Radiogenic Isotope Signatures Research Articles

Investigating the Influence of Crustal Contamination on the Stillwater Complex, Montana Using Sr, Nd, and Pb Isotopes

Abstract The presence of pegmatoid bodies in the Stillwater Complex is poorly understood, but they have been suggested to have resulted from the presence of fluids in the complex. To better understand the origin of the pegmatoids and to trace the possible influence of country-rock-derived fluid in the Stillwater Complex, bulk rock Rb-Sr, Sm-Nd, and Pb-Pb isotopes for samples from the Archean Stillwater Complex and its metamorphic aureole are reported. Pegmatoid bodies are compared to spatially associated host rock and the underlying hornfels facies country rocks. Evidence of resetting of radiogenic isotopes during regional metamorphism at 1700 Ma is not observed, and the initial radiogenic isotopic ratios in Stillwater Complex rocks overlap those of the underlying hornfels. Despite the isotopic similarity of the country rock to the Stillwater Complex, the intrusion is modestly isotopically heterogeneous. In Stillwater samples, the average εNd,2710Ma = −1.1 ± 6.9, 206Pb/204Pb2710 Ma = 15.24 ± 2.26, and 87Sr/86Sr2710Ma = 0.703043 ± 0.002747 (1σ). The similarity between country rock and intrusive rock isotopic compositions at Stillwater contrasts with the data reported for the Bushveld Complex, South Africa, where the country rock is isotopically distinct from the intrusion. The variability in radiogenic isotope signatures in Stillwater rocks show a noisy but decreasing influence of country rock up through the Lower Banded series interpreted to reflect variable crustal contamination, in part from <1.0 wt % country rock fluids released during intrusion of the Stillwater Complex. The influence of crustal fluid contamination as compared to more traditional crustal assimilation models or simple magmatic heterogeneity suggests that hydrothermal fluids modified the isotopic compositions of more fluid-mobile elements and can explain aspects of isotopic heterogeneity in layered intrusions.

Age and Geochemistry of High Arctic Large Igneous Province Tholeiitic Magmatism in NW Axel Heiberg Island, Canada

AbstractThe Cretaceous High Arctic Large Igneous Province (HALIP) in Canada involved extrusion of continental flood basalts (CFBs) at 130–120 Ma and 100‐95 Ma and emplacement of an extensive sill and dike network that intersected the Carboniferous to Paleogene Sverdrup Basin. In this paper, we present new 40Ar/39Ar ages, major and trace elements, and Sr‐Nd‐Pb isotope ratios for HALIP lava, dikes, and sills from Bukken Fiord, NW Axel Heiberg Island, Canadian Arctic Islands. Our best constrained 40Ar/39Ar ages yield a weighted average of 124.1 ± 1 (2σ) Ma, coincident with the first pulse of tholeiitic CFB magmatism in the Arctic‐wide HALIP as exemplified by Isachsen Formation flood basalts on Axel Heiberg Island. The Bukken Fiord samples are plagioclase and clinopyroxene‐phyric tholeiitic basalts, are relatively evolved (3.2–6.5 wt% MgO), and share similar major and trace element compositions to typical HALIP tholeiites. Initial 143Nd/144Nd ranges from 0.51260 to 0.51291 and initial 87Sr/86Sr ranges from 0.70362 to 0.70776, while measured 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb range from 18.614 to 19.199, 15.534 to 15.630, and 38.404 to 39.054, respectively. The most primitive sample in this study has Sr‐Nd‐Pb isotope signatures that suggest an enriched plume‐derived mantle source for HALIP tholeiites. Most samples, however, possess relatively radiogenic isotope signatures that can be explained by moderate degrees of assimilation of Sverdrup Basin sedimentary rocks. Magma‐crust interaction in the HALIP plumbing system was likely widespread and may have increased the environmental impact of the HALIP, particularly if crustal carbon was volatilized.

Understanding the provenance and depositional conditions of Triassic sedimentary rocks from the Spiti region, Tethys Himalaya, India

The Spiti region, renowned as the Museum of Indian Geology, is a world-famous sedimentary succession containing well-exposed sequences from Neoproterozoic to Cretaceous age. In this study, Triassic siliciclastic sedimentary rocks of the Lilang Supergroup were chosen to understand weathering history, provenance, paleoclimate, and depositional conditions using a geochemical and isotopic approach. Triassic shales show more or less similar compositions with substantial enrichment in CaO compared to PAAS (Post Archean shales from Australia), which may be attributed to the association with limestones in the region. However, the sandstones display significant depletion in the trace element concentrations signifying the effect of quartz dilution. The relative depletion of mobile elements (Rb, Ba) as against immobile elements (Zr, Nb, Hf) can be noticed in the trace element spider diagram of the shales. The Triassic sedimentary rocks are characterized by enriched LREE and depleted HREE patterns with pronounced negative Eu anomalies. The Chemical Index of Alteration (CIA; 56–86) indicates low to intense chemical weathering in the source area. The unusual decrease in CIA and other weathering indices in the stratigraphically up section is attributed to changes in climate and environmental conditions during the deposition of sediments in the Triassic period. Detangling the signatures is crucial to understanding the mass extinction crisis, particularly the role of anoxia in these events. Triassic black shales represent suboxic to anoxic depositional conditions in the redox-sensitive elemental binary diagrams. The carbon isotope data of the present study is very well supported by the Total Organic Carbon (TOC), which infers that the oceanic biological system tried to recover from the depletion of biological life. The εNd and 87Sr/86Sr systematics record a shift in source terrains from the Early to Late Triassic period. The Early Triassic samples show much older depleted mantle model ages (TDM = 1.94–1.98 Ga) compared to Late Triassic sediments (TDM = 1.76–1.91 Ga). Similar interpretations can be drawn from Th/Sc ratios (from ∼ 6 to ∼ 0.05) and (La/Yb) N ratios (from ∼ 32 to ∼ 5), which record an increase in these ratios from Early Triassic to Late Triassic formations of the Spiti sedimentary rocks. Overall, trace elemental ratios and radiogenic isotopic signatures of the Triassic rocks of the Spiti region point towards Pan African granitic origin with minor impressions from the juvenile mafic-rich sources, such as Panjal Traps, the African craton, and Arabian-Nubian shield.

Deglacial and Holocene sediment dynamics and provenances off Lancaster Sound: Implications for paleoenvironmental conditions in northern Baffin Bay

Since the last deglaciation, Baffin Bay between Greenland and Canada developed from an isolated marginal sea to a major Arctic-Atlantic throughflow closely linked to the North Atlantic circulation. While the initial steps of gateway openings through Lancaster Sound and Nares Strait to northern Baffin Bay are reasonably well documented, far less is known about related regional deglacial-to-Holocene changes in sediment sources and depositional processes due to a lack of continuous and well-dated paleoenvironmental records from northern Baffin Bay. Sedimentological, mineralogical, and radiogenic isotope data of the well-dated sediment core GeoB22336-4 from the mouth of Lancaster Sound provide new insights on the impacts of ice-sheet retreat and opening of the gateways to the Arctic Ocean on the depositional setting. Basal subglacial till deposits point to a grounded ice stream at the mouth of Lancaster Sound before ∼14.5 ka BP. Subsequent glaciomarine sedimentation is characterized by the input of ice-rafted detritus (IRD), bioturbation traces, and foraminifera shells. Decreasing sediment supply and input of IRD through time reflects a period of ice-sheet recession to predominantly land-terminating positions during the Early Holocene. Changes in radiogenic isotope signatures reveal the openings of Lancaster Sound between ∼10.4 and 9.9 ka BP and of Nares Strait between 8.5 and 8.2 ka BP, in alignment with earlier studies. The rapid mid-Holocene (up to ∼5.8 ka BP) deposition of fine-grained sediments is most likely caused by enhanced sea ice-rafted sediment input released under a strong West Greenland Current influence. Finally, a slight increase in IRD input during the last ∼2 ka BP is linked to the neoglacial re-advance of regional glaciers.

Geochemical and radiogenic isotope records of the Weissert Event in south Tethyan sediments

The Cretaceous marine sedimentary record is characterized by time intervals rich in organic matter correlating with positive carbon isotope excursions, often called oceanic anoxic events. The Weissert Event corresponds to the first such event in the Cretaceous during the Valanginian stage. The associated palaeoenvironmental perturbations, which include increasing marine surface water primary productivity, are hypothesized to have been triggered by volcanic activity from large igneous provinces, and the source of nutrients is not well constrained (continental runoff v. oceanic upwelling). We present isotope ratios of Pb, Sr and Nd, together with concentrations of major and trace elements, for sediments from the central Moroccan margin to test these hypotheses. We demonstrate that the nutrient input was dominated by continental weathering. The source of sedimentary material remained stable during the Valanginian interval and originated from an old source, probably the African Sahara region. The radiogenic isotope signatures do not show a significant contribution of volcanic products from any known Valanginian large igneous province to the geochemical budget of sediments deposited on the central Moroccan margin. Although this does not preclude an impact of volcanic activity on the composition of seawater, it demonstrates that the erupted volumes were not sufficient to affect the deposited sediments. Supplementary material: The Supplementary Table contains three sheets: (1) Central Moroccan Margin, the analytical data generated and analysed during this study; (2) Fig. 8 data, large igneous provinces, the data of known Valanginian large igneous provinces used for comparison; and (3) Fig. 9 and S5 data, source areas, the data of potential surrounding source areas used for comparison, available at https://doi.org/10.6084/m9.figshare.c.6333040 .

The isotopic origin of Lord Howe Island reveals secondary mantle plume twinning in the Tasman Sea

Thermochemical convective instabilities in the mantle, often referred to as mantle plumes, cause mantle melting that give rise to ocean island basalts (OIB) in intraplate settings. The array of radiogenic isotope signatures in global intraplate OIB indicate that although plumes are individual entities, they share enriched components that resemble various parts of subducted crust and a hypothetical mantle matrix termed “focal zone” (FOZO). Each plume is expected to rise with an individual buoyancy flux, thereby producing variable volumes of melting with some OIB forming subaerial islands, whereas others produce submarine volcanoes.Here, we report the first radiogenic isotope data (Sr, Nd, Pb and Hf) for lavas of Lord Howe Island (LHI) in the Tasman Sea, the most prominent subaerial expression of the hypothesised Lord Howe mantle plume. Major element data are consistent with a moderate degree of partial melting, and heavy rare earth element depletion indicates melting occurred in the presence of garnet, consistent with other global plume lavas. Radiogenic SrNd isotopic data are similar to those defining the relatively primitive FOZO component with no clear enriched mantle affinity. The nearby Tasmantid Seamounts are also sourced from a mantle plume, and have similar SrNd character. However, combined 208Pb/204Pb and 207Pb/204Pb vs 206Pb/204Pb ratios of LHI lavas are inconsistent with a FOZO-type source. Instead, Pb isotopes overlap with typical enriched mantle 1 (EM1) lavas (e.g., Pitcairn, Tristan da Cunha), trending slightly higher than the Northern Hemisphere Reference Line (NHRL). Hafnium isotopes follow trends observed in lavas of the archetypal EM1 Pitcairn-Gambier islands, and are partially decoupled from 143Nd/144Nd; this decoupling is rare among OIB. The combined element-isotope data indicate that LHI forms the most recent expression of a mantle plume track in the Tasman Sea. Albeit unique in its 208Pb*/206Pb* isotope code among global mantle plumes, the resemblance of LHI lavas with the spatially related and contemporaneous Taupo seamount of the parallel Tasmantid plume track in the Tasman sea supports a genetic relation of both plumes. The proposed common origin of the two parallel plume tracks requires a larger plume underpinning the Tasman sea with individual plume fingers rising towards the surface, possibly in a triple plume network considering the on-shore Cosgrove track in eastern Australia. The subaerial expression of LHI contrasts the submarine Tasmantid Seamounts; this expression is only possible due to the continental ribbon that forms the island's foundation, suggesting secondary plume fingers display similar low buoyancy fluxes. Plain language summaryLord Howe Island is predominantly composed of basaltic rocks, which are created when mantle underneath the crust melts and the subsequent magma rises to the surface and erupts out of a volcano. The chemical composition of these volcanic rocks can provide us with valuable information about geologic processes. Moreover, we can compare the composition of islands like Lord Howe to better investigate and understand volcanic networks that exist deep within the Earth. The composition of Lord Howe Island basalts was investigated for the first time using specialised instruments that can separate individual elements into different isotopes. The ratios of these different isotopes reflects the composition of the mantle source which can be enriched by the presence of subducted crustal rocks, oceanic rocks and sediments. Our data indicate the distinctive reservoir that Lord Howe Island samples were derived from is the same reservoir as that of the nearby Taupo Seamount volcano. We infer from this that a piece of ancient crust is stuck in the mantle, and the two volcanoes are sampling it. This study supports a theory of a large, linked volcanic network under the Tasman Sea, and adds to the growing body of research investigating ‘double-chain’ volcanism in the Pacific and Atlantic oceans.

Time-space variations in the East African Rift magmatism: the role of different mantle domains

The East African Rift System (EARS) is the classic example of an active continental rift where extensional tectonics and lithospheric thinning have been closely associated to the generation of large volumes of magmas and represents the environment with the largest range of erupted magma types all over the world. The geochemical signature of erupted magmas testifies the involvement of different mantle domains and depths (i.e., subcontinental lithosphere, asthenosphere and deeper mantle sources). Our aim is to investigate the variable contribution of different mantle domains in the genesis of the EARS magmas through space and time, considering not only the geochemical signature of erupted magmas but also the geochemical message of mantle xenoliths. The main goal is to provide a large-scale view of the common process driving the origin of magmas in the EARS beyond the local peculiarities linked to specific settings. To this aim, we screened an exhaustive geochemical database of basalts and mantle xenoliths from the EARS, and we report original trace element and Sr-Nd isotope data of new samples collected from the Main Ethiopian Rift and Turkana depression. The data were subdivided according to spatial and temporal criteria. From a spatial point of view, the samples were ascribed to five groups, namely: Afar, Ethiopia, Turkana, Eastern Branch, and Western Branch; from a temporal point of view, the magmatic activity of the EARS was subdivided into three main temporal intervals: 45-25 Ma, 25-10 Ma and 10-0 Ma. The geochemical and radiogenic isotope (Sr, Nd, Pb) signature of the selected basalts denotes the variable contributions of a mantle plume, a more depleted asthenospheric mantle (DMM), and different SubContinental Lithospheric Mantle (SCLM) domains, depending on their temporal and spatial distribution. The geochemistry of the selected basalts shows a marked correspondence with the compositional heterogeneity of mantle xenoliths, whose isotopic systematics (Sm-Nd, Re-Os) indicates the formation of the local SCLM in the Archean and during the Pan-African orogeny. Both SCLM domains contributed significantly to magma genesis in the Western Branch (whose signature points towards a contribution of the Pan-African lithosphere) and Eastern Branch (which is also affected by Archean SCLM domains) magmas. The contribution of the SCLM generally increases with time, possibly related to an increase of the geothermal gradient in response to the arrival and flattening of the plume head at the base of the lithosphere and later extension, thinning and shallower melting. Our interpretation supports a pivotal role of the different SCLM domains in magma genesis that is able to fully explain the large compositional heterogeneity of the EARS basalts and represents a reasonable alternative to the putative presence of multiple mantle plumes or a heterogeneous mantle upwelling.

Authigenic and Detrital Carbonate Nd Isotope Records Reflect Pulses of Detrital Material Input to the Labrador Sea During the Heinrich Stadials

AbstractLimited constraints on the variability of the deep‐water production in the Labrador Sea complicate reconstructions of the strength of the Atlantic Meridional Overturning Circulation (AMOC) during the Late Quaternary. Large volumes of detrital carbonates were repeatedly deposited in the Labrador Sea during the last 32 kyr, potentially affecting radiogenic Nd isotope signatures. To investigate this the Nd isotope compositions of deep and intermediate waters were extracted from the authigenic Fe‐Mn oxyhydroxide fraction, foraminiferal coatings, the residual silicates and leachates of dolostone grains. We provide a first order estimation of Nd release via dissolution of detrital carbonates and its contribution to the authigenic ԑNd signatures in the Labrador Sea. During the Last Glacial Maximum the Nd isotope signatures in the Labrador Sea would allow active water mass mixing with more radiogenic ɛNd values (−12.6 and −14) prevailing in its eastern part whereas less radiogenic values (ɛNd ∼ −18.4) were found on the western Labrador slope. The deposition of detrital carbonates during Heinrich stadials (2,1) was accompanied by negative detrital and authigenic Nd isotope excursions (ɛNd ∼ −31) that were likely controlled by dissolution of dolostone or dolostone associated mineral inclusions. This highly unradiogenic signal dominated the authigenic phases and individual water masses in the Labrador Sea, serving as potential source of highly unradiogenic Nd to the North Atlantic region, while exported southward. The Holocene authigenic ɛNd signatures of the coatings and leachates significantly differed from those of the detrital silicates, approaching modern bottom water mass signatures during the Late Holocene.

Petrogenesis of Early Mesozoic volcanic rocks in southeastern NE China: Geochemical and Sr–Nd–Pb–O isotopic evidences

This paper reports a study of whole-rock geochemical, Sr–Nd–Pb isotopic compositions and zircon UPb ages and O isotopic compositions of Late Triassic to Early Jurassic volcanic rocks of the Yanbian–Dongning area, northeastern China. The obtained data are used to constrain the petrogenesis of these rocks and to provide new insights into the timing of transition from the Paleo-Asian to the Paleo-Pacific tectonic regime. The studied Late Triassic to Early Jurassic volcanic rocks can be divided into two periods: Late Triassic–early Early Jurassic (212–198 Ma) and late Early Jurassic (ca. 181 Ma). The former rocks display high SiO2 and total alkalis contents, high Ga/Al ratios, depleted SrNd and highly radiogenic Pb isotopic signatures, and low zircon δ18O values. The former rocks have a genetic relationship with coeval intrusions, and their primitive melt was derived from juvenile crustal materials and underwent subsequent differentiation and assimilation of low-δ18O rocks. In contrast, the latter rocks have SiO2 contents of 49.00–63.30 wt%, are classified as calc-alkaline, and display arc-type trace-element features. Their geochemical and Sr–Nd–Pb–O isotopic compositions imply derivation from the partial melting of depleted mantle wedge metasomatized by oceanic slab-derived fluid and sediment melt. Combining these new results with published data, the Late Triassic–early Early Jurassic volcanic rocks are inferred to have formed in a rift-like extensional setting, whereas the late Early Jurassic volcanic rocks were generated in a subduction-related setting. The transition from the Paleo-Asian to the Paleo-Pacific tectonic regime occurred during the Early Jurassic.

Determining the geochemical fingerprint of the lead fallout from the Notre-Dame de Paris fire: Lessons for a better discrimination of chemical signatures

On 2019, the fire of Notre-Dame de Paris cathedral (“NDdP”) spread an unknown amount of lead (Pb) dust from the roof of the cathedral over Paris. No data describing the geochemical fingerprint of the roof lead, as well as no particle collected during the fire, were available: a post-hoc sampling was performed. To discriminate the potential environmental impact of the fire from multiple Pb sources in Paris, it was mandatory to define unequivocally the fire dust geochemical signature. A dedicated and in hindsight geochemistry-based strategy was developed to eliminate any source of potential contamination due to sampling substrates or previously deposited dust. Radiogenic Pb isotopic signatures (206Pb/207Pb and 208Pb/206Pb ratios) and elemental ratios were determined in 23 Pb-rich samples collected inside NDdP. We determined that the dust collected on wood substrates on the first floor was most representative of fire emissions. These samples were the analyzed for the 4 Pb isotopes (204, 206, 207, 208) and the fire dust signature is characterized by ratio values of 206Pb/207Pb: 1.1669–1.1685, 208Pb/206Pb: 2.0981–2.1095, 208Pb/204Pb: 38.307–38.342, 207Pb/204Pb: 15.633–15.639 and 206Pb/204Pb: 18.242–18.275. In addition, the fire dust presents typical element-to-Pb ratio. This fingerprint was compared to the signatures of the known local Pb sources. The geochemical fingerprint of the fire is significantly different from that of the dominant urban Pb source. This will allow future evaluation of the contribution of the fire to Paris Pb pollution and of the real extent of the area affected by the Pb-containing dust plume. Moreover, the geographical origin of Pb used for the roof restauration and the spire building was identified. These findings open new ways to study the Pb sources in historical monuments for environmental impacts evaluation, as well as for historical perspectives.

Hemispheric Geochemical Dichotomy of the Mantle Is a Legacy of Austral Supercontinent Assembly and Onset of Deep Continental Crust Subduction

AbstractOceanic hotspots with extreme enriched mantle radiogenic isotopic signatures—including low 143Nd/144Nd indicative of subducted continental crust—are linked to plume conduits sampling the southern hemispheric mantle. However, the mechanisms responsible for concentrating subducted continental crust in the austral mantle are unknown. We show that subduction of sediments and subduction eroded material, and lower continental crust delamination, cannot generate this spatially coherent austral geochemical domain. However, continental collisions—associated with the assembly of Gondwana‐Pangea—were positioned predominantly in the southern hemisphere during the late Neoproterozoic appearance of widespread continental ultra‐high‐pressure metamorphic terranes, which marked the onset of deep subduction of upper continental crust. We propose that deep subduction of upper continental crust at ancient rifted‐passive margins during ca. 650‐300 Ma austral supercontinent assembly resulted in enhanced upper continental crust delivery into the southern hemisphere mantle. Similarly enriched mantle domains are absent in the boreal mantle plume source, for two reasons. First, continental crust subducted after 300 Ma—when the continents drifted into the northern hemisphere—has had insufficient time to return to the surface in plumes sampling the northern hemisphere mantle. Second, before the first known appearance of continental ultra‐high‐pressure rocks at 650 Ma, deep subduction of upper continental crust was uncommon, limiting its subduction into the northern (and southern) hemisphere mantle earlier in Earth history. Our model implies a recent formation of the austral enriched mantle domain, explains the geochemical dichotomy between austral and boreal plume sources, and may explain why there are twice as many austral hotspots as boreal hotspots.

The Baogutu gold deposit in west Junggar, NW China: An epizonal intrusion-related gold deposit

The Baogutu gold deposit (∼4 Mt ore @ 5.5 g/t Au) in West Junggar, is hosted by Early Carboniferous volcano-sedimentary rocks and Late Carboniferous dioritic dikes. Both the dikes and gold mineralization are controlled by NE-trending faults. Two diorite dikes in direct spatial association with gold-bearing quartz veins have LA–ICP–MS zircon U–Pb ages of 316 ± 5 Ma and 314 ± 3 Ma, respectively. The emplacement ages of the dikes are a little older than reported gold mineralization ages (312–311 Ma), which is consistent with their crosscutting relationships. Gold mineralization occurs in quartz veins and stockworks in altered tuff, tuffaceous siltstone, silty mudstone, and dikes. Three gold-bearing vein stages have been identified: (I) pyrite–quartz veins in the deep part of the system; (II) stibnite–quartz veins and (III) native arsenic–quartz veins at shallower-level. Fluid inclusion study shows that the pyrite–quartz veins of stage I formed at temperatures between 345 and 265 °C from fluids with salinities of 5.1–9.1 wt% NaCl equiv, whereas the native arsenic–quartz veins of stage III formed at ∼200 °C from fluids with salinities of 1.4–3.9 wt% NaCl equiv. The ore-forming pressure is estimated to be ∼50–62 MPa, corresponding to mineralization depth of ∼3.3–4.0 km. Oxygen and hydrogen isotope compositions of the fluids (δ18OH2O = 7.9 to 11.2 ‰, δD = -88 to −96 ‰) indicate that the initial hydrothermal fluids have a magmatic source, with involvement of meteoric water in the late stage. The in-situ lead isotopic compositions of pyrite in stage I pyrite–quartz veins are similar to those of bulk sulfide minerals in the Baogutu porphyry Cu deposit, and close to the porphyry stocks, whereas the stibnite and native arsenic in stage II and III quartz veins have more radiogenic lead isotopic signatures due to water–rock exchange with the wall rocks. The Baogutu gold deposit shows affinities to epizonal gold deposits in intrusion-related gold systems (IRGS). The IRGS model suggests that, sheeted, disseminated and vein-type Au–As–Bi mineralization might be found hosted in dioritic to granitic stocks and proximal strata, and vein-type Au–As–Sb mineralization in distal faults.

Petrogenesis of low-pressure intra-oceanic arc granitoids: Insights from the late Neoproterozoic Avalonian–Cadomian orogen, Bohemian Massif

The processes leading to the formation of siliceous (intermediate to felsic) magmatic rocks from juvenile sources within intra-oceanic island arcs govern the formation of continental crust and tempos of crustal growth. To unravel the predominant process responsible for the formation of the intermediate to felsic intra-oceanic arc crust, we performed inverse geochemical modelling using major elements and fluid-immobile trace elements. Subvolcanic trondhjemite from a volcanic arc – Davle volcanic complex (DVC) situated in the Teplá–Barrandian unit, Bohemian Massif – that formed during the Neoproterozoic Avalonian–Cadomian orogeny was used as a proxy for the most felsic endmember of the tonalite–trondhjemite suite. The DVC trondhjemite that follows the calc-alkaline trend is enriched in several fluid-mobile elements such as Cs, Ba and U, but depleted in high field strength elements (Nb, Ti) and lithophile elements (e.g., K, Rb, Sr and Th), and exhibits flat rare earth element (REE) primitive mantle-normalized distributions with slightly negative Eu anomalies. Together with the largely radiogenic Hf-Nd isotopic signatures (ε Nd +6.0 to +8.7 and ε Hf +10.2 to +11.7), these data indicate formation in an intra-oceanic arc setting with no significant assimilation of older crustal material. In general, the composition of the studied trondhjemite markedly resembles that of other intra-oceanic arc-derived tonalites and trondhjemites worldwide, formed at low pressures below the garnet stability field. These are herein referred to as “low-pressure intra-oceanic arc granitoids” or “LP IOAG”. The performed inverse geochemical modelling revealed that dehydration partial melting or fractional crystallization of a mafic source can produce a primitive tonalitic melt. Consequently, fractional crystallization of that melt accounts for the compositional heterogeneity observed within the tonalite–trondhjemite suite. We conclude that LP IOAG, commonly forming large portions of intra-oceanic arc middle crust, can form in a nearly closed system with a negligible contribution of subduction-related melts or assimilation of older siliceous crust. Therefore, our findings suggest that the formation of LP IOAG within oceanic crust may represent the predominant process of post-Archean crustal growth. • The DVC trondhjemite has flat REE patterns resembling typical LP IOAG. • The LP IOAG play a key role in the post-Archean crustal growth. • The source composition is established by an inverse geochemical modelling. • The trondhjemite formed by fractional crystallization of a tonalitic source.

Characterization of Saharan and Sahelian dust sources based on geochemical and radiogenic isotope signatures

Mineral dust can significantly impact climate and biogeochemical cycles on Earth. To understand dust provenance, an accurate characterization of dust sources and emission regions is required. In this study, we combine rare-earth element patterns, elemental ratios, and radiogenic Sr-Nd-Pb isotopes to discriminate dust sources from key regions in North Africa responsible for ∼55% of the total dust load emitted annually on Earth. This new dataset, based on fifty-nine analyses of deflatable fine soil material, improves our current knowledge about North African dust sources, especially those underrepresented in Saharan-Sahelian regions. Six potential source areas (PSA) – Libya-Algeria-Mali (PSALAM), Libya-Egypt (PSALE), Bodélé Depression (PSABD), Mali Center (PSAMC), West African Coast (PSAWAC) and Mauritania (PSAMa) – are defined based on several lithogenic tracers as well as on the geological subdivision of North African geological provinces, providing a unique chemical and isotope fingerprint for each PSA. For example, the PSABD – the main dust activation area in North Africa – is clearly distinguished from western African sources using Pb isotopes. Major elements show a large variability within each PSA and are not alone diagnostically useful. Comparison of the newly defined PSA with aerosols collected in remote locations over the North Atlantic shows that their sources are predominantly from western African regions (PSALAM and PSAWAC). Bodélé-derived dust has a limited impact on the bulk dust transported over the tropical North Atlantic, regardless of the season and altitude of dust transport. The low impact of the Bodélé Depression can be explained by a high deposition rate (wet and dry) of aerosols along its southwestward trajectory. A detailed analysis of dust collected over South America during winter will be essential to confirm this observation at a global scale. The present dataset can be efficiently used to track modern dust emissions as well as their changes at the scale of glacial/interglacial cycles or during the development of African Humid Periods as stored in sedimentary archives.

Intensification of Asian dust storms during the Mid-Pliocene Warm Period (3.25–2.96 Ma) documented in a sediment core from the South China Sea

Dust storms are an important component of the global climate system. At the same time, they also bear a risk for human health by causing pulmonary diseases. Today, East Asian dust storms account for as much as half of the global dust emissions and temporarily affect highly populated areas. Therefore, understanding their mechanisms and predicting their evolution under warmer near-future climate conditions is of major interest. The mid-Pliocene Warm Period (mPWP; 3.264–3.025 Ma) is considered one of the best analogues from the past for anthropogenic climate change. Consequently, understanding the climate dynamics and associated environmental change during the mPWP can help with predicting the environmental effects of warmer-than-present climates. In order to reconstruct Asian dust storm evolution during the mPWP we have analyzed a sediment core from the northern South China Sea (SCS) for its elemental composition, grain-size variations and radiogenic isotope signature for the interval spanning from 3.69 to 2.96 Ma. We show that shortly after the first strong northern hemisphere glaciation (Marine Isotope Stage [MIS] M2; 3.25 Ma) atmospherically transported dust appeared in the northern SCS and this dust deposition prevailed throughout the mPWP. Atmospheric dust input further intensified with the onset of the MIS KM2 glaciation at 3.15 Ma, with distinct and strong dust storms occurring periodically from that time onwards. The increase in atmospherically transported dust can be attributed to the cooling and drying of interior Asia over the course of the mPWP along with an intensification of the East Asian Winter Monsoon and a potential southward shift of the westerlies.

Geochemical and radiogenic isotopic signatures of granitic rocks in Chanthaburi and Chachoengsao provinces, southeastern Thailand: Implications for origin and evolution

The Chanthaburi, Pliew, Klathing, Khao Cha Mao, and Khao Hin Son granitic bodies in Chanthaburi and Chachoengsao provinces in southeastern Thailand, which are located on the southwestern side of the Mae Ping Fault and eastern side of the Klaeng Fault, were investigated. In this study, magnetic susceptibility measurements, whole-rock chemical composition and Nd-Sr isotope analyses, and zircon U-Pb dating were conducted on these granitic bodies. The surveyed granitic rocks are classified as I- to A-type granites, are of the ilmenite series, and show clearly negative Eu anomalies, which suggest they formed under reducing conditions. Nd-Sr isotope ratios indicate continental crust material involvement in the formation of these granite bodies. The magnetic and geochemical signatures are similar to those of granite bodies in southwestern Cambodia. The study area is thus considered an extensional area of southwestern Cambodia, corresponding to the Sukhothai Zone (the Chanthaburi-Kampong Chhnang Zone). Zircon U-Pb dating yields ages of 208–214 Ma (the Late Triassic) for granite bodies except for the Khao Cha Mao granitic body, which dates to 55 Ma. The former age corresponds to the collision time of the Sibumasu and Indochina terranes, and the latter age is likely related to the collision time of the Indian and Eurasian continents.

Lead isotopic fingerprinting of 250-years of industrial era pollution in Greenland ice

Emissions from mid-latitude industrial activities (e.g., mining, smelting , coal combustion) result in long-range atmospheric transport of lead (Pb) to the Arctic. While previous measurements of elemental concentrations and Pb isotopic ratios in ice and sediments have been used to suggest potential sources of toxic heavy metal pollution in these regions, high resolution Pb isotope records are largely unavailable due to the low Pb concentrations found in Arctic ice. Here we present and interpret a high-resolution, 1759–2008 record of Pb isotopes measured in a central Greenland ice core; the first high-resolution Pb isotope record for Greenland to include the First Industrial Revolution. Records of past industrial activities coupled with Pb isotopic signatures for regional ores and coals suggest Pb pollution prior to the mid-19th century was dominated by emissions from mining and combustion of coals in England, Scotland, and Wales. Rapid 1860s increases in Pb levels and decreases in 206 Pb/ 207 Pb ratios coincided with expansion of coal consumption in Europe and North America. Significant influence of 20th century smelting of Australian Broken Hill Pb ores in Europe resulted in a less radiogenic Pb isotope signature. The phase-out of leaded gasoline and other emissions reductions following passage of air-quality legislation in the United States had a pronounced effect on 206 Pb/ 207 Pb ratios, with values falling from 1.187 in 1978 to 1.154 in 1983. Increasing 208 Pb/ 207 Pb ratios through the 1990s and 2000s indicate rising influence of long-range transport from Asia countering declines in European emissions. This 250-year high-resolution Pb isotope reconstruction allows attribution of Pb sources to central Greenland with unprecedented detail. • 250-year high resolution Pb isotopic ice core record from central Greenland. • > 95% of Pb in the ice originated from pollution; U.S. pollution Pb peaked in 1968. • Following 1980 s U.S. Pb pollution declines, Pb from Asia dominated. • British Isles emissions lead as the Pb source from 1759 to 1817. • Smelting of Australian Broken Hill Pb ore in Europe seen beginning in the 1890s

Oxidized mantle sources of HIMU- and EM-type Ocean Island Basalts

Oxygen fugacity (fO2) is a fundamental variable in igneous petrology with utility as a potential tracer of recycled high Fe+3/ΣFe surficial materials in the sources of mantle-derived lavas. It has been postulated that ocean island basalts (OIB) have elevated fO2 relative to mid-ocean ridge basalts (MORB) owing to higher average Fe+3/ΣFe in their source regions. To examine this issue, trace-element systematics of olivine grains are reported from OIB lavas with HIMU (high-μ; Mangaia, Canary Islands), enriched mantle (EM; Samoa; São Miguel in the Azores Islands) and depleted MORB mantle (DMM; Pico in the Azores Islands) Sr-Nd-Pb-Os isotope signatures, in order to constrain the fO2 of each magmatic system. Despite sampling distinct mantle reservoirs based on radiogenic isotope systematics, these OIB suites show similar fO2, ranging from +0.8 to +2.6 ΔFMQ, with an average of +1.5 ± 0.8 ΔFMQ, largely higher than MORB using the same oxybarometry method at +0.6 ± 0.2 ΔFMQ. The studied OIB show no correlation between fO2 and bulk rock isotopic ratios or calculated parental magma compositions. The lack of correlations with isotopic signatures likely results from radiogenic isotope signatures being hosted in volumetrically minor trace element enriched mantle lithologies, while Fe+3/ΣFe ratios that act as the primary control on fO2 are controlled by the volumetrically dominant mantle component. Higher fO2 in many OIB relative to MORB implies that they sample a uniformly high Fe+3/ΣFe plume source mantle that may be the result of either a common oxidized oceanic crust-rich reservoir parental to many modern plume lavas, or preservation of un-degassed and oxidized mantle domains formed shortly after core formation.

Forging isotopically juvenile metamorphic zircon from and within Archean TTG gneiss: Whole-rock Sr-Nd-Pb and zircon U-Pb-Hf-REE constraints

The Winnipeg River terrane is one of three plutonic-gneiss terranes that contains the oldest rocks in the Archean Superior Province and is integral to understanding the evolution of Earth's largest Archean craton. We evaluate the evolution of the Winnipeg River terrane using whole-rock Sr-Nd-Pb isotope data for a suite of 17 samples of the Cedar Lake gneiss, along with U-Pb-Lu-Hf isotopes and trace elements in zircon grains from one sample. Most whole-rock Sr-Nd-Pb isotope data trend along 3.25 Ga reference isochrons, which overlap with the dominant population of ca. 3.25 Ga zircon, and are interpreted to mark the igneous crystallization of the protolith to the Cedar Lake gneiss. An older population of zircon aged ca. 3.5–3.4 Ga indicate either Hadean or Eoarchean crust was reworked at ca. 3.25 Ga. At ca. 2.7 Ga, new zircon grew with limited dissolution of older grains resulting in near-chondritic time-integrated Hf isotope signatures and low Th/U ratios relative to the older grains. As a result of the overlap between whole-rock reference isochrons at 3.25 Ga and the dominant zircon population ca. 3.25 Ga, the radiogenic Hf isotope signature of the ca. 2.7 Ga zircon grains imply Hf was recycled from non-zircon constituents within the rock rather than from external input. Additional support is drawn from elevated Nb and Ta concentrations in ca. 2.7 Ga zircon, hornblende and biotite. Our data highlights a complexity in the LuHf isotope systematics of zircon, where metamorphic zircon inherited radiogenic Hf isotope compositions as a result of internal redistribution of Hf from non-zircon phases in the rock rather than juvenile input.

Quantifying the growth of continental crust through crustal thickness and zircon Hf-O isotopic signatures: A case study from the southern Central Asian Orogenic Belt

Abstract Accretionary orogens function as major sites for the generation of continental crust, but the growth model of continental crust remains poorly constrained. The Central Asian Orogenic Belt, as one of the most important Phanerozoic accretionary orogens on Earth, has been the focus of debates regarding the proportion of juvenile crust present. Using published geochemical and zircon Hf-O isotopic data sets for three belts in the Eastern Tianshan terrane of the southern Central Asian Orogenic Belt, we first explore the variations in crustal thickness and isotopic composition in response to tectono-magmatic activity over time. Steady progression to radiogenic zircon Hf isotopic signatures associated with syn-collisional crustal thickening indicates enhanced input of mantle-derived material, which greatly contributes to the growth of the continental crust. Using the surface areas and relative increases in crustal thickness as the proxies for magma volumes, in conjunction with the calculated mantle fraction of the mixing flux, we then are able to determine that a volume of ~14–22% of juvenile crust formed in the southern Central Asian Orogenic Belt during the Phanerozoic. This study highlights the validity of using crustal thickness and zircon isotopic signatures of magmatic rocks to quantify the volume of juvenile crust in complex accretionary orogens. With reference to the crustal growth pattern in other accretionary orogens and the Nd-Hf isotopic record at the global scale, our work reconciles the rapid crustal growth in the accretionary orogens with its episodic generation pattern in the formation of global continental crust.