Pb Isotopic Compositions Research Articles

Lunar farside volcanism 2.8 billion years ago from Chang'e-6 basalts.

Unravelling the volcanic history of the enigmatic lunar farside is essential for understanding the hemispheric dichotomy of the Moon1-3. Cratering chronology established for the lunar nearside has been used to suggest long-lived volcanism on the farside of the Moon3,4, but without sample verification. Here we report two newly recognized episodes of basaltic volcanism with Pb-Pb dating for basalt fragments returned by the Chang'e-6 mission. One high-Al basalt fragment dated at 4,203 ± 4 million years ago (Ma) has a source 238U/204Pb ratio (µ value) of ~1,620, implying a KREEP-rich (K, rare earth elements, and P) source for this oldest-known occurrence of basaltic volcanism among returned samples. The main volcanic episode of the Chang'e-6 basalt documents a surprisingly young eruption age of 2,807 ± 3 Ma, which is not recognized from the nearside of the Moon. The initial Pb isotope compositions of these younger basalts indicate derivation from a source with a µ value of ~360, indicating a KREEP-poor mantle source. Mare volcanism on the lunar farside thus persisted for >1.4 billion years even with a shift to a source depleted in heat-producing elements. The consistency between the 2.8-billion-year basalt age and its crater-counting age indicates that the cratering chronology model established for the lunar nearside is also applicable to the farside of the Moon.

Anthropogenic Lead (Pb) deposition history of the western Indian Ocean from coral-based Pb/Ca ratio and Pb isotope records

Despite the rapid industrial growth and urban expansion along the coastline of the Western Indian Ocean, knowledge of both historical and current levels of anthropogenic lead (Pb) contamination, as well as its impact on the biosphere, remains limited compared to other industrialized regions. We present a twenty-four year long coralline record (1989–2013) of Pb/Ca ratio and Pb isotopes from the Lakshadweep coral reef in the Western Indian Ocean. This new record provides critical insight into source(s), possible transport pathways, and temporal trends in Pb deposition during the studied interval. The long-term trend in the surface seawater Pb concentration ([Pb]SW), reconstructed from the coralline Pb/Ca record, reveals almost doubling in [Pb]SW from ~50 pmol/kg in the year 1990 to ~107 pmol/kg in the year 2013. Bayesian mixing model calculations reveal that among the potential Pb polluting sources to this region, anthropogenic aerosol from the hinterland of the continents was the dominant contributor of Pb (23–89 %). A compilation of available Pb records from the Indian Ocean reveals that Pb isotope distribution patterns in the western and central equatorial Indian Oceans are distinctly different from those observed in the eastern Indian Ocean. The western Indian Ocean records exhibit lower Pb isotope ratios (206Pb/207Pb and 208Pb/207Pb) compared to the East Indian Ocean, suggesting a greater influence of anthropogenic Pb on seawater concentration. These findings highlight the spatio-temporally spread of anthropogenic Pb pollution and its potential impact on the biosphere in the Indian Ocean and therefore emphasize the urgent need for region-specific environmental management strategies. Plain language summaryThis study reconstructs the history of lead (Pb) pollution in the Western Indian Ocean. We analyzed a specimen of coral, collected from Lakshadweep, to create a 24-year-long (years 1989 to 2013) for Pb concentration and isotopic composition of seawater in the Western Indian Ocean. Using the coralline Pb/Ca ratio and Pb isotope data, we have reconstructed surface ocean Pb concentration ([PbSW]) and isotopic composition to understand the sources, transport pathways, and temporal depositional trends over the western Indian Ocean during the past two decades. This reconstruction of [PbSW] reveals a doubling from ~50 pmol/kg in the year 1990 to ~107 pmol/kg in the year 2013. Our investigations to fingerprint the Pb source(s) to our study area reveal that majority of the anthropogenic Pb has been contributed by aerosol deposition sourced from the hinterland of the surrounding continents. Our investigation also revealed that the western Indian Ocean is more contaminated by anthropogenic Pb compared to the eastern Indian Ocean. These findings highlight the need for region-specific monitoring efforts in the Indian Ocean as well as the formulation of environmental strategies to mitigate the impact of Pb pollution.

Geochemical signatures of forearc serpentinites from oceanic to continental subduction

In subduction systems, the forearc (FA) serpentinized mantle plays a key role as a reservoir for fluid-mobile elements, especially those prone to early mobilisation from the slab. FA serpentinites therefore provide an indication of the fluid characteristics that have been steamed out of the slab, and allow better quantification of elemental recycling and fluid flow. FA serpentinites outcropping along the Indus Suture Zone are formed by hydration of mantle peridotites by fluids derived from the Indian slab, providing a geochemical record of fluid-rock interactions during the India-Eurasia convergence. Systematic trace and multi-isotope (Sr and Pb) analyses of serpentinites from three major sites, namely Shergol-Tingdo, Kargil and Tso Morari, along the Indus Suture Zone in Ladakh (NW Himalaya) show that these rocks record a major change in the origin of metasomatic agents, from oceanic to continental subduction. The Shergol-Tingdo and Kargil FA serpentinites, formed under low temperature and pressure conditions, are characterised by high B enrichment and As and Sb depletion, high alkali/U ratios and relatively unradiogenic Sr and Pb isotope ratios. These geochemical characteristics are identical to those reported for modern oceanic subduction zones (e.g., Mariana forearc) and are consistent with input by fluids derived from subducted oceanic crust. In contrast, FA serpentinites from the Tso Morari ultra-high pressure unit show enrichment in fluid mobile elements such as As, Sb and U, low alkali/U ratios and radiogenic Sr and Pb isotopic compositions. These features suggest a change in the metasomatizing agent with a strong influence from subducted continental material. Consistent with this scenario, the Sr-Pb isotopic composition of the Kargil FA serpentinites can be reproduced by mixing between an Indian MORB-depleted mantle source and fluids derived from dewatering of blueschist facies oceanic metasediments, whereas in the case of Tso Morari a fluid end-member derived from various eclogitic continental metasediments is required. We propose that the observed geochemical signature of the Indus Suture Zone FA serpentinites can be used to reconstruct the geochemical exchanges between the slab and the overlying mantle from intra-oceanic to continental subduction. This successful systematic approach could be applied to other collisional contexts

Mesozoic lithospheric mantle evolution in South China: Evidence from Ar-Ar dating, geochemistry and Sr-Nd-Pb isotopes of lamprophyres

Understanding the Mesozoic lithospheric mantle beneath South China is challenging, primarily because mantle-derived magmatic rocks are rare, whereas granitic magmatism is far more abundant. In this study, we present new 40Ar/39Ar geochronology, major and trace elemental composition, and Sr-Nd-Pb isotopic data for lamprophyres from northern and central Guangxi Province, South China. Our results reveal two intrusion phases: an earlier phase at 207.7 ± 1.2 Ma, and a later phase between 95.6 ± 0.6 Ma and 102.3 ± 0.7 Ma. The earlier phase, represented by the Danyang lamprophyres, is marked by low Cr (330–355 ppm) and Ni (241–258 ppm) contents and strongly fractionated REE patterns with (La/Yb)n ratios of 57.5 to 62.8. The later phase, represented by the Leidong, Nongchang, and Longchang lamprophyres, shows higher Cr (490–781 ppm) and Ni (237–377 ppm) contents, with less fractionated REE patterns and (La/Yb)n ratios of 20.2 to 27.8. Despite their temporal differences, the lamprophyres share similar isotopic characteristics, including high initial 87Sr/86Sr (0.707940–0.721310), negative εNd(t) (−9.19 to −5.31), and radiogenic Pb isotopic compositions (206Pb/204Pb: 18.200–18.331, 207Pb/204Pb: 15.62–15.72, 208Pb/204Pb: 38.53–38.84). These features suggest derivation from low-degree partial melting of a phlogopite-bearing, refractory peridotite lithospheric mantle, modified by olivine and clinopyroxene fractionation. This refractory mantle was metasomatically enriched by Proterozoic subduction processes beneath the Yangtze Craton. By synthesizing published Sr-Nd-Pb isotopic data, we propose that the lithospheric mantle beneath South China shifted from enriched to depleted mantle between ~220 Ma and ~ 100 Ma, possibly due to extension associated with the subduction and rollback of the Paleo-Pacific Plate.

ISOTOPE COMPOSITION OF Pb AND Nd OF THE LATE NEOPROTEROZOIC–EARLY PALEOZOIC GRANITOIDS IN EASTERN TUVA (Case study of the Kaa-Khem batholith)

We present the results of Nd and Pb isotope studies of late Neoproterozoic–early Paleozoic granitoids of the Kaa-Khem batholith, which formed in different geodynamic settings within the early Caledonian structures of Eastern Tuva (Altai–Sayan sialic area). Based on the obtained isotope data, we assess the role of different source materials in the formation of melts for these granitoids and compare these rocks with coeval granitoids from the Lake Zone of Mongolia. The plagiogranitoid associations of the island arc stage of the Kaa-Khem batholith formation (572–562 Ma) are characterized by mantle Nd isotope values. In contrast, the Pb isotope characteristics of these intrusive associations correspond to an upper-crust source formed predominantly by terrigenous sediments. The granitoid associations of the accretion–collision stage of the Kaa-Khem batholith formation (512–450 Ma) have a less radiogenic primary Pb isotope composition as compared with the island arc granitoids. Evidently, during the magma formation at this stage, the relative role of terrigenous material decreased, while the role of depleted mantle material increased. At the same time, the decrease in ɛNd values in the granitoids as compared with the depleted mantle indicates that, along with the upper-mantle component, the role of the lower-crust component increased. These Nd and Pb isotope characteristics of the accretionary granitoids of the Kaa-Khem batholith indicate a significant contribution of the lower-crust source to their formation. This source is the crystalline basement of the Tuva–Mongolian microcontinent. The Pb isotope characteristics of the accretion–collision granitoids of the Kaa-Khem batholith are shifted to the upper-crust compositions relative to those of the Lake Zone granitoids. The reason is that the latter formed predominantly through the transformation of early Caledonian island arc complexes, which were generated with a higher portion of mantle component.

Geochronology and Origin of Quaternary Dacites from the Daliuchong Volcano in the Tengchong Volcanic Field (TVF), SE Tibetan Plateau

Quaternary volcanoes from the southeastern Tibetan Plateau occur at the Tengchong volcanic field (TVF). The Daliuchong volcano is the largest volcano in the TVF, which has the most felsic compositions with explosive eruptions. The eruption history and origin of the Daliuchong volcano are a matter of debate. In the present paper, we report the groundmass K-Ar ages, whole-rock Sr-Nd-Pb-Hf isotopes, zircon U-Pb ages, and Hf-O isotopic compositions for the Daliuchong volcano to constrain its eruption history and petrogenesis. The groundmass K-Ar ages and zircon U-Pb ages indicate mid-Pleistocene (0.6 Ma to 0.3 Ma) eruptions. The presence of zircon phenocrysts with enriched mantle-like O-Hf isotopes (δ18O < 6‰, and εHf about −2) suggests the involvement of mantle-derived basaltic magmas. The whole-rock Pb isotope compositions and Sr-Nd isotope modeling reveal the involvement of magma from the lower crust. The zircon xenocrysts reveal previously unrecognized 20-Ma magmatic activity at the TVF and contamination of late Cretaceous (66–80 Ma) S-type granites during the formation of the Daliuchong dacites. The dacite magma at Daliuchong was formed by mixing of the mantle-derived magma and lower-crust-derived magma and subsequently contaminated by upper crustal materials, including late Cretaceous S-type granitic rocks.

Vertical transport of anthropogenic lead by reversible scavenging in the South Atlantic Ocean

Stable lead (Pb) isotopes have been regarded as tracers of ocean circulation, both in the present time and geological past. Here we present a new dataset of seawater Pb concentrations and isotope compositions for ten depth profiles from the South Atlantic Ocean (GEOTRACES cruises GA02 and GA10). By comparing Pb isotope data collected on the two cruises, and by modelling the distribution of Pb with an extended optimum multiparameter analysis, we find evidence of vertical transport of anthropogenic Pb pollution due to reversible scavenging. Surface to depth transfer of polluted Pb is aided by high suspended particulate matter loads at the Brazil – Malvinas Confluence and along ∼40°S in the South Atlantic. Overall, our findings caution the use of Pb isotope ratios as ventilation tracers in the South Atlantic and emphasize the importance of particle-seawater interaction for biogeochemical cycles.

Source apportionment and emission projections of heavy metals from traffic sources in India: Insights from elemental and Pb isotopic compositions

The study investigates the sources of metals in urban road dusts using elemental concentration and Pb isotopic ratios. The elemental concentrations are also utilized to determine the present heavy metal emissions as well as projected emissions till 2045. Bayesian mixing model for source apportionment highlights the significant contributions of both exhaust and non-exhaust sources to the metal-enriched urban road dusts, with each contributing approximately 40 %. Emission analysis reveals that India’s projected electric vehicle (EV) penetration may not be sufficient to suppress the metal emissions from vehicular exhausts. Further challenge is posed by high metal concentrations in the non-exhaust sources, that dominates the emission of some metals compared to exhaust sources. If the metal concentrations remain unchanged, the emission analysis predicts alarming increases in total emissions from all the exhaust and non-exhaust sources by 174 %, 176 %, 163 % and 184 % for Ni, Cu, Zn and Pb, respectively, from 2022 to 2045. Thus, it is crucial to reduce the metal concentrations in traffic emission sources and also impose better regulatory measures to improve the urban metal pollution scenario.

Orogenic Gold Deposits of Northern Transbaikalia, Russia: Geology, Age, Sources, and Genesis

The paper presents newly obtained and summarizes preexisting data of long-term geochronological and isotope studies of orogenic mesothermal gold deposits in the world’s largest Lena metallogenic province and reviews interpretations of their genesis. Geochronologic data indicate that the gold mineralization was formed during three ore-forming Paleozoic events. The early Late Ordovician–Early Silurian (450–430 Ma) event produced the abundant veinlet—disseminated gold–sulfide mineralization in Neoproterozoic carbonaceous terrigenous–carbonate rocks of the Baikal–Patom foldbelt (BPB). The mineralization was formed simultaneously with regional metamorphic processes. The rejuvenation of hydrothermal activity in the BPB resulted in gold-bearing quartz veins, which was produced in the Middle Carboniferous (340–330 Ma) in relation to postcollisional granitoid magmatism. The latest ore-forming event occurred in the Early Permian (290–280 Ma) and affected exclusively in Precambrian structures of the Baikal-Muya foldbelt (BMB). It was synchronous with the development of intraplate alkaline and subalkaline magmatism in the region. Newly obtained and preexisting isotopic-geochemical (87Sr/86Sr, 143Nd/144Nd, Pb–Pb, and δ34S) data indicate that the mesothermal ore-forming systems of northern Transbaikalia were heterogeneous in their isotopic characteristics, which distinguishes them from the ore–magmatic (intrusion-related type) systems. Comparison of the Sr, Nd, and Pb isotopic composition of the ores and rocks, including magmatic rocks coeval with the gold mineralization, indicates that the Precambrian continental crust was the dominant source of mineral-forming components for the hydrothermal systems of the gold deposits. The contribution of the magmatic source to the genesis of the orogenic gold mineralization was limited and is identifiable only for the Early Permian ore-forming systems of the BMB, for which the input of metals from alkaline mafic melts was suggested.

Clustering and interfacial segregation of radiogenic Pb in a mineral host-inclusion system: Tracing two-stage Pb and trace element mobility in monazite inclusions in rutile

Abstract Accessory minerals like zircon, rutile and monazite are routinely studied to inform about the timing and nature of geological processes. These studies are underpinned by our understanding of the transfer processes of trace elements and the assumption that the isotopic systems remain undisturbed. However, the presence of microstructures or Pb-bearing phases in minerals can lead to the alteration of the Pb isotopic composition. To gain insight into the relationship between Pb isotopic alterations from inclusions and microstructures, this study focused on inclusions from an ultra-high-temperature metamorphic rutile. The studied inclusions are submicrometer monazites, a common mineral rich in Pb but normally not present in rutile. The sample is sourced from Mt. Hardy, Napier Complex, East Antarctica, an ultra-high-temperature (UHT) metamorphic terrane. By applying correlative analytical techniques, including electron backscatter diffraction mapping, transmission electron microscopy (TEM), and atom probe tomography, it is shown that monazite inclusions are often in contact with low-angle boundaries and yield no preferred orientation. TEM analysis shows the monazite core has a mottled texture due to the presence of radiation damage and nanoclusters associated with the radiation damage defects that are rich in U, Pb, and Ca. Some monazites exhibit a core-rim structure. The rim yields clusters composed of Ca- and Li-phosphate that enclose Pb nanoclusters that are only present in small amounts compared to the core, with Pb likely diffused into the rutile-monazite interface. These textures are the result of two stages of Pb mobility. Initial Pb segregation was driven by volume diffusion during UHT metamorphism (2500 Ma). The second stage is a stress-induced recrystallization during exhumation, leading to recrystallization of the monazite rim and trace element transport. The isotopic signature of Pb trapped within the rutile-monazite interface constrains the timing of Pb mobility to ca. 550 Ma.

Traceability and dispersion of highly toxic soluble phases from historical mine tailings: insights from Pb isotope systematics

Dispersion of potentially toxic elements associated with efflorescent crusts and mine tailings materials from historical mine sites threaten the environment and human health. Limited research has been done on traceability from historical mining sites in arid and semi-arid regions. Pb isotope systematics was applied to decipher the importance of identifying the mixing of lead sources involved in forming efflorescent salts and the repercussions on traceability. This research assessed mine waste (sulfide-rich and oxide-rich tailings material and efflorescent salts) and street dust from surrounding settlements at a historical mining site in northwestern Mexico, focusing on Pb isotope composition. The isotope data of tailings materials defined a trending line (R2 = 0.9); the sulfide-rich tailings materials and respective efflorescent salts yielded less radiogenic Pb composition, whereas the oxide-rich tailings and respective efflorescent salts yielded relatively more radiogenic compositions, similar to the geogenic component. The isotope composition of street dust suggests the dispersion of tailings materials into the surroundings. This investigation found that the variability of Pb isotope composition in tailings materials because of the geochemical heterogeneity, ranging from less radiogenic to more radiogenic, can add complexity during environmental assessments because the composition of oxidized materials and efflorescent salts can mask the geogenic component, potentially underestimating the influence on the environmental media.

Elemental Geochemistry and Pb Isotopic Compositions of the Thick No. 7 Coal Seam in the Datun Mining Area, China

Thick coal seams recorded abundant petrological, geochemical, and mineralogical information regarding their formation, which in turn can reflect the characteristics of the coal-forming environments, provenance attributes, paleoclimate, and so on. In order to explore the geochemical and lead isotope characteristics of thick coal seams, the No. 7 coal seam in the Datun mining area, Jiangsu Province of China, was selected as the research object. In this work, 29 samples (including coal, roof, and floor rock samples) were collected from three coal mines in the Datun mining area. Through an analysis of the mineral composition and element geochemical characteristics in the coal samples, the enrichment degree of trace elements and modes of rare earth elements were determined. The genetic mechanism of abnormal enrichment of enriched elements is discussed, especially the modes of occurrence and isotope characteristics of Pb. The results showed the following: (1) The main minerals in the coal samples include quartz, potassium feldspar, plagioclase, calcite, dolomite, pyrite, gypsum, and clay minerals, with clay minerals, calcite, quartz, and dolomite being the most common. (2) The major element oxides in coal mainly include SiO2, Al2O3, Fe2O3, MgO, CaO, Na2O, K2O, TiO2, P2O5, and FeO. In the vertical direction, the variation of SiO2, Al2O3, Fe2O3, MgO, K2O, and FeO in coal samples from the three coal mines is consistent. The average value of Al2O3/TiO2 in the samples of Kongzhuang, Longdong, and Yaoqiao coal mines is 28.09–50.52, which basically locates the samples in the felsic source area, such that the sediment source is considered to be felsic source rock. (3) Elements U, La, Pb, and other elements are more enriched in Kongzhuang coal mine samples; elements Th, U, La, Pb, and other elements are more enriched in the Longdong coal mine samples; and elements Th, U, La, Pb, and other elements are more enriched in the Yaoqiao coal mine samples. Furthermore, W is enriched in Yaoqiao mine samples and is highly enriched in Longdong mine samples. The mining area is generally rich in the elements U, La, and Pb. The distribution curves of rare earth elements in the three mines are inclined to the right, with negative Eu anomalies. The enrichment is of the light rare earth enrichment type. (4) Pb isotope data show that the samples from the three mines are mainly distributed in the orogenic belt and the subduction zone lead source areas, where the upper crust and the mantle are mixed, with individual sample points distributed in the mantle and upper crust lead source areas.

Trace Element Composition of Discordant Zircon as a Reflection of the Fluid Regime of Paleoproterozoic Granulite Metamorphism (Khapchan Terrane, Anabar Shield)

New data on the U–Pb age (SHRIMP-II) and trace element composition (SIMS) of zircon from gneisses of the Khapchan Group of the Khapchan terrane of the Anabar Shield are presented. Zircon grains contain relicts of magmatic zircon, the protolith and source areas of which are difficult to specify. The only zircon in this group with the least altered core preserved the 207Pb/206Pb protolith age of 1971 ± 19 Ma. During granulite metamorphism, the zircon was subjected to the impact of fluid enriched in incompatible elements. The disurbances affected both the U–Pb isotope system (zircon age was “reset” at metamorphic age of 1920–1930 Ma) and composition of zircon, magmatic cores of which were significantly recrystallized in a solid state or dissolved by fluid up to the practically complete disappearance of primary zircon. In both the cases, zircon was sharply enriched in incompatible elements (Ca, Ti, Pb, Sr, Ba, and some others) owing to fluid effect, while preserved cores seemed to be rimmed by a new zircon population (CL-black). The REE distribution patterns in the recrystallized cores acquired a “bird’s wing” profile atypical for zircon. When the fluid lost its reactivity, the main part of the zircon grain crystallized, typical of granulite zircon. This zircon population is less enriched in incompatible element than the cores are. The horizontal pattern of HREE distribution is consistently repeated, which indicates the co-crystallization of zircon and garnet. The estimated crystallization temperature of the main part of zircon varies in a narrow range of 800–830°C. All zircon domains in the Wetherill concordia diagram form a single trend with a zero lower intercept and an upper intercept confirmed by concordant cluster with an age around 1920–1930 Ma. This value corresponds to the age of regional Paleoproterozoic granulite metamorphism. A unique feature of zircon from the Khapchan gneisses is that its cores did not retain the age marks of the protolith, but were completely reset during metamorphism both in terms of the U-Pb system and the trace element composition, which can be explained by the extremely high intensity of fluid impact during the granulite facies metamorphism superimposed on the rocks of the Khapchan terrane of the Anabar Shield.

A Two‐Stage Geodynamic Model for Post‐Collisional Potassic‐Ultrapotassic Magmatism in Southeast Tibet

AbstractPost‐collisional potassic‐ultrapotassic rocks can provide key clues to the change of the recycled material type and/or tectonic transition in subduction‐related zones. Despite continental materials widely recognized in their sources, it remains unclear whether such continental materials were contributed by former oceanic subduction or recent continental subduction. Here we address this issue by systematically investigating previously reported and our new chemical and Sr–Nd–Pb isotopic compositions of the post‐collisional K‐rich rocks in Southeast Tibet. Kink‐like compositional variations provide solid evidence for a primary control of fractional crystallization on the evolution of these K‐rich magmas. Their primary melts are demonstrated to have been produced by partial melting of phlogopite‐bearing peridotites in subcontinental lithospheric mantle (SCLM). The trace element and Sr–Nd–Pb isotopic signatures argue against involvement of the deeply subducted Indian continent but suggest a great contribution from sediments in oceanic slabs. A thinned (∼70–100 km) and hot (∼55–70 mW/m2) lithosphere is also unraveled beneath Southeast Tibet during the potassic‐ultrapotassic magmatism. Together with geophysical data, here we suggest a two‐stage geodynamic model for post‐collisional potassic‐ultrapotassic magmatism in Southeast Tibet: (a) Before the Indian‐Asia continental collision, phlogopite/K‐richterite‐bearing SCLM sources were formed through oceanic subduction‐related metasomatism; (b) After the Indian‐Asia continental collision, asthenosphere upwelling induced by post‐collisional tectonic extension or deep subduction of the Indian continental slab caused lithospheric thinning, partial melting of pre‐existing phlogopite/K‐richterite‐rich SCLM and thus K‐rich magmatism. This study provides new insights into the role of oceanic subduction and continental collision in post‐collisional potassic‐ultrapotassic magmatism.

Magmatic evolution and sources of metals and fluids in the large granite-related Xiasai vein-type Ag–Pb–Zn(–Sn) deposit, northern Yidun terrane, SW China

The Xiasai Ag–Pb–Zn–(Sn) deposit in the northern Yidun terrane (eastern Tethys), hosted in Late Triassic metasandstones, is genetically associated with Late Cretaceous granites. We present chemical and Sr–Nd–Pb–Li–B isotope data of Late Cretaceous granites and their mafic microgranular enclaves (MME), late aplites, and Late Triassic metasedimentary rocks. The narrow range of 87Sr/86Sr99 (0.70627–0.70953) and εNd99 (–6.1 to –5.2) values of most Late Cretaceous granites reflects a mixed source with contributions from the mantle and Paleoproterozoic felsic crust. These granites have high SiO2 contents (>69 wt%) and strong depletions in Sr, Ba, and Eu. Major and trace element contents correlate with the fractionation index 1/TiO2, indicating that they experienced extensive fractionation. Local late aplite dikes were derived from similar source rocks, but seem to be younger.The Pb isotopic compositions reflect that the metal Pb was derived from the granite with variable contributions from the Late Triassic sedimentary rocks. Weakly and moderately altered metasandstones have similar δ7Li and δ11B values as most granites (δ7Li: 0.00 to 2.68 ‰; δ11B values: –14.93 to –12.36 ‰), indicating magmatic fluid sources. The magmatic fluids resulted in the initial enrichment of Sn, Cu, Pb, and Zn. Strong alteration of feldspar and biotite resulted in the metasandstones in negligible change of Li concentrations, significant loss of Sr, Na2O, CaO, MgO, and Fe2O3, but significant addition of B. This alteration lowered δ7Li from 1.01 to –3.65 ‰ and lowered δ11B from –9.43 to –19.18 ‰. The low δ7Li values and extremely low δ11B values reflect external fluids from the Late Triassic sedimentary wall rocks, which interacted with granites and mobilized metals (e.g., Sn, Ag, Pb, and Zn) from the granite and/or the sedimentary wall rocks. The location of the Xiasai deposit is controlled by regional NNW-trending faults induced by Late Cretaceous extension.

Origin of the Dongga Au deposit in the giant Xiongcun porphyry Cu–Au district, Tibet, China: Constraints from multiple isotopes (Re, Os, He, Ar, H, O, S, Pb) and fluid inclusions

The Dongga Au deposit (9.55 t Au; average grade = 6.9 g/t [up to 61.6 g/t]) is located in the Xiongcun giant porphyry Cu–Au district of the Gangdese porphyry Cu belt, Tibet. Its mineralization age, ore-forming processes, and relationship to adjacent porphyry mineralization remain unclear. To address this, we conducted a geological, pyrite Re–Os dating, He–Ar–H–O–S–Pb isotopic, and fluid inclusion study of the Dongga deposit. Pyrite samples from ore-bearing chlorite–sulfide veins yielded a weighted-mean Re–Os age of 178.4 ± 2.6 Ma (MSWD=0.01), implying the deposit formed during the Early Jurassic. Fluid inclusion analyses yielded homogenization temperatures of 237–360 °C for quartz–sulfide veins and 125–201 °C for late quartz veins, with corresponding salinities of 2.7–43.8 and 0.9–9.9 wt% NaCleq, respectively. Fluid inclusion analyses of pyrite samples yielded 3He/4He and 40Ar/36Ar values of 0.50–1.08 Ra and 325.1–559.4, respectively, and δD and δ18Ofluid values of –86.2 ‰ to –72.1 ‰ and –5.6 ‰ to 3.9 ‰, respectively. The He–Ar–H–O isotopic data suggest the mineralizing fluids were derived mainly from a crustal source with a small mantle contribution, and also contained a mixture of magmatic and meteoric waters. The S (δ34S = –1.57 ‰ to –0.55 ‰) and Pb (206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb are 18.17–18.41 ‰, 15.55–15.63 ‰ and 38.15–38.37 ‰, respectively) isotopic compositions of pyrite suggest the ore-forming metals were derived from a magma source in the mantle containing minor amounts of subducted sediment. Based on the geology, and isotopic and fluid inclusion data, we infer that mixing between magmatic and meteoric waters was the main trigger of Au precipitation. In addition, based on the geochronological, spatial, and genetic relationships with the adjacent No.2 deposit in the Xiongcun ore district, we propose that the Dongga Au deposit is a sub-epithermal deposit, which represents the transition between porphyry and epithermal deposits. The two deposits constitute a porphyry system and record the continuous evolution of hydrothermal fluids transported outwards in a porphyry system.

Provenance changes of the holocene deposits of Oga and Tsivolki bays (Novaya Zemlya archipelago) according to SR, ND, PB isotope data

The paper is devoted to the Sr-, Nd-, Pb-isotope data obtained for two cores of bottom sediments taken in the Oga and Tsivolki bays of the Severny Island of the Novaya Zemlya archipelago. The studied sequence of sediments from Oga Bay has been accumulated over the last thousand years. The 87Sr/86Sr ratio decreases from top to bottom down the section from 0.72225 to 0.71995, the value of εNd varies from –6.1 to –5.5. The Pb isotopic composition varies within narrow limits: the 206Pb/204Pb ratio from 19.107 to 19.139, the 207Pb/204Pb ratio from 15.632 to 15.635, and the 208Pb/204Pb ratio from 38.568 to 38.635. A rapid decrease in the 87Sr/86Sr ratio at a relatively stable neodymium and lead isotope composition indicates a change in the source of the clastogenic material. This can be explained by the fact that the material of the destruction of Permian clay shales, and then the Devonian-Silurian sedimentary carbonates, first entered the area of glacier abrasion and further, respectively, into the sedimentation zone. The sediment column from the Tsivolki Bay was formed over a little more than 10 thousand years. Based on the Sr, Nd, and Pb isotope ratios, these bottom sediments are divided into lower and upper parts: before and after 150 cm (or ~3500 years). In the lower part of the column, the 87Sr/86Sr ratio increases from 0.72055 to 0.72580, the value of εNd remains approximately the same and varies around –8.2. In the upper part, the 87Sr/86Sr ratio drops to 0.72049 in the near-surface layer; at the same time, the value of εNd increases to –6.4. At the boundary of these two units, the 206Pb/204Pb ratio abruptly changes from about 18.0 in the lower part to 19.3 in the upper part and 208Pb/204Pb from about 36.5 in the lower part to 38.7 in the upper part of the section. The change in the Sr, Nd, and Pb isotope characteristics is likely a reflection of changes in the composition of the rocks in the area where the basin was removed, which is now being eroded by the glacier. Comparison with modern sources supplying clastic material to the Kara Sea showed that the material inputs the Oga and Tsivolki bays only from Novaya Zemlya.

Magnesium isotope fractionation during magmatic differentiation in the lower continental crust

The Mg isotopic compositions display a significant disparity between the lower continental crust and the mantle. However, due to the limited availability of representative samples from the lower crust, our understanding remains incomplete regarding whether Mg isotope fractionation occurred during its formation and its potential influence on the observed isotopic differences. This study addresses this gap by presenting Mg isotopic data for juvenile lower crustal mafic xenoliths from Daoxian, Southeast China, providing new insights into this poorly understood process. The xenoliths, classified as websterites and two-pyroxene granulites, equilibrated at 818–912 °C and 10.6–15.4 kbar. They exhibit high Mg# (77–85), Cr (154–1451 ppm), and Ni (143–312 ppm), as well as trace element patterns resembling those of island arcs and evolved Sr–Nd–Pb isotopic compositions (87Sr/86Sri = 0.7046–0.7071; εNd(t) = −2.44 to +4.62; 206Pb/204Pbi = 18.26–19.03), indicating their derivation primarily from an enriched mantle source. Despite this, these xenoliths display a diverse range of Mg isotopic compositions. Websterite xenoliths exhibit mantle-like δ26Mg values (−0.27‰ to −0.29‰), indicating no influence from hybridized mantle source or pyroxene fractional crystallization/accumulation on their Mg isotopes. In contrast, granulite xenoliths show lower and variable δ26Mg values (−0.26‰ to −0.49‰), which positively correlate with Mg#, Cr, and V, but negatively correlate with Na2O + K2O, indicating that magmatic processes likely influenced Mg isotope fractionation in these samples. Petrological features, coupled with correlations between MgO and CaO/Al2O3, Cr, Co, and Ni, indicate that fractional crystallization and/or accumulation of olivine, spinel, clinopyroxene, orthopyroxene, and plagioclase played crucial roles in the formation of the granulite protoliths. Furthermore, the segregation of spinel during magmatic differentiation could have enriched residual melts in light Mg isotopes inherited by the cumulate Cpx + Opx + Pl (i.e., the granulite protoliths). Overall, the weighted average δ26Mg value of the lower crust in South China is estimated to be −0.34 ± 0.04‰. This finding, together with previous studies on granulite xenoliths in North China and Australia, strongly indicates heterogeneous Mg isotopic compositions in the lower continental crust relative to the mantle.

Middle Triassic Cu–Pb–Zn Skarn mineralization in the Wulonggou gold ore field, Eastern Kunlun Orogen, NW China: Insights from phlogopite Ar–Ar and zircon U–Pb dating and Sr–Nd–Pb–Hf isotopes

The skarn is one of the most significant Cu–Pb–Zn mineralization worldwide. The timing of the magmatic-hydrothermal activity is of importance for understanding the Cu–Pb–Zn mineralization process. The Cu–Pb–Zn skarn mineralization has been recognized in the northwestern section of the Wulonggou gold ore field, Eastern Kunlun Orogen (EKLO), NW China. However, no precise geochronological data has been measured for the Cu–Pb–Zn mineralization, and the source of ore-forming materials is also not well studied. In this study, we present new LA-ICPMS U–Pb ages and Sr–Nd–Hf isotopic compositions for granodiorite and diorite in the northern section of the Wulonggou gold ore field, as well as an Ar–Ar age for phlogopite and Pb isotopic compositions for sulfides and intrusions, further to constrain the ages of mineralization and magmatism, and the source of ore-forming materials. Two granodiorite samples yield mean ages of 245 ± 2 to 243 ± 2 Ma and a diorite has a mean age of 245 ± 2 Ma. A phlogopite sample, which is associated with Cu–Pb–Zn mineralization, has been chosen to precisely date the formation of Cu–Pb–Zn ores by Ar–Ar method and yields a well–defined plateau age of 240.4 ± 1.2 Ma. These results indicate that the Cu–Pb–Zn skarn mineralization formed in the Middle Triassic and is closely related to the coeval magmatism. The εHf(t) values of the ore-related granodioritic and dioritic rocks range from −2.8 to +2.5, whereas an un-mineralized granodiorite sample (WHFG–1) shows lower εHf(t) values of –9.3 to –3.4, indicating that these magmatic rocks are derived from the mixing of magma with different proportions of mantle- and crust-components. A mixing model with ∼40–70 % mantle contribution involved in the formation of ore-related granodiorite and diorite was revealed by Sr–Nd–Hf isotopic compositions. Combined with Pb isotopic compositions for sulfides, feldspar, and bulk rocks, we consider a common source for the ore-forming materials and magmatism. In addition, higher εHf(t) and εNd(t) values for ore-related magmatism implicate that mantle-derived melts show a vital role in the Cu–Pb–Zn skarn mineralization process.

Early Cretaceous fayalite-, ferrosilite-, and biotite-bearing rhyolitic porphyries in the Baishuizhai area, South China: Formation by fractional crystallization in the shallow crust

Fayalite- and ferrosilite-bearing felsic igneous rocks are a distinctive category of A-type granitic magmas, and the origins of these rocks have proved controversial. In addition, the shallow-crust high-silica rhyolitic magmas responsible for such rocks are considered to have higher viscosities than their plutonic equivalents, which casts doubt on whether fractional crystallization could have occurred during the evolution of these high-silica melts. Here, we report a study of earliest Early Cretaceous (145–142 Ma) fayalite- and ferrosilite-bearing rhyolitic porphyries (FFBRPs) and closely associated biotite-bearing rhyolitic porphyries (BBRPs) from the Baishuizhai area, Guangzhou, Guangdong Province, South China. The Baishuizhai suite provides an excellent opportunity to establish the magmatic origins and evolutionary processes responsible for the FFBRPs. Rocks from the Baishuizhai suite have SiO2 contents of 73.2–77.7 wt%, Na2O + K2O contents of 7.83–9.42 wt%, and Fe2O3T contents of 1.52–2.97 wt%. They have A-type granite features including mineral assemblages (euhedral fayalite and ferrosilite in addition to anhedral amphibole and biotite) and geochemical characteristics [e.g., high 10,000 × Ga/Al values (3.7–4.7), high FeOT/MgO ratios (13.0–34.8), and high Zr (245–514 ppm) and Nb (56.4–96.5 ppm) contents]. The rocks have slightly enriched in situ plagioclase Sr [(87Sr/86Sr)i = 0.7070–0.7076] and whole-rock Nd [εNd(t) = −2.3 to −2.0] compositions and depleted zircon Hf isotopic compositions [εHf(t) = −0.9 to +7.0] relative to chondrite, as well as higher zircon δ18O values (6.3‰–7.8‰) and intermediate in situ plagioclase Pb isotopic compositions [(206Pb/204Pb)i = 18.368–18.727, (207Pb/204Pb)i = 15.488–15.673, and (208Pb/204Pb)i = 38.049–38.801] compared with depleted mantle. We infer that the primary magmas of this rock suite were formed by mixing of mantle-derived magmas with subordinate (20%–50%) metasedimentary-rock-derived magmas. Fayalite and ferrosilite in the FFBRPs were most likely crystallized from dry, hot, and reduced magma in a lower (<16.5 km depth) chamber. The lower magma gradually evolved during ascent to a water-enriched magma in an intermediate chamber at a shallow depth (3–4 km), forming the FFBRPs (elevation: 129–168 m). Finally, the residual melts migrated upward and resided in an upper (<3 km depth) chamber, forming the BBRPs (elevation: 196–824 m). According to the established temporal–spatial distribution of Early Cretaceous A-type granites and adakitic rocks in South China, we suggest that the Paleo-Pacific plate underwent diachronous rollback, starting during the earliest Early Cretaceous. This rollback enhanced the degree of crust–mantle interaction in the Guangzhou area.