Guangzhou Institute of Geochemistry
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Mantle metasomatism by subducted Indian continental crust: Evidence from post-collisional basaltic ultrapotassic rocks in southern Tibetan plateau
No full textAt continental collisional zones, the continental crust is known to subduct, penetrating the deep lithospheric mantle and undergoing partial melting. However, the extent to which these melts can effectively alter, or metasomatize, the overlying mantle remains a contentious issue. This debate is especially pronounced in the Cenozoic Himalayan-southern Tibet collisional orogen. The complexity arises from the fact that previous research has predominantly concentrated on the post-collisional intermediate ultrapotassic rocks. However, these rocks could have originated either from the subducted Indian continental crust through a process of mantle assimilation or from melange rocks, thereby primarily reflecting crustal recycling rather than mantle metasomatism by subducted continental crust. In this study, we shift our focus to the less-well-studied post-collisional basaltic ultrapotassic rocks (BUPRs) from the Sailipu region in the Lhasa block of southern Tibet. Our aim is to evaluate the characteristics of the orogenic mantle and the dynamics of crust-mantle interactions. In this study we report major and trace elements and Sr-Nd-Pb-Mo-B isotopes for these Miocene post-collisional BUPRs. The rocks are typically characterized by alkaline affinity, arc-like trace element distribution patterns, and enriched Sr-Nd-Pb isotopes, which point to their derivation from an enriched mantle source. Moreover, the Mo-B isotope ratios of these BUPRs - delta 98/95Mo ranging from -0.85 %o to -0.27 %o and delta 11B from -20.2 %o to -14.4 %o - are notably lower than those found in Mid-Ocean Ridge Basalts (MORBs) and arc lavas. Instead, they are similar to the isotopic compositions of basalts associated with continental subduction, as well as intermediate ultrapotassic rocks from western Anatolia and the Lhasa block, and the gneisses and schists of the Himalayas. These similarities strongly suggest the input of subducted Indian continental crust to the mantle source for these rocks. In conclusion, our study supports the metasomatism of the mantle beneath southern Tibet by subducted Indian continental crust. These results show that continental subduction zones, much like their oceanic counterparts, are key regions for mantle metasomatism, and thus expands our understanding of the geological processes at work in these dynamic areas
The characterization of ambient levoglucosan in Beijing during summertime: Dynamic variation and source contributions under strong cooking influences
No full textParticulate levoglucosan is an important tracer for biomass burning emission in ambient air. However, recent studies question its reliability as a biomass burning tracer in Chinese mega cities due to important contribution from potential non-biomass burning sources, such as cooking. To address this, we examined the dynamic variation and sources of levoglucosan using a chemical ionization mass spectrometer and other advanced instruments during Beijing's summer of 2021. The average mass concentration of levoglucosan and its isomer (C6H10O5) was 0.025 +/- 0.014 pg/m(3) , constituting 0.55 % +/- 0.32 % of total organic carbon (OC) in this campaign. Despite cooking emissions contributing significantly to the organic aerosol (OA, 20 %), levoglucosan and its isomers correlated more strongly with biomass-burning related tracers ( R > 0.6), black carbon (R = 0.72) and less so with cooking-related sources (R = 0.3). This indicates that levoglucosan is primarily dominated by biomass-burning emis- sions rather than cooking in Beijing's urban areas during summertime. The diurnal varia- tion of levoglucosan concentrations highlighted the importance of daytime and nocturnal biomass burning emissions during polluted periods in Beijing. Using levoglucosan as a tracer to quantify the biomass burning OC (BBOC), we found good agreement on the time series of BBOC between the tracer method and other independent source apportionment method. This reaffirms the reliability of levoglucosan as a biomass burning tracer. Biomass burn- ing contributed an average of 7 %-8 % to OC, highlighting its significant impact on Beijing's summer air quality. Our study enhances understanding of biomass burning influences on ambient aerosol in typical urban areas. (c) 2025 The Research Center for Eco-Environmental Sciences, Chinese Academy of Sciences. Published by Elsevier B.V
Study on hydrocarbon retention and expulsion of kerogen based on centrifugal swelling method
No full textCentrifugal swelling experiments using n-hexadecane and 1-methylnaphthalene were carried out to explore the connection between hydrocarbon retention and expulsion in type I kerogen. A driving force-hydrocarbon expulsion model was innovatively established through the results of differential centrifugation experiments and low field nuclear magnetic resonance. Additionally, the hydrocarbon expulsion process of kerogen was categorized into three distinct stages based on the driving force, including free, intergranular state and immovable state (adsorption and swelling) hydrocarbon expulsion. A linear correlation also was established between the T1/T2 signal and the liquid hydrocarbon content during the swelling process, revealing that low- field NMR technology could effectively detect the retention of alkane compounds in kerogen. The study revealed that when the driving force is greater than 647.4 g N, most of the shale oil retained in the kerogen is in an immobile state, which cannot be exploited. Additionally, the hydrocarbon expulsion model based on the centrifugal experiment presents several advantages, including low equipment requirements, straightforward operation, and a broad range of applications. This model can effectively support various types of laboratories conducting shale oil retention assessment work, especially those in oilfields with relatively simple experimental setups
The factors affecting mineralization potential of arc magma: Insights from silicate melt inclusions and zircon composition of igneous lava at Eastern Taiwan Island
No full textSubduction-related Cu-(Au) deposits which represent giant geochemical anomalies of metals and S in the upper crust are commonly associated with arc magmas. However, the fundamental differences between barren and fertile magma producing these deposits still remain highly controversial. In this study, we report the chemical compositions of zircon and silicate melt inclusions (SMIs) from barren arc lavas at eastern Taiwan Island aiming to increase our knowledge on the factors that affect the mineralization potential of arc magma systems. The zircon U-Pb dating shows the magmatism occurred at similar to 0.7 Ma and the andesitic lava formed at similar to 900-950 degrees C with a reduced magmatic environment. The plagioclase-hosted SMIs show an andesitic melt composition and variable Cu content features. The calculated H2O content of parental melt is similar to 3-4 wt.%. The evidence presented above suggests that oxidation state (fO(2)) and H2O content are probably the key controls of Cu-(Au) fertility of arc magmas. We interpret that a reduced and relatively dry magma may be a potential hinderance to cause the absence of porphyry Cu deposits at the eastern Taiwan Island
Paired mass-dependent and mass-independent Hg isotope excursions through non-euxinic intervals: Signals of post-depositional Hg isotope fractionation?
No full textMercury (Hg) in sedimentary deposits has become a widely used proxy for reconstructing paleo-volcanic activity. However, a comprehensive understanding of Hg accumulation pattern and the isotope variation during early diagenesis is still lacking. A thorough diagenetic evaluation of sedimentary rocks is crucial prior to the interpretation of geochemical data, as diagenetic changes can lead to considerable differences between the original seawater signatures and the sedimentological records. We observed a Hg isotopic pattern characterized by a negative trend in S202Hg with a concurrent gradual positive shift in p199Hg throughout non-euxinic intervals in multiple marine sedimentary successions. We propose that the synchronous Hg mass dependent fractionation (MDF) and mass independent fractionation (MIF) were driven by diagenetic events. The release of organic-bound Hg and its subsequent diffusion downward to sites where it complexed with pyrite in early diagenetic stages may induce S202Hg shift towards more negative values in the Hg precipitate, and in turn, contribute to the concurrent positive shift of p199Hg observed in these sections
The development of a pull-apart basin in response to collision of the irregular South China continental margin with Indochina
No full textThe collision of Indochina and South China has played a significant role in shaping the current tectonic configuration of ics are still debated. To better constrain the development and nature of the collision, we conducted a study of the Shiwandashan basin in the southwestern South China block that employed integrated basin analysis methods including detrital zircon prov similar to 200 km long and 60 km wide, and features a V-shaped cross-sectional profile. Our study reveals that the Lopingian Series in the basin thickens rapidly from the northwestern and southeastern edges toward the center of the basin. The formation of the basin appears to have been influenced by intense compressive strike-slip faults during the Lopingian Epoch. However, the sedimentary structures within the basin also display characteristics consistent with an extensional setting, including slump deposits and soft-sediment deformation, along with rapid depositional rates and abrupt changes in depositional facies. Furthermore, Lopingian to Early Triassic igneous rocks within the basin exhibit geochemical signatures that are indicative of an extensional setting. Provenance analyses reveal two distinct source areas controlled by basin contains detrital zircons dating from 297 Ma to 253 Ma, with a peak of 270 Ma, while a similar detrital zircon age peak is not found in the southeastern part of the basin. These findings suggest the coexistence of extensional and compressive structures and distinct sediment sources during the Lopingian. Our analysis provides new insights into the tectonic evolution of the Shiwandashan basin and suggests that it may have been a pull-apart basin influenced by the diachronous collision of the Indochina block with the irregular margins of the South China block
Temporal Variations and Source Apportionment of Biomass Burning and Biogenic Organic Aerosols in the Pearl River Delta: Effects of the Monsoon, Plant Phenology, and Anthropogenic Activities
No full textBiomass burning (BB) and biogenic emissions significantly influence the chemical compositions of organic aerosols (OAs). In this study, aerosol samples collected throughout the year from a regional receptor site in the Pearl River Delta (PRD) were analyzed for BB tracers, primary biological aerosol particle (PBAP) tracers, and biogenic secondary organic aerosol tracers. The BB aerosol concentrations were highest from November to January, and this was mainly attributed to domestic biomass burning in southern China. Biomass burning emissions in Southeast Asia affected the PRD through long-range transport between May and August. Two primary sugar concentration peaks, one in September and October and the other in March, may reflect regional plant phenology, and the constant concentrations for most of the year indicated regional PBAP background emissions. Concentration and correlation analyses indicated that the use of arabitol, erythritol, and inositol as PBAP tracers may not be appropriate because of strong interference by intensive BB emissions. Positive matrix factorization source apportionment showed that plant debris and fungal spores (46%) were the dominant contributors to BB and biogenic organic aerosols (BOAs) during the marine monsoon, but that BB (28%) was the most important contributor during the continental monsoon. The positive matrix factorization results agreed well with the molecular-level radiocarbon-based source apportionment results. Our work indicates that geographic and anthropogenic factors, including the monsoon, plant phenology, and anthropogenic activities, combine to control the chemical compositions of BB and BOAs in the PRD.
Temporal Variations and Source Apportionment of Biomass Burning and Biogenic Organic Aerosols in the Pearl River Delta: Effects of the Monsoon, Plant Phenology, and Anthropogenic Activities
No full textBiomass burning (BB) and biogenic emissions significantly influence the chemical compositions of organic aerosols (OAs). In this study, aerosol samples collected throughout the year from a regional receptor site in the Pearl River Delta (PRD) were analyzed for BB tracers, primary biological aerosol particle (PBAP) tracers, and biogenic secondary organic aerosol tracers. The BB aerosol concentrations were highest from November to January, and this was mainly attributed to domestic biomass burning in southern China. Biomass burning emissions in Southeast Asia affected the PRD through long-range transport between May and August. Two primary sugar concentration peaks, one in September and October and the other in March, may reflect regional plant phenology, and the constant concentrations for most of the year indicated regional PBAP background emissions. Concentration and correlation analyses indicated that the use of arabitol, erythritol, and inositol as PBAP tracers may not be appropriate because of strong interference by intensive BB emissions. Positive matrix factorization source apportionment showed that plant debris and fungal spores (46%) were the dominant contributors to BB and biogenic organic aerosols (BOAs) during the marine monsoon, but that BB (28%) was the most important contributor during the continental monsoon. The positive matrix factorization results agreed well with the molecular-level radiocarbon-based source apportionment results. Our work indicates that geographic and anthropogenic factors, including the monsoon, plant phenology, and anthropogenic activities, combine to control the chemical compositions of BB and BOAs in the PRD.
Transitional chromitites within the Maqsad crust-mantle transition zone of the Semail ophiolite formed above the nascent forearc setting
No full textThe Semail ophiolite is a classic example of a well-preserved ophiolite, hosting over 450 documented podiform chromite deposits. This study investigates a transitional chromitite occurrence in the crust-mantle transition zone (CMTZ) of the Maqsad area within the Sumail massif of the Semail ophiolite. The chromitites, enveloped by dunites, exhibit massive and disseminated textures. Host harzburgites, representing upper mantle residues, underwent similar to 20-35 % partial melting followed by metasomatism from subducting slab-derived melts/fluids. These peridotites show geochemical affinities with Izu-Bonin-Mariana (IBM) forearc peridotites, indicating a forearc tectonic setting. The Maqsad chromitites, with Cr# values of 54.3-66.2, are classified as transitional chromitites. Their geochemical features resemble chromites crystallized from mid-ocean ridge basalt (MORB) melts. Elevated oxygen fugacity (f(O2)) values compared to abyssal peridotites further support their formation in a supra-subduction zone (SSZ)environment. Based on the petrogeological and geochemical characteristics of the host rocks, we propose that the parental magma of the Maqsad transitional chromitites derived from a nascent forearc mantle above an SSZ setting, with MORB-like affinity
Early miocene adakitic porphyries in the Chagai arc, Pakistan: Petrogenesis, tectonic setting and implications for porphyry Cu-Au mineralization
No full textNumerous porphyry Cu-Au deposits were formed in the Chagai arc, western Pakistan, but their origin remains debated. To address this issue, we conducted zircon U-Pb dating, zircon Hf-O and plagioclase Sr isotopic analyses, and geochemical characterization of ore-bearing biotite granitic porphyries (BGP) from the Saindak area in the Chagai arc. These BGP, formed during the early Miocene (21.9-21.7 Ma), exhibit adakite-like geochemical characteristics, including high SiO2 (60.5-67.9 wt%) and low Y (6.41-9.13 ppm) contents, coupled with high Sr/ Y (26.2-116) ratios. The depleted Hf (epsilon Hf(t) = 2.42-9.73) and mantle-like O (delta 18O = 4.99-6.06 parts per thousand) isotopic compositions of zircon grains, along with depleted Sr ((87Sr/86Sr)i = 0.70297-0.70482) isotopic compositions and high anorthite (An = 56.5-69.4 %) contents of plagioclase crystals, suggest that these BGP rocks were derived from an oceanic slab. Slab melting may cause the exceptionally high Cu (667-3060 ppm, average of 1416 ppm) contents of the studied BGP samples, which facilitates the formation of porphyry Cu-Au deposits