Although the deep recycling of carbon has been proposed to play a key role in producing intraplate magmatism, the question of how it controls or triggers mantle melting remains poorly understood. In ...addition, generation of incipient carbonated melts in the mantle and their subsequent reaction with the mantle are critical processes that can influence the geochemistry of intraplate basalts, but the details of such processes are also unclear. Here we present geochemical evidence for the existence of pervasive carbonate melt in the mantle source of Cenozoic continental intraplate highly alkali basalts (SiO2 < 45 wt%), which are volumetrically minor but widespread in eastern China. The primary magma compositions of these basalts cannot be explained by either partial melting of a single mantle source lithology or mixing of magmas derived from distinct mantle sources, but can be adequately explained by carbonate-fluxed melting of eclogite and subsequent reaction between silica-rich melts and peridotite that ultimately transformed the initial carbonated silica-rich melts into silica-undersaturated alkalic magmas. The source of the carbonate is in subducted eclogites associated with the Pacific plate, which stagnated in the mantle transition zone (MTZ). The spatial distribution of the alkali basalts is in accord with large-scale seismic low-velocity anomalies in the upper mantle above the MTZ. Similar scenarios in central-western Europe and eastern Australia lead us to propose that reaction between carbonated silica-rich melt and peridotite may be a pivotal mechanism for the generation of continental intraplate alkali basalts elsewhere in the world.
•Pervasive carbonate melt in the mantle source of Cenozoic intraplate basalts in eastern China.•These basalts were generated by carbonate-fluxed melting of eclogite and melt-peridotite reaction in the asthenosphere.•Melt-peridotite reaction is a pivotal mechanism for genesis of continental intraplate alkali basalts.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
A novel preconcentration method is presented for the determination of Mo isotope ratios by multi‐collector inductively coupled plasma‐mass spectrometry (MC‐ICP‐MS) in geological samples. The method ...is based on the separation of Mo by extraction chromatography using N‐benzoyl‐N‐phenylhydroxylamine (BPHA) supported on a microporous acrylic ester polymeric resin (Amberlite CG‐71). By optimising the procedure, Mo could be simply and effectively separated from virtually all matrix elements with a single pass through a small volume of BPHA resin (0.5 ml). This technique for separation and enrichment of Mo is characterised by high selectivity, column efficiency and recovery (~ 100%), and low total procedural blank (~ 0.18 ng). A 100Mo‐97Mo double spike was mixed with samples before digestion and column separation, which enabled natural mass‐dependent isotopic fractionation to be determined with a measurement reproducibility of < 0.09‰ (δ98/95Mo, 2s) by MC‐ICP‐MS. The mean δ98/95MoSRM 3134 (NIST SRM 3134 Mo reference material; Lot No. 891307) composition of the IAPSO seawater reference material measured in this study was 2.00 ± 0.03‰ (2s, n = 3), which is consistent with previously published values. The described procedure facilitated efficient and rapid Mo isotopic determination in various types of geological samples.
Une nouvelle méthode de pré‐concentration est présentée pour la détermination des rapports isotopiques du Mo par spectrométrie de masse à source plasma et multi‐collection (MC ‐ICP‐MS) dans des échantillons géologiques. La méthode est basée sur la séparation du Mo par Chromatographie d'extraction en utilisant du N‐benzoyle‐N‐ phénylhydroxylamine (BPHA) supporté par une résine polymère d'ester acrylique microporeuse (Amberlite CG‐ 71). En optimisant la procédure, le Mo peut être simplement et efficacement séparé de la quasi‐totalité des éléments de la matrice avec un seul passage à travers un petit volume de résine BPHA (0,5 ml). Cette technique de séparation et d'enrichissement du Mo est caractérisée par une grande sélectivité, une grande efficacité et une grande récupération de la colonne (~ 100%), et un faible total de la procédure de blanc (~ 0,18 ng). Un double spike Mo100‐Mo97 a été mélangé avec les échantillons avant la digestion et la séparation sur colonne, ce qui a permis la détermination du fractionnement isotopique naturel dépendant de la masse avec une reproductibilité de mesure de < 0,09 ‰ (δ98/95Mo, 2s) par MC ‐ICP‐MS. La composition moyenne δ98/95MoSRM3134 (matériau de référence NIST SRM 3134 Mo; Lot n ° 891307) de l'eau de mer de référence IAPSO mesurée dans cette étude est de 2,00 ± 0,03 ‰ (2s, n = 3), ce qui est cohérent avec les valeurs précédemment publiées. La procédure décrite facilite une détermination efficace et rapide des rapports isotopiques du Mo dans divers types d’échantillons géologiques.
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FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
The middle-lower reaches of the Yangtze River is one of the most important metallogenic belts in China, hosting numerous Cu–Fe–Au–Mo deposits. Previous chronological and geochemical investigations ...mostly focused on the magmatic rocks and associated deposits in the lower reaches of the river, but the timing and genesis of the Cu–Au–(Mo) mineralized porphyries in the middle reaches are yet to be well constrained. In this study we carried out precise SIMS U–Pb zircon age determinations for a number of Cu–Au–(Mo) mineralized porphyries and barren granitoid intrusions in the Jiurui and Edong mining districts in the middle reaches of the Yangtze River valley. Our new age results reveal two discrete magmatic and mineralization events at ca. 145–146
Ma and ca. 140
Ma in the study area. The ca. 145–146
Ma granitoids signify the start of the Cretaceous magmatism in the Yangtze River Metallogenic Belt, coinciding temporally with a regional tectonic switching from a Late Jurassic transpressive tectonic regime to an earliest Cretaceous extensional regime in eastern China.
►The Cu–Au–(Mo) mineralized porphyries and barren granitoids in the Jiurui and Edong mining districts in Yangtze River Metallogenic Belt were precisely dated at ca. 145–146
Ma and ca. 140
Ma. ►The ca. 145–146
Ma mineralized porphyries and barren granitoids signify the initiation of the Cretaceous magmatism in the Yangtze River Metallogenic Belt. ►These igneous rocks are most likely generated in response to the regional tectonic switch from the Late Jurassic transpressive to earliest Cretaceous extensional regimes.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
We report an improved procedure for the determination of the platinum‐group elements (PGE) and Re, and Os isotopes from a single sample aliquot by isotope dilution (ID) using inductively coupled ...plasma‐mass spectrometry (ICP‐MS) and negative thermal ionisation mass spectrometry (N‐TIMS), respectively. A two‐stage column method was used to purify PGE‐Re from their sample matrix and interfering elements (e.g., Mo, Zr and Hf) after Os had been separated by CCl4 solvent extraction. The first column separation step used cation exchange resin (AG50W‐X8) to concentrate PGE‐Re and some potential interfering elements (e.g., Mo, Zr and Hf). In the second step, N‐benzoyl‐N‐phenylhydroxylamine (BPHA) extraction resin was used to separate PGE‐Re from the remaining interfering elements, which all remained strongly absorbed to the resin. The method was used to determine the PGE and rhenium, and Os isotope ratios in a range of geochemical reference materials (TDB‐1, WGB‐1, BHVO‐2 and UB‐N). The obtained results agree well with those previously published. This new method enables PGE‐Re abundances and Os isotopic ratios to be determined on the same sample digestion, and circumvents the problems created by sample heterogeneity when comparing PGE and Re‐Os isotope data.
Nous décrivons une procédure améliorée pour la détermination des abondances des éléments du groupe du platine (EGP), du Re et des isotopes de l'Os à partir d'un même aliquot d'échantillon par dilution isotopique (ID) en utilisant respectivement la spectrométrie de masse à source plasma (ICP‐MS) et la spectrométrie de masse par ionisation thermique négative (N‐TIMS). Une méthode sur colonne en deux étapes a été utilisé pour purifier les EGP et le Re de la matrice de l'échantillon et des éléments interférents (par exemple, Mo, Zr et Hf) et ceci après que l'Os ait été séparé par extraction par solvant CCl4. La première étape de séparation sur colonne utilise une résine échangeuse de cations (AG50W‐X8) pour concentrer les EGP et le Re et certains éléments interférents potentiels (par exemple, Mo, Zr et Hf). Au cours de la seconde étape, la résine d'extraction N‐benzoyl‐N‐phénylhydroxylamine (BPHA) a été utilisée pour séparer les EGP et le Re des éléments interférents restants, qui sont tous fortement absorbé par cette résine. La méthode a été utilisée pour déterminer les concentrations en PGE et Re, et les rapports isotopiques de l'Os dans une gamme de matériaux de référence géochimiques (TDB‐1, WGB‐1, BHVO‐2 et UB‐N). Les résultats obtenus sont en bon accord avec ceux publiés antérieurement. Cette nouvelle méthode permet la détermination des abondances en PGE et Re et des rapports isotopiques de l'Os sur la même portion d'échantillon digérée, et contourne ainsi les problèmes liés à l'hétérogénéité des échantillons lorsqu'on compare les données élémentaires des EGP et les données isotopiques Re‐Os.
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FZAB, GIS, IJS, IZUM, KILJ, NLZOH, NUK, OILJ, PILJ, PNG, SAZU, SBCE, SBMB, UL, UM, UPUK
Thermal ionization mass spectrometry is considered as the "gold standard" for obtaining precise
87
Sr/
86
Sr isotopic ratios. However, the conventional TIMS technique is laborious because it requires ...the purification of Sr using expensive and complex resin column techniques to overcome severe
87
Rb isobaric interference on
87
Sr and inhibition on ionization of Sr originating from matrix elements. To overcome this issue, we have developed a hydrofluoric acid coprecipitation method specifically designed for red wine samples. This method effectively captures Sr while eliminating
87
Rb isobaric interference and removing the majority of matrix element inhibition for red wine samples. Notably, our method offers several advantages, including low cost, rapidity, simplicity, and high sample throughput. We have applied this method to analyze nine red wine samples sourced from China, Spain, Chile, Argentina, France, Italy, and Australia. Our method greatly simplified the sample preparation process for red wine samples without compromising the analytical accuracy and precision.
Thermal ionization mass spectrometry is considered as the "gold standard" for obtaining precise
87
Sr/
86
Sr isotopic ratios.
The composition of the deep crust is a key to understanding the formation of the low-velocity zone in the middle to lower crust of the Tibetan Plateau. The Suyingdi rhyolites exposed in the northern ...Qiangtang Terrane have high Sr (296–384 ppm) and low Y (5.81–7.93 ppm), with therefore high Sr/Y ratios (42–56), showing geochemical features of adakitic rocks. Zircon U–Pb dating yields an eruption age of 38.2±0.8 Ma (MSWD = 0.78). These adakitic rhyolites are high-K calc-alkaline in composition, displaying a weakly peraluminous character. They have low MgO content (0.20–0.70 wt.%) and Mg# values (24–39), as well as low Sc (2.25–2.76 ppm), Cr (8–14 ppm), Co (1.6–3.5 ppm) and Ni (2–3 ppm) concentrations. The rocks are LREE-enriched ((La/Yb)N = 50–62) and display weakly negative Eu anomalies (Eu/Eu⁎ = 0.82–0.95) and pronounced negative Nb and Ta anomalies. They have low initial (87Sr/86Sr)i ratios (0.707860 to 0.708342) and enriched Nd isotopic compositions with εNd(t) values ranging from −8.4 to −5.0, which are indistinguishable from those of Cenozoic potassic and ultra-potassic lavas exposed in northern Tibet. Their much higher SiO2 and lower Fe2O3 contents, yet similar MgO, Cr, Co, Ni, and Mg# values to the potassic and ultra-potassic lavas, however, indicate that the rhyolites are unlikely to have formed by fractional crystallization of these lavas. Because of their low Nb/Ta ratios and similar Sr–Nd isotopic compositions to granulite xenoliths within the Cenozoic potassic rocks, we infer that the Suyingdi adakitic rhyolites were most likely produced by partial melting of a thickened lower crust in the garnet stability field. The magma source is most likely dominated by granulite facies metabasalts and clay-poor metamorphosed sedimentary rocks which indicate that the lower crust of northern Tibet is heterogeneous. In combination with data from previously-reported peraluminous and metaluminous adakitic rocks in the same region, the age and petrogenesis of the Suyingdi adakitic rhyolites suggest that the low-velocity zone in the deep crust of central and northern Tibet was most likely the result of partial melting of thickened crust.
•The Suyingdi rhyolites erupted at 38.2 Ma and show geochemical characteristics of adakitic rocks.•The rhyolites were most likely produced by partial melting of a thickened lower crust.•The lower crust of central Tibet is heterogeneous.•The low-velocity zones in the deep crust of central and northern Tibet were generated by crustal melting.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UL, UM, UPCLJ, UPUK
The behavior of molybdenum (Mo) isotopes during chemical weathering was investigated through analysis of saprolite and soil samples from two weathering profiles developed on Neogene basalts of Hainan ...Island in southern China. The Penglai and Nanyang profiles exhibit strong (mean ∼60 wt.% kaolinite) and extreme (mean ∼85 wt.% kaolinite) degrees of weathering, respectively. Our results show that light Mo isotopes are selectively retained in residual minerals, including kaolinite and hematite, whereas heavy Mo isotopes are preferentially leached out. Relative to the Mo-isotopic composition of the underlying unaltered parent basalt –0.38 ± 0.05 ‰ (2σ); vs. NIST 3134), the δ98/95Mo values of the overlying saprolite and surface soil at Penglai are modestly lower (–0.81 to –0.39‰) and higher (–0.35 to –0.12‰), respectively. In this profile, both δ98/95Mo and total organic carbon (TOC) decrease with depth, and their positive correlation in the soil layer (r = +0.99, p(α) < 0.01, n = 6) implies that heavier Mo isotopes are fractionated into organic matter. In the more intensely weathered Nanyang profile, the δ98/95Mo of the underlying unaltered parent basalt is lower –0.56 ± 0.05‰ (2σ) than that at Penglai, and the weathered samples show a larger range of relatively lower values (–1.39 to –0.33‰), which may reflect adsorption of Mo by Fe oxides. Throughout the weathering profile, Fe2O3 correlates positively with Mo concentration (r = +0.86, p(α) < 0.01, n = 32) but negatively with δ98/95Mo (r = –0.69, p(α) < 0.01, n = 32), implying that adsorption onto Fe-oxides played a key role in Mo mobility and isotopic fractionation. Extremely negative δ98/95Mo values (to –1.39‰) in the middle saprolite were likely linked to hydrothermal activity, as indicated by unusual enrichments of trace metals (Ni, Cu, Mo), low Hf/Y ratios, and a positive Eu/Eu* anomaly. Furthermore, the overlying soil layers of both profiles are characterized by enrichment of heavier Mo isotopes and higher Mo concentrations relative to their parent rocks. This may have been caused by additional inputs of Mo from atmospheric precipitation (average δ98/95Mo = +0.94 ± 0.14‰, 1σ, n = 3). Based on mass balance modeling of Mo isotopes and concentrations through the integrated profile, we infer Mo inputs from volcanic ash in the basalt-derived soils of northern Hainan Island. This study highlights the range of potential factors controlling Mo retention and isotopic fractionation as a function of variable weathering intensity, which may help to explain the large range of Mo isotopic compositions observed in river catchments at a global scale.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
Incompatible element ultra‐depleted melt (UDM) has occasionally been found as olivine‐hosted inclusion in plume‐related oceanic island basalts and mid‐oceanic ridge basalts and is genetically linked ...with the involvement of lower oceanic gabbroic crust. However, it has rarely been reported in intracontinental magmatism, and the contribution of such a recycled component to the origin of intracontinental magmas remains unclear. Here, we first report UDM inclusions hosted in olivine from Late Cretaceous intracontinental basaltic andesites in South China. These UDM inclusions are characterized by low SiO2 (46.4–47.9 wt%) and extremely high Al2O3 (18.47–21.79 wt%) contents, positive Sr‐Eu anomalies, and low 207Pb/206Pb and 208Pb/206Pb ratios. Such geochemical features resemble those of high‐pressure melts derived from young lower oceanic gabbro. These results, in combination with geochemical modeling, plate reconstruction, and seismic topography, suggest that tearing or fragmentation of the subducted paleo‐Pacific slab during Late Cretaceous may trigger melting of the refractory lower oceanic cumulates. Our results demonstrate that lower oceanic gabbroic crust could also play an important role in the generation of intracontinental magmatism, which may previously have been underestimated.
Plain Language Summary
The diversity of recycled oceanic slabs, including marine sediments, upper oceanic crust, lower oceanic gabbro, and oceanic mantle, plays an important role in producing chemical heterogeneity of Earth's mantle. Melt inclusions trapped in olivine crystals are isolated from external melt and have the potential to preserve information on the heterogeneous composition of mantle‐derived magmas, which is completely or partially homogenized in bulk rocks. Incompatible element ultra‐depleted melts (UDMs) have been occasionally found as olivine‐hosting inclusions in plume‐related oceanic island basalts and mid‐oceanic ridge basalts and are considered as evidence for the involvement of lower oceanic gabbroic crust in origin. Here, we are the first to document olivine‐hosted UDM inclusions from Late Cretaceous intracontinental basaltic andesites in South China. Their geochemical features resemble those of high‐pressure melts by partial melting of a young lower oceanic gabbroic crust. Partial melting of the refractory lower oceanic gabbro was likely attributed to slab tearing or fragmentation, as revealed by seismic tomographic images that the torn paleo‐Pacific slabs are still stagnated between 1,000 and 1,500 km in the lower mantle. Our results reveal the role of lower oceanic crust in producing the geochemically heterogeneous mantle even beneath the intracontinental regions.
Key Points
This study is the first to document ultra‐depleted melt (UDM) inclusion in intracontinental basaltic andesites from South China
The geochemistry of UDM inclusions resembles those of high‐pressure melts of lower oceanic gabbroic crust
The newly identified UDM inclusions may reflect the contribution of recycled paleo‐Pacific slab to the mantle source of intracontinental magmas
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BFBNIB, FZAB, GIS, IJS, KILJ, NLZOH, NUK, OILJ, SBCE, SBMB, UL, UM, UPUK
As the most polar component of crude oils, asphaltenes are characterized by their complex molecular structures and their sequestration of small organic molecules. These organic compounds are often ...protected from various secondary alteration processes, and thus might serve as useful molecular markers. However, little is presently known about the mode of occurrence of hydrocarbons isolated from asphaltenes by conventional approaches. In the current study, we collected veined bitumen samples from the Changjianggou Formation of the Longmen Mountains on the northwestern rim of the Sichuan Basin, extracted soluble organic matter with dichloromethane, precipitated asphaltenes with petroleum ether, and isolated hydrocarbons from the asphaltenes by centrifugation and hydrous pyrolysis. The results indicated that the n-alkane profiles of saturated hydrocarbons obtained by centrifugation and by hydrous pyrolysis at 300 °C not only were similar to each other, but also resembled those of typical crude oils more closely than the n-alkane profiles of the saturates fraction in veined bitumen extracts did. The asphaltene-bound aromatic hydrocarbons and resins were found to be too difficult to extract thoroughly by the current separation methods. A possible explanation is that saturated hydrocarbons are mostly associated with the peripheral substituents of asphaltenes via hydrophobic association, whereas aromatics and resins tend to interact with the asphaltene cores through π-π stacking, which is much stronger. Therefore, the dissociation or diffusion of the loosely bound saturated hydrocarbons would alter the geochemical information that they had originally recorded, whereas the covalently bonded or more tightly associated hydrocarbons are better shielded from secondary alteration processes.
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GEOZS, IJS, IMTLJ, KILJ, KISLJ, NLZOH, NUK, OILJ, PNG, SAZU, SBCE, SBJE, UILJ, UL, UM, UPCLJ, UPUK, ZAGLJ, ZRSKP
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