 parts per thousand similar to -33.13 +/- 0.14 parts per thousand, and PCB174: -26.30 +/- 0.09 parts per thousand similar to -26.01 +/- 0.07 parts per thousand), which further supported the generation of other non-dehalogenation products and/or stable intermediates with enrichment or depletion of C-13. These findings provide deeper insights into the anaerobic microbial transformation behaviors of PCBs
Comprehensive exploration of the anaerobic biotransformation of polychlorinated biphenyls in<i> Dehalococcoides</i><i> mccartyi</i> CG1: Kinetics, enantioselectivity, and isotope fractionation
No full textAnaerobic microbial transformation is a key pathway in the natural attenuation of polychlorinated biphenyls (PCBs). Much less is known about the transformation behaviors induced by pure organohalide-respiring bacteria, especially kinetic isotope effects. Therefore, the kinetics, pathways, enantioselectivity, and carbon and chlorine isotope fractionation of PCBs transformation by Dehalococcoides mccartyi CG1 were comprehensively explored. The results indicated that the PCBs were mainly dechlorinated via removing their double-flanked meta-chlorine, with their first-order kinetic constants following the order of PCB132 > PCB174 > PCB85 > PCB183 > PCB138. However, PCBs occurred great loss of stoichiometric mass balance during microbial transformation, suggesting the generation of other non-dehalogenation products and/or stable intermediates. The preferential transformation of (-)-atropisomers and generation of (+)-atropisomers were observed during PCB132 and PCB174 biotransformation with the enantiomeric enrichment factors of -0.8609 +/- 0.1077 and -0.4503 +/- 0.1334 (first half incubation times)/-0.1888 +/- 0.1354 (second half incubation times), respectively, whereas no enantioselectivity occurred during PCB183 biotransformation. More importantly, although there was no carbon and chlorine isotope fractionation occurring for studied substrates, the delta C-13 values of dechlorination products, including PCB47 (-28.15 +/- 0.35 parts per thousand similar to -27.77 +/- 0.20 parts per thousand), PCB91 (-36.36 +/- 0.09 parts per thousand similar to -34.71 +/- 0.49 parts per thousand), and PCB149 (-28.08 +/- 0.26 parts per thousand similar to -26.83 +/- 0.10 parts per thousand), were all significantly different from those of their corresponding substrates (PCB85: -30.81 +/- 0.02 parts per thousand similar to -30.22 +/- 0.21 parts per thousand, PCB132: -33.57 +/- 0.15 parts per thousand similar to -33.13 +/- 0.14 parts per thousand, and PCB174: -26.30 +/- 0.09 parts per thousand similar to -26.01 +/- 0.07 parts per thousand), which further supported the generation of other non-dehalogenation products and/or stable intermediates with enrichment or depletion of C-13. These findings provide deeper insights into the anaerobic microbial transformation behaviors of PCBs