GIG OpenIR

Origin of the Site U1504 alkaline basalts in the South China Sea continental margin: Insights on deep mantle diversity and subduction dynamics under continental arcs

Thu, 08 May 2025 03:47:54 GMT

题名: Origin of the Site U1504 alkaline basalts in the South China Sea continental margin: Insights on deep mantle diversity and subduction dynamics under continental arcs 作者: Sun, Liheng; Zhang, Yunying; Sun, Zhen; Miao, Xiuquan; Li, Ruilong; Zhang, Wanfeng 摘要: Alkaline basalts produced in continental arcs should contain information different from the arc tholeiite-calc-alkaline-series magmas, and their origin could provide unique constraints on deep mantle composition and material cycling. However, due to their sparse occurrence, alkaline basalts in continental arcs have not been studied thoroughly, which hinders our understanding of the mantle diversity and subduction dynamics under continental arcs. In this study, we present new 40Ar/39Ar ages, major and trace elements, and Sr-Nd-Hf isotopic data for the International Ocean Discovery Program Site U1504 alkaline basalts in the continental arc developed on the South China Block (SCB). These alkaline basalts were generated at ca. 121 Ma and display typical oceanic-island basalt geochemical characteristics. Their relatively high epsilon Nd(t) (3.5-3.7) and low (87Sr/86Sr)i (0.7034-0.7040) and La/Nb (0.5-1.0) values suggest that they were mainly derived from an asthenosphere mantle source. Compared to alkaline basalts in the SCB inland, U1504 alkaline basalts exhibit lower K2O/Na2O, Zr/ Sm, Zr/Y, epsilon Nd(t), and epsilon Hf(t) values, indicating the addition of minor sub-continental lithospheric mantle. The enrichment of Nb, Ta, light rare earth elements, and slight depletion of Zr, Hf, and Ti, as well as elevated Fe/Mn and Sm/Yb and low CaO, indicate that their mantle lithology was mainly garnet pyroxenite. Based upon these findings and previous studies, the garnet pyroxenite was probably formed by the interaction of upwelling asthenosphere with slab edges in the scenario of break-off of the Paleo-Pacific Plate, and such interaction diversified the mantle chemistry beneath continental arcs. In conjunction with other reported alkaline basalt data, it is proposed that the enriched asthenosphere mantle beneath the SCB had formed sequentially from inland to coastal since the late Mesozoic, and this may be related to lateral and vertical flow in the deep asthenosphere controlled by the break-off of subducted plates.

Facet-specific photoreduction and immobilization of Cr(vi) on hematite nanocrystals

Thu, 08 May 2025 03:47:53 GMT

题名: Facet-specific photoreduction and immobilization of Cr(vi) on hematite nanocrystals 作者: Chen, Chaorong; Xian, Haiyang; Liu, Jing; Chen, Qingze; Liang, Xiaoliang; Zhu, Runliang; Hochella Jr, Michael F. 摘要: Photochemical and adsorptive reactions occurring at the mineral-water interface are of great importance in controlling the fate and transport of contaminants in nature, but the effects of the mineral surface structure on the interplay of these two processes and the underlying mechanisms remain largely unknown. In this work, two different structurally well-defined hematite nanocrystals were employed to explore the coupled photoreduction and immobilization of Cr(vi) on various facets of hematite under light irradiation. Compared with hematite quasi-nanocubes (HNCs) exposing only {012} facets, hematite nanoplates (HNPs) with exposed {001} and {113} facets exhibited higher immobilization ability for Cr(vi) under light irradiation, indicating that the coupled process of photoreduction and adsorption of Cr(vi) strongly relied on the particularly exposed facets of hematite. X-ray photoelectron spectroscopy measurements indicated that the removed Cr(vi) from solution was mainly immobilized on hematite surfaces in the form of Cr(iii) (the reduced products of Cr(vi)). Transmission electron microscopy and electron energy-loss spectroscopy results revealed that the accumulation of Cr species was predominantly on the {113} facets of HNPs and the {012} facets of HNCs. Theoretical calculations show that the work function difference between the {113} and {001} facets was directly linked with the effective separation of photogenerated electron-hole pairs in HNPs, which is beneficial for the reduction of Cr(vi). The results of desorption experiments indicate that the coupling of photoreduction and adsorption processes significantly promotes the long-term immobilization of Cr species and effectively reduces their environmental risks (the desorption of Cr(vi) from the HNP and HNC samples decreased from 35.9% and 33.5% to 0.3% and 1.7%, respectively). Overall, the exposed facets can significantly affect the electron transfer properties of hematite as well as the binding affinity for Cr species, thereby governing the reduction and immobilization behavior of Cr(vi) on various facets of hematite. These results advance our understanding of the speciation and immobilization of redox-sensitive contaminants at the surfaces of semiconducting minerals in nature.

Stratigraphic and U-Pb zircon age constraints on the timing of the Yanliao Biota in northern China

Thu, 08 May 2025 03:42:05 GMT

题名: Stratigraphic and U-Pb zircon age constraints on the timing of the Yanliao Biota in northern China 作者: Zou, Zu-Yang; Ma, Qiang; Xu, Yi-Gang; Liu, Liang; Xia, Xiao-Ping; Yang, Chuan-Mao 摘要: The Yanliao Biota from northern China is one of the most famous Mesozoic terrestrial lagerst & auml;tten in the world, with well-preserved fossil records in the Jurassic volcanic-sedimentary Ningcheng and Jianchang basins. However, the temporal evolution of the Yanliao Biota remains controversial, mainly due to the confusing stratigraphic framework and the unclear age of fossiliferous deposits in the Ningcheng basin. To address this issue, we carried out detailed field investigations and secondary ion mass spectrometry (SIMS) U-Pb age dating of zircons on fossiliferous and related strata in the southern part of the Ningcheng basin. The Jurassic infill of this area is composed of, in ascending order, fossil-bearing tuffaceous clastic rocks (unit 1), andesite and pyroclastic rocks (unit 2), fossil-bearing clastic rocks interlayered with minor volcanic rocks (unit 3), and andesite and pyroclastic rocks (unit 4). Integration of stratigraphic and geochronological data indicates that the Yanliao Biota in the Ningcheng basin appeared no later than ca. 163 Ma and lasted until ca. 156 Ma. A comparison with other analogous basins in northern China (e.g., the Luanping, Jianchang, and Beipiao basins) leads to the conclusion that the Yanliao Biota evolved in two phases and lasted for more than 11 m.y.: the first phase from ca. 167 Ma to ca. 163 Ma in the middle Bathonian to middle Callovian, followed by the second phase between ca. 161 Ma and ca. 156 Ma in the lower and middle Oxfordian.

Ultrahigh-pressure to high-pressure eclogite in Cuban ophiolitic melange reveals proto-Caribbean spreading ridge subduction

Thu, 08 May 2025 03:36:58 GMT

题名: Ultrahigh-pressure to high-pressure eclogite in Cuban ophiolitic melange reveals proto-Caribbean spreading ridge subduction 作者: Rui, Hui-Chao; Duan, Wen-Yong; Yang, Jing-Sui; Maresch, Walter V.; Schertl, Hans-Peter; Cai, Peng-Jie; Llanes-Castro, Angelica I.; He, Hong-Ping 摘要: Proto-Caribbean oceanic crust produced during ocean-floor spreading between diverging North and South American plates was subsequently subducted beneath the Caribbean plate. However, the timing and spatial configuration of proto-Caribbean spreading ridge subduction remain subjects of debate. High-pressure (HP) basaltic metamorphic rocks, representing relics of the sub ducted proto-Caribbean oceanic crust, commonly occur in Cuban ophiolitic m & eacute;langes. In this study, an integrated set of petrological, geochemical, and geochronological data is presented for eclogite from the Las Villas m & eacute;lange, central Cuba. The typical geochemical signature of mid-ocean-ridge basalt (MORB) indicates that the protolith of eclogite formed at the proto-Caribbean spreading ridge. Based on pressure-temperature (P-T) estimates obtained by pseudosection analysis as well as Zr-in-rutile and Ti-in-zircon thermometry, the following P-T paths for representative samples can be derived: a prograde path from 24-25 kbar and 510-520 degrees C to peak conditions of 29-31 kbar and 525-575 degrees and a complex retrograde path initially following almost isothermal exhumation to 25-27 kbar, followed by near-isobaric heating to 610-640 degrees C before final exhumation. This is the first documentation of prograde oceanic ultrahigh-pressure (UHP) metamorphism in the northern Caribbean area. U-Pb dating of magmatic zircon with steep heavy rare earth element (HREE) patterns and negative Eu anomalies yielded a protolith age of 126.3 +/- 0.7 Ma. In contrast, metamorphic zircon with flat HREE patterns and without an Eu anomaly yielded a weighted mean age of 118.6 +/- 1.6 Ma. The short time interval of >8 m.y. between MORB magmatism and UHP metamorphism suggests that the oceanic crust was subducted to great depth (similar to 100 km) shortly after generation in an oceanic ridge, which provides robust evidence for subduction of the proto-Caribbean spreading ridge. Furthermore, this work demonstrates high potential to trace ancient spreading ridge subduction by joint petrological, geochemical, and geochronological study of oceanic eclogite.

Iron isotope fractionation during transcrustal magmatic differentiation: Implications for continental crustal formation in subduction zones

Thu, 08 May 2025 03:36:57 GMT

题名: Iron isotope fractionation during transcrustal magmatic differentiation: Implications for continental crustal formation in subduction zones 作者: Xu, Jian; Xia, Xiao-Ping; Wang, Qiang; Zhang, Le; Cui, Ze-Xian; Yang, Qing; Zhang, Wan-Feng 摘要: Earth's continental crust is hypothesized to originate from mantle melting in subduction zones. However, mantle melts are typically Fe-rich, with light Fe isotopes (delta Fe-56 = 0.05 parts per thousand), unlike the Fe-depleted and heavy Fe isotope (delta Fe-56 can be up to 0.2 parts per thousand) compositions of continental crust. The mechanisms responsible for this paradox remain elusive. Here, we report Fe isotope data from a suite of co-magmatic Middle Triassic continental arc rocks in the Ailaoshan tectonic zone (Yunnan, SW China). Gabbroic diorites have light Fe isotopes (delta Fe-56 = 0.04 parts per thousand-0.09 parts per thousand), while granodiorites and normal-arc granites have slightly heavier values (delta Fe-56 = 0.07 parts per thousand-0.14 parts per thousand). Adakitic granites, with high Sr/Y (61-183) and La/Yb (47-90) ratios, exhibit the heaviest delta Fe-56 (0.16 parts per thousand-0.19 parts per thousand). Rayleigh fractionation modeling results suggest that fractionation of isotopically light Fe minerals (e.g., olivine, pyroxene, and amphibole) can reproduce the Fe isotopic variations in most samples, but fails to explain the high Sr/Y, La/Yb, and delta Fe-56 values of adakitic granites, which would require Fe-rich garnet fractionation. Notably, the different Fe isotopes of the adakitic granites and non-adakitic rocks reveal two distinct transcrustal magmatic differentiation pathways: lower crustal, high-pressure, garnet-dominated fractionation; and middle-upper crustal, amphibole-dominated fractionation. The heavy Fe isotopic and Fe-depleted characteristics of the Ailaoshan magmatic arc suites are more comparable to the calc-alkaline arc magmas from normal to thick crust arcs (e.g., the Andean arc), but remarkably different from the tholeiitic arc magmas from thin crust arcs (e.g., the Mariana arc). This study highlights the role of amphibole and garnet fractionation in magmatic calc-alkaline differentiation. We propose that a thickened crust would permit a longer magmatic differentiation column, thus facilitating felsic and Fe-depleted continental crustal formation. The fractionated Fe-rich, garnet-bearing arc cumulates with light Fe isotopes are density-unstable and may be recycled into the mantle by lower crustal foundering.

Coexisting hematite induces and accelerates the transformation of ferrihydrite: Pathway and underlying mechanisms

Thu, 08 May 2025 03:36:56 GMT

题名: Coexisting hematite induces and accelerates the transformation of ferrihydrite: Pathway and underlying mechanisms 作者: Wei, Hongyan; Liu, Jing; Chen, Qingze; Yang, Yixuan; Xian, Haiyang; Liang, Xiaoliang; Yang, Yiping; Zhu, Jianxi; Zhu, Runliang 摘要: Crystallization induced by heterogeneous surfaces is an important process in geochemistry, biomineralization, and material synthesis, but the effects of heterogeneous surfaces on the transformation of metastable phases into new crystals remain poorly understood. In this work, we studied the transformation behaviors of ferrihydrite (Fhy) in the presence of hematite (Hem) nanoplates with specific exposed facets ({001} and {113}) at different pH (4, 7, and 12). Our results reveal that the Hem nano-plates can induce the transformation of Fhy to Hem/Gth (goethite) and accelerate the transformation rate. This effect is primarily achieved by modulating the dissolution-recrystallization process, i.e., accelerating the dissolution of Fhy and promoting the heterogeneous crystallization (to form new Hem/Gth) at the surface of added Hem nanoplates, and solution pH plays a crucial role in these processes. Specifically, a relatively low supply of dissolved Fe3+ from Fhy at pH 4 favors island growth of new Hem at the {001} facets of Hem nanoplates and layer-by-layer growth at the {113} facets, which eventually results in the formation of thermodynamically stable pseudo-cubic morphology (exposing {012} facets). Because of the very low solubility of Fhy at pH 7, the induced transformation of Fhy by Hem nanoplates is relatively weak. While at pH 12, a high supply of dissolved Fe3+ from Fhy benefits the layer-by-layer growth at {001} facets of Hem and the significant heteroepitaxial growth of Gth at the {113} facets. Besides the induced transformation, the direct solid-state transformation of Fhy into Hem and the homogeneous crystallization of dissolved Fe3+ also contribute to the transformation of Fhy. This study reveals the mechanisms of induced transformation of Fhy in the presence of Hem nanoparticles, which will advance our understanding of the significant effects of heterogeneous surfaces in modulating metastable phases and supplement the transformation mechanisms of Fhy.

Mineralogical fingerprints of crustal silica contamination in the Bayan Obo carbonatite

Thu, 08 May 2025 03:36:55 GMT

题名: Mineralogical fingerprints of crustal silica contamination in the Bayan Obo carbonatite 作者: Xiao, Yulun; Yang, Wubin; Yang, Xiaoyong; Cao, Yonghua; Tian, Pengfei; Zhao, Zhuang 摘要: Carbonatites are carbonate-rich and silica-undersaturated igneous rocks. The presence of various silicates in carbonatites has sparked discussions about the source of silica. In this study, abundant fluorbritholite-(Ce) and humite group minerals are identified in the No. 1 carbonatite dike at the Bayan Obo REE-Nb-Fe deposit. These silicates are relatively rare and poorly understood in carbonatite systems. Mineral textures, in situ EPMA, and LA-ICP-MS analyses have been combined to constrain the mineral genesis in the carbonatite. Fluorbritholite-(Ce), a member of the apatite super-group, occurs as euhedral to subhedral crystals in the dike. They are characterized by remarkably high concentrations of REE2O3 (56.0-63.7 wt%), SiO2 (19.6-21.2 wt%), and F (2.47-3.47 wt%), along with relatively lower P2O5 (0.25-3.69 wt%) and CaO (10.3-14.2 wt%) contents compared to common fluorapatite species. Additionally, their high-Y (961-3435 ppm) and low-Sr/Y (0.59-2.70) values suggest a hydrothermal origin from a fluid rich in SiO2, REE, and F. Humite group minerals, mainly chondrodite and humite, display irregularmineral textures. They also exhibit elevated SiO2 (32.5-34.7 wt%) and F content (3.59-7.32 wt%) with notably low TiO2 content (0.02-0.08 wt%), indicating a hydrothermal origin induced by fenitization in the shallow crust. Our results favor a model of crustal silica contamination for the fenitization fluids enriched in F, LREE, and SiO2. More importantly, the fluid-assisted silica contamination from wall rocks within carbonatites is likely to be a critical trigger of REE deposition in the carbonatite ore-forming systems.

Molybdenum isotopes demonstrate that multistage upgrading is required to generate heavy rare earth element-enriched carbonatites

Thu, 08 May 2025 03:36:53 GMT

题名: Molybdenum isotopes demonstrate that multistage upgrading is required to generate heavy rare earth element-enriched carbonatites 作者: Zhao, Xiao-Chen; Yan, Shuang; Niu, He-Cai; Yang, Wu-Bin; Li, Ning-Bo; Li, Jie; Zhang, Le; McCoy-West, Alex J. 摘要: Carbonatites with heavy rare earth element (HREE) enrichment are a rare and intriguing prospect for economic geology research, due to the growing global demand for HREEs in various industries. However, debate persists over the mechanism responsible for HREE enrichment in carbonatites, with the mantle source, magmatic-hydrothermal evolution, or a combination of these factors proposed to be responsible. This study examines three adjacent Late Triassic carbonatites (from the Huanglongpu, Huayangchuan, and Jialu carbonatite dike systems) in the Lesser Qinling of Central China and uses Mo isotope systematics to provide unique insights into the HREE enrichment process of these magmas. All three carbonatites exhibit elevated total REE (Sigma REE) concentrations (up to 4600 ppm), along with significant HREE enrichment (Sigma HREE/Sigma REE = 0.1-0.4). Notably, Jialu carbonatite stands out for having the highest total HREE concentrations (>= 360 ppm) and Sigma HREE/Sigma REE ratios (0.2-0.4). Regardless of their variable degrees of HREE enrichment, the three carbonatites display similar Sr-Nd-Pb isotope signatures, which indicates a shared enriched mantle source. The Huanglongpu and Huayangchuan carbonatites mostly display significantly lighter delta Mo-98/95 (-1.71 parts per thousand to -0.15 parts per thousand) values than the depleted mantle, which indicates an origin from an enriched mantle influenced by recycled pelagic clays and Fe-Mn nodules. Both types of marine sediments are enriched in REEs and would have undergone initial HREE enrichment during slab dehydration and metamorphism, resulting in an HREE-enriched mantle source region. In contrast, Jialu carbonatite possesses significantly heavier delta Mo-98/95 (0.13 parts per thousand-1.89 parts per thousand), which is indicative of the subsequent influence of hydrothermal processes. Additional evidence of this hydrothermal influence at Jialu is preserved in calcite crystal fluid inclusions, elevated delta O-18 (8.71 parts per thousand-10.72 parts per thousand), non-charge-and-radius-controlled (CHARAC) Y/Ho ratios (36-41), and low Sr concentrations (<4800 ppm). Secondary upgrading of HREEs at Jialu occurred due to preferential complexation and transportation during hydrothermal exsolution. This study demonstrates that maximum HREE enrichment in carbonatites is achieved through a two-stage process that involves both a refertilized mantle source and late-stage hydrothermal exsolution.

Early miocene adakitic porphyries in the Chagai arc, Pakistan: Petrogenesis, tectonic setting and implications for porphyry Cu-Au mineralization

Thu, 08 May 2025 03:36:52 GMT

题名: Early miocene adakitic porphyries in the Chagai arc, Pakistan: Petrogenesis, tectonic setting and implications for porphyry Cu-Au mineralization 作者: Miao, Xiu-Quan; Xue, Chuan-Dong; Yang, Xue-Ping; Zhang, Yun-Ying; Chen, Jian-Lin 摘要: Numerous 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.

Transitional chromitites within the Maqsad crust-mantle transition zone of the Semail ophiolite formed above the nascent forearc setting

Thu, 08 May 2025 03:36:51 GMT

题名: Transitional chromitites within the Maqsad crust-mantle transition zone of the Semail ophiolite formed above the nascent forearc setting 作者: Bo, Rong-Zhong; Cai, Peng-Jie; Lian, Dong-Yang; Yang, Jing-Sui; Nasir, Sobhi; Rui, Hui-Chao; Lu, Yu-Xiao; Cao, Chu-Qi; Masoud, Ahmed E. 摘要: The 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.