Radiogenic Isotopic Compositions Research Articles

Geodynamic Evolution of the Proto-Tethys Ocean in the West Kunlun Orogenic Belt, NW Tibet: Implications from the Sub-Arc Crust and Lithospheric Mantle Modification

Abstract Radiogenic isotopes serve as a crucial tool for investigating crustal evolution, playing a pivotal role in revealing magma sources and petrogenesis. However, they can be ineffective in distinguishing distinct magmatic sources with similar radiogenic isotopic compositions, a common occurrence in nature. This paper addresses this challenge in the Ordovician igneous rocks from the West Kunlun orogenic belt (WKOB), to distinguish between two potential magmatic sources (i.e., the Tarim Craton and the Tianshuihai terrane) with similar isotopic compositions, by using appropriate thermodynamic and geochemical modeling based on mineral and whole-rock geochemistry. Zircon U–Pb dating yields ages of 483 ± 3 Ma, 469 ± 2 Ma and 461 ± 2 Ma for the Pushou gabbros and the Datong monzogranites and syenites, respectively. The Pushou gabbros exhibit low SiO2 (47.4–49.1 wt.%), high MgO (5.5–6.9 wt.%), high large ion lithophile elements (LILEs, e.g., Rb, Ba, Th, and K) and low high field-strength elements (HFSEs, e.g., Nb, Ta, Zr, Hf, P, and Ti), which suggest an origin in subduction-modified mantle. They display high whole-rock (87Sr/86Sr)i ratios (0.7156 to 0.7192), negative whole-rock εNd(t) values (−7.1 to −7.8), as well as high zircon δ18O values (7.6–7.9 ‰) and enriched zircon Hf isotopic compositions (εHf(t) = −5.3 to −7.7), consistent with 1–5% subducted sediments in an enriched mantle source. Trace element models further confirm that the gabbros are most likely derived from low-degree (~15 %) partial melting of subduction-modified Tarim mantle in the spinel-garnet facies, rather than from the Tianshuihai mantle. The Datong syenite samples belong to the shoshonitic series and are characterized by medium SiO2 (59.5–61.4 wt.%), relatively low MgO (0.9–1.2 wt.%) and Mg# (37–42), enrichment in LILEs and depletion in HFSEs. They have high whole-rock (87Sr/86Sr)i ratios (0.7103 to 0.7105) and negative whole-rock εNd(t) values (−3.8 to −4.3), along with negative to slightly positive zircon εHf(t) values (–3.8 to +2.6), similar to coeval mafic rocks. Thermodynamic and geochemical modeling suggests that the Datong shoshonitic rocks likely originated via crystal fractionation of shoshonitic basaltic magmas in the SW Tarim Craton. The Datong monzogranites have high SiO2 (69.7–72.6 wt.%), low MgO (0.6–0.7 wt.%) and demonstrate a typical enrichment in alkalis, Zr, and Nb with depletion in Sr, P, and Ti, consistent with A-type granites. They are characterized by high whole-rock (87Sr/86Sr)i ratios (0.7321 to 0.7323), negative whole-rock εNd(t) (−11.3 to −11.8), negative zircon εHf(t) (−11.0 to −16.5) and high zircon δ18O (7.2–8.0 ‰), indicating derivation from the remelting of an ancient crustal source. Thermodynamic, major, and trace element modeling indicate that their parent magma may have been generated by water-deficient (~2 wt.%) partial melting of ancient crustal material beneath the SW Tarim Craton rather than that of the Tianshuihai terrane, under high-temperature (T > ~950 °C), low-pressure (P = 5–8 kbar) conditions. Based on the tectonic framework of the WKOB, we propose that the original mantle and crust beneath the southern Kunlun terrane may have been modified or partially replaced by that beneath the SW Tarim Craton during the Ordovician. Therefore, this evidence for Tarim-derived magmatism, when combined with regional sedimentary and structural records, indicates that Ordovician magmatism in the southern Kunlun terrane is most consistent with episodic northward subduction of the Proto-Tethys Ocean, commencing at ~485 Ma. Middle Ordovician slab break-off can explain the formation of the A-type granites, but re-instated northward subduction is required for the generation of late Ordovician Datong syenites.

Mascarene basin asthenosphere reservoir source and composition revealed by Central Indian Ridge basalt geochemistry

Along the Central Indian Ridge (CIR), the geochemical and isotopic signature of mid-ocean ridge basalts (MORB) from three segments between 8° and 12°S show a FOZO-like enrichment with elevated 3He/4He (R/RA) (> 10 RA), suggesting small-scale upwelling with a deep primordial mantle component. Based on complementary major and trace elements analysis as well as Sr, Nd and Pb radiogenic isotope compositions of MORB samples from the same location, we confirm the presence of a FOZO/C-like enriched signature characterized by high Pb isotope ratios (206Pb/204Pb = 18.1134–19.1481; 207Pb/204Pb = 15.4710–15.6146 and 208Pb/204Pb = 37.8625–39.0332), relatively low 87Sr/86Sr (0.702767–0.702974) and high 143Nd/144Nd (0.512989–0.513118). In agreement with recent seismological studies highlighting an asthenospheric anomaly centered under the Mascarene Basin and flowing beneath the CIR, we propose that this plume-like anomaly named Mascarene Basin Asthenosphere Reservoir (MBAR) is the source of the enriched FOZO/C-like signature observed along this portion of the CIR. Furthermore, our analyses reveal that the source of the MBAR anomaly may have a unique geochemical signature in the Indian Ocean, distinct from other Indo-African plumes such as the Réunion plume. Moreover, the primordial characteristics, and the location of the MBAR (next to the margin of the African LLSVP) suggest that this plume-like anomaly may be a disconnected blob/proto-plume of a broad mantle structure formed by different Indo-African plumes/upwelling (e.g., Afar and Bouvet plumes) and anchored in the lower mantle.

New discovery of the Early Paleozoic carbonatite in Bayan Obo (China): Insights into the giant REE accumulation

The Bayan Obo deposit in China hosts the world’s largest rare earth element (REE) resource. Age dating results for Bayan Obo range across approximately one billion years (from ∼1.4 to 0.3 Ga), with three age groups (∼1.3 Ga, ∼0.45–0.40 Ga, and 0.28–0.26 Ga). Carbonatite dated to around 1.3 Ga and granitoids to approximately 270 Ma have been identified. The Early Paleozoic vein-type mineralization is widespread in the deposit. However, magmatic activity related to the highest age peak at ∼0.4 Ga has not well been identified in the deposit, complicating the interpretation of the ore genesis. In this study, we report a zircon 232Th-208Pb age of 435.2 ± 4.6 Ma (MSWD = 1.9, N = 26) from carbonatite in Bayan Obo. The zircon morphology, characterized by bipyramidal crystal forms, low U concentrations (mostly < 1 ppm), high Th/U ratio, and carbonatite-like oxygen isotopes (5.92 ± 0.31 ‰), demonstrates that the zircon crystallized from carbonatitic magmas, thus their ages represent the timing of magmatic activity. Although remelting of the Mesoproterozoic carbonatite during Paleozoic carbonatite has been previously documented, the newly identified Early Paleozoic carbonatite derives from a different mantle source, as indicated by their more radiogenic Hf isotope compositions. This suggests that the Bayan Obo deposit experienced multiple carbonatitic activities. The Early Paleozoic carbonatitic activity may have facilitated element mobilization and mineral recrystallization, improving the exploitable levels of the world’s largest REE deposit.

Petrographic and Geochemical Evidence for a Complex Magmatic Plumbing System beneath Bagana Volcano, Papua New Guinea

Abstract Bagana is a persistently active stratovolcano located on Bougainville Island, Papua New Guinea. Characteristic activity consists of prolonged lava effusion over months to years, with occasional shifts to explosive vulcanian or sub Plinian eruptions that threaten surrounding communities. Satellite observations have shown that Bagana is a major SO2 emitter, particularly during eruptive intervals. Despite persistent and potentially hazardous activity, no previous geophysical, petrological, or geochemical studies have constrained the magma storage conditions and reservoir processes at Bagana. To address this knowledge gap, we present new bulk rock major, trace element, and radiogenic isotope data, plus mineral phase major element compositions, for Bagana lavas erupted in 2005 and 2012 and ash erupted in 2016. We use our new data to understand the magmatic processes controlling the typical effusive activity and provide the first estimates of magma storage conditions beneath Bagana. The basaltic andesite bulk rock compositions (56–58 wt% SiO2) of our Bagana lavas reflect accumulation of a plagioclase + clinopyroxene + amphibole + magnetite + orthopyroxene crystal cargo by andesitic-dacitic (57–66 wt% SiO2) carrier melts. Constraints from clinopyroxene and amphibole thermobarometry, amphibole hygrometry, and experimental petrology suggest that the high-An plagioclase + clinopyroxene + amphibole + magnetite assemblage crystallizes from basaltic-basaltic andesite parental magmas with &amp;gt;4 wt% H2O, over a temperature interval of ~1100–900°C, at pressures of ~130–570 MPa, corresponding to ~5–21 km depth. Continued crystallization in the magma storage region at ~5–21 km depth produces andesitic to dacitic residual melts, which segregate and ascend towards the surface. These ascending melts entrain a diverse crystal cargo through interaction with melt-rich and mushy magma bodies. Degassing of carrier melts during ascent results in crystallization of low-An plagioclase and the formation of amphibole breakdown rims. The radiogenic isotope and trace element compositions of Bagana lavas suggest that parental magmas feeding the system derive from an enriched mantle source modified by both slab fluids and subducted sediments. Our findings suggest that the prolonged lava effusion and persistently high gas emissions that characterise Bagana’s activity in recent decades are sustained by a steady state regime of near-continuous ascent and degassing of magmas from the crustal plumbing system. Our characterisation of the Bagana magmatic plumbing system during effusive activity provides a valuable framework for interpreting ongoing monitoring data, and for identifying any differences in magmatic processes during any future shift to explosive activity.

Early Triassic high-K granitoids and enclaves of the Daheba pluton, West Qinling (China): Implications for relative contributions of crust and mantle

Early Triassic granitoids are widespread in the northwestern West Qinling Orogen, China, but their petrogenesis and geodynamic implications remain unclear. In this study, we integrated new field and petrological observations, mineralogical compositions, zircon U-Pb dating, Hf isotopic and whole-rock geochemical analyses for the early Triassic granitic pluton in the Daheba area to determine its magma source and geodynamic scenario. The pluton is composed of granodiorite and syenogranite and carries microgranular enclaves (MEs) which formed at ca. 253–249 Ma. The granitoids show wide SiO2 contents of 63.73–77.49 wt% (av. 69.89), high K2O contents of 3.4–5.4 wt% (av. 4.1) and moderate Mg# of 16–51 (av. 36), belonging to high-K, calc-alkaline I-type granites. These rocks have high radiogenic but uniform Hf isotopic compositions with 176Hf/177Hf and ɛHf(t) of 0.282511–0.282658 and −3.85 – +1.25, respectively. The MEs hosed within the granite are characterized by high Mg# of 35–58 (av. 46) and variable εHf(t) of −4.64 – +9.35, which likely represent a hybridized melt derived from a slab-modified mantle and lower crust. In combination with coeval magmatic rocks in the western Gonghe-East Kunlun area, we propose a genetic model where mafic magmas derived from an enriched mantle and underplated beneath the overlying lower crust are considered to have produced the high-K felsic magma. Further, the hybridized melt ascended to shallower crustal levels to generate a series of rocks ranging from dioritic MEs to granodiorite to syenogranite. Mass balances modeling suggests that the generation of these rocks involved 36 % of the lower crustal-derived melt and 64 % of the SCLM (R2 = 0.9). Our new data, in tandem with published results suggest that the Daheba pluton formed during the subduction stage of the Paleo-Tethyan Ocean and that a local extensional episode occurred at 253–249 Ma in the western Gonghe area.

The oxygen fugacity of intermediate shergottite NWA 11043: implications for Martian mantle evolution

Shergottite meteorites, classified as depleted, intermediate, or enriched based on incompatible trace elements and specific radiogenic isotope compositions (Sr, Nd, and Hf isotope ratios), point to multiple Martian mantle source regions. The oxygen fugacity (fO2) of these mantle regions, determined from early crystallizing minerals using the olivine-pyroxene-spinel oxybarometer, appears to correlate with incompatible trace element enrichment and isotope compositions. However, values derived from the vanadium-in-olivine oxybarometer challenge this correlation, hinting at potential biases in oxybarometry or complexities in the redox conditions of the Martian mantle. By analyzing the intermediate shergottite Northwest Africa (NWA) 11043 with various oxybarometers, this study deduced its origin from a reduced mantle source, with an average fO2 value of −0.77 ± 0.35 relative to the iron-wüstite (IW) buffer. Notably, these values coincide with those of depleted shergottites, which represent the depleted Martian mantle region. This redox similarity between intermediate and depleted shergottites contrasts with earlier notions that postulated intermediate shergottites as a mix of depleted and enriched mantle derivatives. Moreover, intermediate shergottites such as NWA 11043, Elephant Moraine (EETA) 79001A, and Allan Hills (ALH) 77005 display 176Hf/177Hf values akin to those of depleted shergottites, suggesting that intermediate mantle components can be separated from the depleted mantle source at approximately 2.2 Ga based on model age calculations. Therefore, there presents a consistent redox state between mantle magma sources of both intermediate and depleted shergottites since the Hesperian period, while enriched shergottites lean toward more oxidized conditions past source formation.This study prompts a reassessment of conventional theories, emphasizing the nuanced redox evolution of the Martian mantle across distinct mantle source regions and underscoring the complexity of the redox evolution of the Martian mantle. The emergence of chemically diverse mantle reservoirs might predominantly arise from early magma ocean differentiation processes, albeit with inherent oxidation nuances. The differences in fO2 observed between intermediate and depleted shergottites underscore the need for more in-depth studies to decipher Martian mantle differentiation and evolution.

Petrogenesis of volcanic rocks from the Quaternary Eifel volcanic fields, Germany: detailed insights from combined trace-element and Sr–Nd–Hf–Pb–Os isotope data

Quaternary rocks from the East and West Eifel volcanic fields in western Germany are a key suite of intraplate volcanic rocks that can provide insights into volcanism of the Central European Volcanic Province (CEVP) and into continental intraplate volcanism in general. We present a comprehensive dataset for Eifel lavas including isotope as well as major and trace element data for 59 samples covering representative compositions of the different volcanic fields. In line with previous studies, the lavas are all SiO2-undersaturated, alkaline-rich and mainly comprise primitive basanites, melilitites, and nephelinites (Mg# ≥ 57). Geochemical compositions of samples from both volcanic subfields display distinct differences in their trace-element as well as radiogenic isotope compositions, largely confirming previous subdivisions. Coupled trace-element and radiogenic Sr–Nd–Hf–Pb–Os isotope compositions can now provide firm evidence for spatially heterogeneous mantle sources and compositionally distinct magmatic pulses. Within the West Eifel Field, Sr–Nd–Pb isotope compositions of the younger (≤80 ka), ONB-suite (olivine-nephelinite-basanite) are similar to FOZO (FOcal ZOne) or the EAR (European Asthenospheric Reservoir) and resemble compositions that have been previously reported from plume-sourced ocean island basalts (OIB). In marked difference, older (700 Ma to 80 ka) volcanic rocks from the F-suite (Foidite) in the West Eifel field and from the entire east Eifel Field tap a more enriched mantle component, as illustrated by more radiogenic Sr isotope (86Sr/87Sr up to 0.705408) and variable Pb isotope compositions (206Pb/204Pb = 18.61–19.70, 207Pb/204Pb = 15.62–15.67 and 208Pb/204Pb = 38.89–39.76). Combined trace-element compositions of ONB-suite samples are in good agreement with results from batch melting models suggesting a hybrid composition of Eifel magmas formed through mixing 10% of a FOZO-like melt with 90% of a DMM-like melt, similar to melts from the Tertiary HEVF. However, radiogenic Sr–Nd–Pb isotope compositions of F-suite and EEVF and some ONB lavas require the admixture of melts from lithospheric mantle sources. Elevated Nb/Ta and Lu/Hf ratios in combination with variable 187Os/188Os ratios can now demonstrate the presence of residual carbonated eclogite components, either in the lithosphere or in the asthenospheric mantle. Finally, by combining geochemical and temporal constraints of Tertiary and Quaternary volcanism it becomes evident that CEVP volcanism in central and western Germany has resulted from compositionally distinct magmatic pulses that tap separate mantle sources. Although the presence of a mantle plume can neither be fully confirmed nor excluded, plume-like melt pulses which partially tap carbonated eclogite domains that interact to variable extents with the lithosphere provide a viable explanation for the temporal and compositional cyclicity of CEVP volcanism.

Post-Imbrium Pb–Pb isochron ages for Apollo basaltic impact melt samples 14078 and 68415

The Apollo 14 and 16 missions returned several samples commonly interpreted as crystalline impact melt, with ages of approximately 3800–3850 Ma. Previous work has suggested that these rocks formed in one or more pre-Imbrium basin forming impact(s). By contrast, recent ages determined for a range of lunar breccias provide compelling evidence that the Imbrium basin was formed at approximately 3920 Ma. Using an approach previously demonstrated in lunar basalts, Pb–Pb isochron ages are determined for two of these proposed impact melt samples (14078: 3848 ± 4 Ma; and 68415: 3834 ± 11 Ma). In the case of 14078, the least radiogenic Pb isotopic compositions measured here are interpreted as representing the initial Pb isotopic composition of the sample. This value indicates derivation from a source (or sources) with high 238 U/ 204 Pb ratios (approx. 2400), similar to those predicted for the Apollo 14 high-Al and very high-K basalts. It was not possible to determine an equivalent initial Pb isotopic composition for 68415, but Pb isotope evolution models indicate that the sample would have been derived from lithologies with lower 238 U/ 204 Pb source ratios (approx. 1000). In both cases, the samples are interpreted as having been formed by impacts local to the Apollo 14 and Apollo 16 landing sites.

The Origin and Evolution of Rare Earth Element Mineralization in the Muluozhai Deposit (Sichuan, China): Insights from Mineralogical, Trace Element, and Sr-Nd-Pb-C-O-Ca Isotope Data

Abstract Carbonatite complexes are globally significant sources of rare earth elements (REEs); however, mechanisms governing REE deposition in various tectono-lithologic settings, encompassing host rocks, wall rocks, ore-controlling structures, and metasomatism, remain inadequately understood. The Zhengjialiangzi mining camp, situated within the extensive Muluozhai deposit (containing 0.45 million metric tons [Mt] at 4.0 wt % REE2O3) in the northern segment of the Mianning-Dechang belt, Sichuan (southwestern China), is characterized by a complex vein system that evolved within metamorphosed supracrustal rocks of the Yangxin and Mount Emei Formations. The mineralization is coeval with Oligocene intrusions of carbonatite and nordmarkite at ~27 Ma. The major gangue minerals include fluorite, barite (transitional to celestine), and calcite, with bastnäsite serving as the primary host for REEs in all analyzed orebodies. Several other accessory to minor minerals were identified in the ore veins, including some that had not previously been known to occur in the Muluozhai deposits (e.g., thorite and pyrochlore). The stable isotopic (C-O-Ca) and trace element compositions of calcite, along with whole-rock data, suggest that carbonate material was derived from the mantle and subsequently reequilibrated with the Yangxin marbles. The radiogenic isotope (Sr-Nd-Pb) compositions of vein material remained unaffected by wall-rock contamination and suggest a mantle source influenced by crustal recycling, consistent with other REE deposits hosted by carbonatite and nordmarkite in the region. The combined petrographic and geochemical evidence suggests derivation of Muluozhai mineralization from a carbonatitic source and interaction of carbonatite-derived fluids with wall rocks, xenoliths, and early-crystallizing mineral phases, particularly barite.

Bioavailability, risk assessment, and source traceability of potentially toxic elements in bivalve species widely consumed in the emblematic Acapulco Bay, Mexico

The concentration of potentially toxic elements (PTE) was determined in edible tissue, digestive system, and shells of two bivalve species (Hyotissa hyotis and Spondylus limbatus) highly consumed in Acapulco Bay (southern Mexico) to evaluate the impact on marine ecosystems and human health risk associated with the consumption of potentially contaminated bivalves. PTE concentrations (Ba–Cd–Cu–Co–Cr–Mn–Ni–Pb–Sr–V–Zn) in edible tissues were lower than in the digestive system and shells in both bivalve species. Except for Cr, Pb, and Cd, PTE concentrations in the edible tissue of both species did not exceed international threshold limits. In addition, the biota-sediment accumulation factor values indicate bioaccumulation in edible tissues (Ni, Cd, Pb, Co, and Zn), digestive system (V, Ni, Cu, Cd, Pb, Co, and Zn), and shells (V, Cr, Pb, Co, and Mn), evidencing availability of pollutants in the Acapulco Bay. Contents of Pb and Cd found in edible tissue can threaten human health. Target Hazard Quotient values indicate that the consumption of edible tissue does not represent any risk of developing non-cancerogenic effects. However, the Target Cancer Risk values indicate a moderate risk of developing carcinogenic effects due to the accumulation of Cr and Pb and an even greater risk due to the accumulation of Cd. Additionally, although local rocks and sediments should be the only source of PTE in uncontaminated marine ecosystems, a low radiogenic Pb isotope composition was detected in the bivalve shells, which partially overlap with Pb isotope compositions of paints and gasoline, indicating anthropogenic lead sources participation in the Acapulco Bay.

Paleoproterozoic Dacite Dikes of the Vorontsovka Terrane, Volga–Don Orogen: Geochemistry, Age, and Petrogenesis

Metamorphosed dacitic porphyry dikes were first found in the western part of the Vorontsovka terrane, which is located in the Paleoproterozoic Volga–Don orogen at the margin of Archean Sarmatia and Volga–Ural cratons. The magmatic protolith age for the metadacites is ca. 2.07 Ga. These are ferrous, metaluminous calc-alkali I-type granitoids. The sodium specialization of the rocks and their low concentrations of Mg, Cr, Ni, and incompatible elements, with significant REE fractionation, the absence of Eu* anomalies, high Sr/Y ratio, remarkably high (Gd/Yb)n values (>10), and the radiogenic Nd isotopic composition indicate that the dacitic melts were derived from a juvenile mafic source. According to petrogenetic estimations, such conditions could be caused by the partial melting of depleted N-MORB basites in equilibrium with an eclogitic residue. The dacitic magmas were likely generated by the partial melting of mafic rocks at lower levels of the significantly thickened crust (>60 km) in relation to collision processes.

Young KREEP-like mare volcanism from Oceanus Procellarum

The Moon’s mare volcanism predominantly occurs within the Procellarum KREEP Terrane (PKT), which is widely thought to be associated with KREEP components within the lunar interior. The Chang’e-5 (CE-5) mission sampled a young (2 Ga) mare basalt Em4/P58 unit of northern Oceanus Procellarum. The geochemistry of the CE-5 mare basalt enables assessment of mantle source compositions which are essential to understand the thermo-chemical mechanism for prolonged volcanism during secular cooling of the Moon. Geochemical compositions of the CE-5 bulk soil, breccias, and basalt clasts from various depths within the drill core consistently display high concentrations of incompatible trace elements (ITE: ∼ 0.3 × high-K KREEP; ∼ 5 μg/g Th) with KREEP-like inter-element ratios, for example for La/Sm, Nb/Ta, and Zr/Y. Exotic impact ejecta, extensive magma differentiation (<70 % fractional crystallization) and significant assimilation of KREEP materials during magma transit and eruption cannot account for the ITE contents and ratios or radiogenic isotope compositions (e.g., εNdinitial of + 8 to + 9 and εHfinitial of + 40 to + 46) of the CE-5 basalts; instead, partial melting of their mantle source played a dominant role. The Chang’e-5 basalt is a chemically evolved low-Ti mare basalt (Mg# of ∼ 34) with enriched KREEP-like ITE compositions but high long-term time-integrated Sm/Nd and Lu/Hf ratios, which represent a hitherto unsampled type of mare basalt. It formed by melting of an augite-rich mantle source (late-stage magma ocean cumulates containing > 30–60 % augite, and little or no ilmenite), with a small amount of late-stage interstitial melt that resembles KREEP (∼1–1.5 modal %, equivalent to 0.2–0.3 μg/g Th in the mantle source). The voluminous mare basalts making up the Em4/P58 unit (>1500 km3) provide compelling evidence for large-scale, ITE enriched young mare magmatism within Oceanus Procellarum. In combination with remote sensing data and with the unique Th-rich Apollo 12 basalt fragment 12032,366–18 (impact ejecta likely from Oceanus Procellarum), this implies that significant portions of the FeO- and Th-rich mare regions of the western PKT may also have formed in a similar way.

A temporal control on the isotopic compositions of the Antarctic Peninsula arc

Radiogenic isotopic compositions of arc magmas are a key tool for studying active margin evolution. They have two isotopic end-members: melts formed mostly from juvenile asthenosphere and melts sourced from evolved continental crust/continental lithospheric mantle. Cordilleran-margins are typically more isotopically juvenile near the trench, and conversely, increasingly evolved landward. However, this model has not been tested on the ~1,500 km long Mesozoic-Cenozoic arc of the Antarctic Peninsula. Here we show that while geochemical compositions remain largely constant, radiogenic isotopes become increasingly juvenile with time. Unlike other continental arcs, there is no association between isotopic composition and spatial distribution. This is attributed to: (i) slow subduction of young oceanic lithosphere, resulting in narrowing of the arc and reduced capacity to incorporate continental crust into melts, and (ii) the Cenozoic decrease in convergence rate, which reduced the friction in the slab-overriding plate interface, allowing the arc melts to increasingly source from young juvenile asthenosphere.

Metallogeny and Genesis of Fault-Filling Barite-Sulfide Veins (Ougnat, Morocco): Petrography, Fluid Inclusion, and Sr-S Isotopic Constraints

The Ougnat Massif of the eastern Anti-Atlas (Morocco) hosts barite and sulfide vein-type deposits of vital economic importance. With over 150 mineralized structures reported in the Ougnat Massif, the ore-bearing ones are predominantly composed of barite, quartz, calcite, and minor portions of sulfides. The mineralized veins are driven by NW-SE and NE-SW to E-W oblique-slip opening faults that cross both the Precambrian basement and its Paleozoic cover. The mineralized structures occur as lenses and sigmoidal veins that follow stepped tension fracture sets oblique to the fault planes. These geometries and kinematic indicators of these structures point to a predominantly normal-sinistral opening in a brittle-ductile tectonic setting. The S isotopic compositions of barite from the Ougnat Massif (+10.8 to +19.5‰) fall mostly within the range of δ34S values of Late Triassic to Jurassic seawater, thus suggesting that some of the SO2− in barite comes from seawater sulfate. This range of δ34S values also corresponds approximately to the hydrothermal barite context. The 87Sr/86Sr ratios of barite, which range from 0.710772 to 0.710816, lie between the radiogenic strontium isotopic compositions of deposition by hydrothermal solutions, and also coincide with the non-radiogenic isotopic signature of Triassic to Jurassic seawater. Based on a fluid inclusions study, the ore-forming fluids were a mixture of two or more fluids. A deep hot fluid with an average temperature of 368 °C leached the granodiorites and volcanic-sedimentary complex of the Ouarzazate Group. This fluid provided the hydrothermal system with most of the Ba, radiogenic Sr, and some of the dissolved S. A second, shallow fluid with an average temperature of 242 °C was derived from Late Triassic to Jurassic seawater. The barite mineralization of the Ougnat Massif constitutes a typical example of vein-type mineralization that occurred along the northern margin of the West African Craton and regionally tied to the central Atlantic opening.

Calcium isotope variability among ocean islands reveals the physical and lithological controls on mantle partial melting

Major geodynamic processes, including mantle convection and plate tectonics, govern the exchange of mass between Earth's interior and its surface. These processes drive mass-transfer mechanisms such as partial melting and crustal recycling, which have generated a lithologically and compositionally heterogeneous mantle over time. Ocean island basalts (OIB) directly sample these heterogeneities, making them excellent tools to constrain the mantle’s dynamic geochemical evolution. In this study, we probe the connection between crustal recycling, mantle mineralogy, and partial melting using the stable Ca isotope compositions of OIB, ultimately contextualizing their Ca isotope variability using conventional mantle “endmember” components including DMM, HIMU, EM-1, and EM-2. Using the state-of-the-art Nu Sapphire collision cell MC-ICP-MS, we measured the Ca isotope compositions of 27 well-characterized OIB samples from 11 different ocean island groups. By supplementing our new data with additional high-precision Ca isotope measurements from the literature, we create an expanded dataset consisting of 15 ocean islands, facilitating a global-scale investigation of Ca isotope variability among hotspots. Our findings reveal that island-averaged Ca isotope compositions form two distinct groups on the basis of their radiogenic isotope compositions. The first group, consisting of OIB with un-enriched radiogenic isotope compositions (non-EM-type), exhibits robust negative correlations with both lithospheric thickness and primitive TiO2 contents, providing compelling evidence that Ca isotopes are sensitive to the degree of partial melting (F) as well as to the effects of crustal recycling. This joint signal appears to trace the process of decompression melting beneath oceanic lithosphere of variable thickness, as recycled lithologies rich in garnet and pyroxene (i.e., pyroxenites)—which generate large solid-melt Ca isotope fractionations—disproportionally control the Ca isotope composition of the final aggregate melt when F is limited by thick overlying lithosphere. In contrast, the island-averaged Ca isotope compositions of OIB with enriched radiogenic isotope compositions (EM-type) form a distinctly light negative correlation with primitive TiO2 contents without exhibiting a relationship with lithospheric thickness. In addition to lithology-dependent partial melting effects, the Ca isotope compositions of EM-type OIB appear to be sensitive to the compositional effects of crustal recycling, which may explain their decoupling from the physical controls of partial melting. Calcium isotopes show exceptional potential for elucidating the origin of both enriched and ultra-depleted mantle sources, emphasizing the need to prioritize localities with these signatures in future studies.

Evaluation of two methods allowing the full preparation in a single day of silicate rocks in view of radiogenic isotope (Nd, Sr, and Pb) analyses

In this paper, two approaches are described aiming to circumvent the bottle-neck associated with sample decomposition of silicate samples and subsequent isolation of three elements (Nd, Sr, and Pb) of interest for their radiogenic isotope compositions.

Buried Paleoproterozoic orogen of the East European Craton: Age and origin of the Vyatka terrane

We studied the composition, isotopic geochemistry and zircon U–Pb dating of Paleoproterozoic supracrustal rocks of the Uni Formation and granitoids of the Talitsky Complex of the Vyatka terrane located on the northeastern margin of the East European Craton. The obtained data were used to constrain the age, origin and tectonic setting of the Vyatka terrane, which is thought to be a part of the Paleoproterozoic Vyatka–Kama orogenic belt.Most of the gneisses and schists of the Uni Formation have greywacke protoliths, while metatuffites of andesite to dacite compositions are subordinate. All of these rocks have similar suprasubduction geochemical characteristics and highly radiogenic Nd isotopic compositions. The age of magmatic zircons from the felsic metatuffites is 2.08 Ga, and individual detrital grains from these rocks date back to ca. 2.12 Ga. Detrital zircons from the metasedimentary rocks have well-preserved crystals and multimodal U–Pb age distributions peaking at 2.08, 2.10, and 2.14 Ga. Some zircons from migmatized gneisses have 2.05-Ga-old rims, corresponding to the age of the Talitsky granitoids. The obtained data suggest that the Vyatka terrane could be considered an accretionary complex of the Vyatka–Kama orogen. The Uni Formation was formed by the accretion of sediments derived from an eroding Paleoproterozoic magmatic arcs (2.08–2.14 Ga). The completion of the accretionary tectonics was marked by the intrusion of the ca. 2.05-Ga-aged post-tectonic Talitsky granitoids.The Vyatka terrane accretionary complex contains chronologic counterparts of the West Sarmatian and Volga–Don orogens. This complements the model of a simultaneous amalgamation of Archean blocks of the Volgo–Sarmatia dating back to 2.2–2.0 Ga. Data from the Vyatka terrane confirm the model of the superposition of the East European and Siberian cratons in the Paleoproterozoic Columbia/Nuna supercontinent reconstruction. The Vyatka–Kama and Batomga belts may have been parts of a single extended Paleoproterozoic orogen that combined the Archean Volgo–Uralia and Aldan composite terranes.

Genesis of tungsten mineralization in the Yangla copper-polymetallic deposit, northwest Yunnan, China: Evidence from in situ U-Pb dating, trace elements, Sr isotopes, fluid inclusions, and H-O isotopes of scheelite

Scheelite is one of the main ore minerals in tungsten deposits, and its geochemical features can be used to examine and constrain tungsten mineralization. The Yangla deposit is one of the most important Cu-polymetallic skarn-porphyry deposits in the “Sanjiang” polymetallic metallogenic domain, Southwest China, hosting 150 Mt of Cu ores with economic concentrations of Pb, Zn, and Sb. The newly discovered tungsten mineralization in the Yangla deposit remains poorly understood. Hence, in order to ascertain the timing of tungsten mineralization, investigate its genetic relationship with magmatism and ore genesis, and elucidate the source, nature, and evolution of the ore-forming fluids, we performed a combined study of geochronology, in-situ trace elements, fluid inclusions, H-O isotopes, and in-situ Sr isotopic compositions of scheelite. In-situ U-Pb dating of scheelite showed that the timing of tungsten mineralization was 30 Ma. This mineralization age is significantly later than the published Cu mineralization age (∼230 Ma), skarn (∼231 Ma), and granitoid emplacement age (∼230 Ma) in the Yangla ore district. In-situ trace element analyses revealed that scheelite is depleted in Mo, Mn, and Ta and slight enriched in Sr, Y, REEs, and Au. It is also characterized by HREE-depleted chondrite-normalized patterns with strongly positive Eu anomalies. Our data reveal that 3Ca2+ = (2REE)3++□Ca (site vacancy) is the main substitution mechanism that controls REEs in scheelite. The low concentrations of Mo, negative Ce anomalies, and positive Eu anomalies of scheelite indicate a reduced condition. Preliminary REEs features indicate that the genesis of scheelite may be related to vein-type W (Au) deposits. The homogenization temperatures for fluid inclusions present a strongly decreasing trend (328 to 173 ℃), indicate that cooling is the predominant mechanism for the formation of scheelite. The δD and δ18Ofluid values from fluid inclusions of scheelite vary broadly from −161.70 ‰ to −133.30 ‰ and from 6.32 ‰ to 9.80 ‰, respectively, indicating a mixture of magmatic and meteoric water, with possible involvement of magma degassing and fluid–rock interaction. The initial 87Sr/86Sr ratios of scheelites (0.71931– 0.72117) are significantly higher than those of the granodiorite (0.71149 – 0.71990) and basalt (0.70562 – 0.70995) in the Yangla ore district, indicating that the Oligocene magmatism and Devonian marble with radiogenic Sr isotopic compositions contributed to tungsten mineralization. The fluid-rock interaction between the ore-forming fluids derived from the concealed granitoids and the Devonian wall rocks may play an important role in tungsten mineralization and provide significant Ca for scheelite precipitation. Thus, the tungsten mineralization should be associated with Oligocene magmatism, ruling out the genetic relationship with Late Triassic magmatism and being significantly different from the skarn-porphyry Cu-Pb-Zn mineralization at Yangla.

Tracing the provenance of mineral dust over the northern and southern Indian Oceans during the GEOTRACES-India (GI-01, GI-02) expeditions

Aeolian transport of mineral dust is an important carrier of nutrients and trace metals to the surface ocean and, hence, affects the biogeochemical cycles of C and N therein. Studies focusing on the provenances of dust across the latitudinal gradients over the open ocean waters are essential for better constraining the biogeochemical effects of dust deposition. Here, we analyzed the radiogenic isotope composition of Sr (87Sr/86Sr) and Nd (εNd(0)) in the silicate fraction of mineral aerosols collected over the pelagic northern and southern Indian ocean waters during the GEOTRACES-India research expeditions (GI-01: March-April 2014 and GI-02: April-May 2014). Both tracers, 87Sr/86Sr and εNd(0), exhibited marked spatial variability for dust deposition to different basins, including the Bay of Bengal (BoB), Arabian Sea (AS), equatorial Indian Ocean (EIO), and the southern sector of the Indian Ocean (SIO). The dust deposited in the AS showed a narrow spread in both 87Sr/86Sr (0.71520–0.71642) and εNd(0) (−10.9 to −10.1), along with the air mass back trajectories, indicating transport from the Arabian Peninsula. Likewise, the dust collected over the BoB was mainly sourced from the Thar Desert (87Sr/86Sr > 0.72, −16 < εNd(0) 〈−13), whereas the EIO received mineral dust from the Thar Desert and sand dunes in northern Australia (87Sr/86Sr: 0.72, εNd(0): −11) and Indonesian Archipelago. In contrast, the SIO appeared to receive dust deposition from three different source regions, including dust from Antarctica, western Australia (87Sr/86Sr: 0.712, −20 < εNd(0) <−16, and South Africa/South America (87Sr/86Sr: 0.708–0.710; −24.5 < εNd(0) >−8.4). These results have important implications for modelling the dust transport to pelagic Indian and southern Indian Ocean waters.

Late Cretaceous basalts in the eastern South China Block: The result of slab tearing during retreating subduction of the Paleo-Pacific plate

Cretaceous arc-like, ocean island basalt (OIB)-like, and transitional basalts erupted on the eastern South China Block (SCB), and their petrogenesis was possibly related to the subduction of the Paleo-Pacific plate. However, the detailed petrogenesis in terms of slab subduction, magma sources, and the spatial distribution of the different types in the coastal and inland regions is poorly understood. Here we conduct a comparative study on the Late Cretaceous basalts from both coastal and inland regions. Our compiled age data demonstrate that the basalts in the coastal regions erupted in the period of 104–81 Ma. They show low Nb/La (<0.8), Nb/Y (<0.7), and other typical arc-like signatures, e.g., negative Nb-Ta anomalies, variable enrichment of LILE and LREE, and more radiogenic Sr-Nd isotopic compositions. In contrast, the basalts in the inland regions erupted at 86–71 Ma. They are characterized by high Nb/La (>0.9), slightly negative to positive Nb-Ta anomalies; they mostly possess positive εNd(t) values and geochemical features transitional to OIB-like basalts. The systematic geochemical differences may imply the spatial heterogeneity in their magma sources. The three end-member mixing model suggests that the isotopic signatures of arc-like basalts largely resulted from partial melting of the mantle wedge metasomatized by the subducted sediments, while those of transitional basalts likely resulted from depleted mantle source metasomatized by melts from altered oceanic crust (AOC) with minor sediments. We suggest that the retreating of the Paleo-Pacific plate promoted the melting of sediment-hybridized mantle wedge, and then triggered the opening of a slab window and asthenosphere-slab interactions.