Lower Pb Isotopic Ratios Research Articles

Sampling Earth’s mantle at intra-transform spreading ridges

The Doldrums transform system, located in the Equatorial Atlantic at 7–8°N, is a 110 km-wide multi-fault shear zone, with five active transform faults separated by four short intra-transform ridge segments (ITRs). The two central ITRs, ITR-2 and ITR-3, are significantly deeper than the peripheral ridge segments, suggesting differences in the thermal conditions of the sub-ridge mantle. New chemical and radiogenic isotope data from on-axis lavas erupted across the transform domain reveal that the basalts from ITR-3 are enriched in alkalis (Na2O + K2O = 4.3 wt%; Na8 up to 3.7) and light rare earth elements (La/Sm)N = 0.86–0.97). However, these basalts have lower Sr and Pb isotope ratios than MORB from the Equatorial Atlantic (i.e., 87Sr/86Sr ∼ 0.70237 and 206Pb/204Pb ∼ 18), and relatively high Nd and Hf isotope ratios (143Nd/144Nd = 0.51315–0.51325; 176Hf/177Hf = 0.2832–0.28325). Hence, the mantle underlying ITR-3 is, on average, depleted in highly incompatible elements. Thermal models of the sub-ridge mantle show that it is also the coldest mantle region under the Doldrums system with the lowest crust production. Considering its location in the central transform domain, it is likely that the local mantle underwent melting beneath adjacent segments of the Mid-Atlantic Ridge (MAR) before remelting below ITR-3. We propose that the initial melting event beneath the MAR selectively removed the most fusible, geochemically enriched mantle components, leaving behind a predominantly peridotitic source, characterized by comparatively low Sr and Pb isotope ratios. Therefore, MORB from intra-transform ridge segments, such as those within the Doldrums transform system, provide a rare opportunity to constrain the isotopic composition of highly incompatible element depleted peridotitic mantle, which is a ubiquitous, but otherwise often camouflaged component of Earth’s mantle.

Middle-Late Miocene to Pleistocene Post-Collisional Magmatism in the Arabia-Eurasia Collision Zone, an Example from Northwest Iran

Abstract Post-collisional volcanism contains important clues for understanding the processes that prevail in orogenic belts, including those in the mantle and the uplift and collapse of continents. Here we report new geochronological and geochemical data for a suite of post-collisional Miocene to Pleistocene volcanic rocks from northwest Iran. Four groups of volcanic rocks can be distinguished according to their geochemical and isotopic signatures, including: (1) Miocene depleted lavas with high Nd and Hf but low Pb and Sr isotopic ratios, (2) less depleted lavas with quite variable Pb isotopic composition, (3) lavas with non-radiogenic Nd and Hf isotopic values, but highly radiogenic Sr and Pb isotopic composition, and (4) Pleistocene adakitic rocks with depleted isotopic signatures. The isotopic data reveal that the Miocene rocks are derived from asthenospheric and highly heterogeneous sub-continental lithospheric mantle sources. Evidence suggests that the lithospheric mantle contains recycled upper continental material and is isotopically similar to the enriched mantle two (EMII) end-member. Analysis of Sr-Nd-Pb-Hf-O isotopes in both mineral and rock groundmass, in conjunction with energy-constrained assimilation and fractional crystallization (EC-AFC) numerical modeling, demonstrates that the incorporation of continental crust during magma fractionation via AFC had an insignificant impact on the isotopic composition of the Miocene lavas. Moreover, adakites are the youngest rocks and show a geochemical signature consistent with the partial melting of a young and mafic continental lower crust. Both seismological data and geochemical signatures on these Miocene to Pleistocene volcanic rocks indicate the initiation of asthenospheric upwelling and orogen uplift in the Arabia-Eurasia collision zone, which occurred after slab break-off, following the Neotethyan closure.

Multiple-Stage Neoproterozoic Magmatism Recorded in the Zhangbaling Uplift of the Northeastern Yangtze Block: Evidence from Zircon Ages and Geochemistry

The Yangtze Block records Neoproterozoic magmatism and sedimentation related to the breakup of Rodinia and is an important piece in the reconstruction of the supercontinent. However, the tectonic setting and position of this block in Rodinia remain a subject of debate. In the present study, we report the zircon U-Pb ages and Hf isotopic composition of zircon and geochemical and Nd-Pb isotopic compositions for meta-volcanic rocks exposed in the Zhangbaling uplift of the NE Yangtze Block. The volcanic rocks, dominated by rhyolite and dacite, belong to the calc-alkaline series and show geochemical characteristics of arc rocks. Zircon U-Pb isotopic ages show that volcanic rocks in the Xileng Formation formed at ca. 790 Ma and ca. 760–700 Ma peaking at ~740 Ma. The late-stage volcanism was widely exposed in the uplift, characterized by a temporal-spatial trend becoming younger southwards. The old volcanic rocks have low initial εNd (−11.0) and εHf (−19.7 to −8.2) values and low Pb isotopic ratios, likely indicating an origin from ancient basement rocks underneath the Yangtze Block. The younger ones, being similar to continental arc andesite in trace element compositions, have relatively high initial εNd (mostly −4.6 to 0.5) and εHf (−0.4 to 8.8) values and high Pb isotopic ratios. These isotopic features point to an origin from the partial melting of juvenile crustal rocks. Sedimentary rocks of the Xileng Formation and the overlying strata also contain numerous zircon grains of ~700 Ma to ~630 Ma. The volcanic rocks in the Zhangbaling uplift might demonstrate long-lasting subduction along the northeastern margin of the Yangtze Block, probably active until ca. 700 Ma.

Gold mineralized diorite beneath the Linglong ore field, North China craton: New insights into the origin of decratonization-related gold deposits

Gold deposits in Precambrian cratons were mostly generated during the formation and stabilization of the cratons, but the North China craton is unusual in that its gold deposits were mainly formed ∼1.7 b.y. after its stabilization. A magmatic-hydrothermal origin or mantle-derived fluid source has been proposed for the giant gold deposits of the Jiaodong District in the eastern North China craton, but direct evidence is sparse, and the mineralization processes remain controversial. Here, we present the results of a comprehensive geological, geochronological, and geochemical study of the gold mineralized Xiejia diorite beneath the Linglong ore field at Jiaodong to link the gold mineralization to underlying magmatism. Magmatic zircon and titanite grains from the Xiejia diorite have laser ablation−inductively coupled plasma−mass spectrometry (LA-ICP-MS) U-Pb ages of 121.3 ± 0.9 Ma to 120.8 ± 1.1 Ma and 121.7 ± 3.9 Ma, respectively, which are indistinguishable from the time of gold deposition throughout the Jiaodong District as constrained by previous studies. The diorite has a shoshonitic composition and is characterized by strong enrichment in large ion lithophile elements (LILEs) and light rare earth elements (LREEs) along with significant depletion in high field strength elements (HFSEs) and heavy rare earth elements (HREEs). Samples of the diorite have high initial 87Sr/86Sr ratios, but low εNd(t) and ɛHf(t) values and low Pb isotope ratios. These geochemical characteristics are akin to those of contemporaneous mafic dikes in most gold mines at Jiaodong, indicating that the Xiejia diorite was most likely derived from an enriched lithospheric mantle source. The upper part of the diorite intrusion is pervasively altered and mineralized, containing an average of 0.32 g/t Au, but locally up to 7.59 g/t. Hydrothermal titanite from the mineralized diorite has a LA-ICP-MS U-Pb age of 122.3 ± 4.3 Ma, which is consistent with the gold-bearing pyrite Re-Os isochron age of 122.5 ± 6.7 Ma. Ore-related sericite aggregates from the Dongfeng gold deposit above the Xiejia diorite have a 40Ar/39Ar plateau age of 122.6 ± 1.3 Ma. Pyrite from the mineralized diorite yielded δ34SCDT values of 2.1‰−9.7‰, which are comparable with those of pyrite (δ34SCDT = 5.8‰−8.1‰, where CDT indicates the Canyon Diablo troilite standard) from gold ores of Dongfeng. Pyrite grains from both groups also have similar Pb isotope compositions. The S and Pb isotope data are consistent with values of mafic dikes that are spatially and temporally associated with gold veins in the Linglong ore field. The results presented here thus indicate a possible genetic link between gold mineralization in the Xiejia diorite and underlying magma presumably represented by the Xiejia diorite. The auriferous fluids exsolved from that magma and migrated upward along the Potouqing fault to form the Dongfeng gold deposit above the Xiejia diorite. The mineralized diorite thus links shallow gold mineralization to deep-seated mantle-derived magmatism generated during the extensive destruction of the North China craton induced by the rollback of the subducted paleo-Pacific plate.

Pre-eruption magmatic processes and magma plumbing system at Hachijo-Nishiyama volcano, Izu–Bonin arc, Japan

Nishiyama volcano on Hachijojima Island is an active basaltic volcano located in the Izu–Bonin arc. In this study, petrological and geochemical analyses were conducted on mafic lavas and pyroclastics to understand the magma plumbing system and pre-eruption magmatic processes. Whole-rock major element compositions show significant variations (49.4–54.9 wt.% SiO2), and the samples contain variable amounts of plagioclase phenocrysts (1–40 vol.%). The whole-rock Sr, Nd, and Pb isotopic compositions of samples from the youngest volcanic stage (< 0.7 ka) are homogeneous, whereas some samples from the older stage (3–1 ka) have relatively low Pb isotopic ratios. This observation suggests that the younger magmas were derived from a single parental magma, but another parental magma with distinct geochemical features was involved in the magmatic system before 1 ka. The temporal variation in the FeO*/MgO ratios of the volcanic products is complex and is considered to reflect the intermittent injection of primitive magmas into the main magma chamber in which fractional crystallization occurred. Two-pyroxene geobarometry suggests that the main magma chamber was located at a depth of 9–12 km. The core region of some plagioclase phenocrysts consists of a glass inclusion-free inner core and an inclusion-rich outer mantle, suggesting that some plagioclase crystallized in the main magma chamber, which was followed by overgrowth during magma ascent because of increasing liquidus temperatures due to decompression-induced water exsolution from the melt. The whole-rock compositions of some eruption units with different Al2O3/MgO ratios exhibit distinct plagioclase-controlled trends, which negates the possibility that plagioclase accumulation occurred in a stable magma chamber. In addition, the density of plagioclase was higher than that of the melt during the magma ascent to the surface. From these observations, it is suggested that the accumulation of plagioclase phenocrysts occurred in ascending magmas as the plagioclase settled relative to the surrounding melt. The estimated depth of 9–12 km for the main magma chamber coincides with the depth range over which earthquake swarms occurred in 2002, suggesting that the magma chamber is still active, and that the earthquake swarms may reflect the injection of primitive magma into the magma chamber.Graphical

Геохимическая характеристика источника пород вулкана Дзотол на Юго-Востоке Монголии: финальная генерация расплавов позднего кайнозоя в зоне транстенсии на границе литосферы–астеносферы

The purpose of this study is to establish relationship between a source for final eruptions on the Dzotol volcano and those for the whole preceded Late Cenozoic volcanism in the Dariganga field. Major oxides, trace elements, and Pb isotopic ratios in rocks of the volcanic field are indicative for generation of magmatic melts in a homogeneous mantle region. In final eruptions of the Dzotol volcano, rocks show, on the one hand, intermediate geochemical signatures relative to those of other Dariganga volcanic rocks, and, on the other hand, lower Pb isotope ratios. We infer that Dariganga volcanic rocks were derived due to deep degassing of the about 4.47 Ga protomantle and were dispersed at the asthenospheric–lithospheric boundary affected by transtension. The source for the final Dzotol rocks corresponded in depth to the central part of the magma-generating region and displayed the Pb-isotopic protomantle component, previously designated in rare Quaternary Dariganga rocks.

Asthenospheric mantle metasomatized by subducted marine sediments: Li isotopic evidence from Dagze mafic rocks, southern Tibet

Lithium (Li) isotopes show large isotopic fractionation during low temperature processes and have been widely used to trace subduction components at convergent margins. To identify recycled subducting components in the asthenospheric mantle, we report the lithium concentrations and isotopic compositions of the ca. 90 Ma Dagze mafic rocks from the eastern Gangdese Arc, southern Tibet, along with their corresponding whole-rock analyses and Sr-Nd-Pb isotope data. They mainly comprise basalts and diabases with high MgO (7.2–8.3 wt%) and low K 2 O (0.02–0.68 wt%) contents. These rocks show slightly variable initial 87 Sr/ 86 Sr ratios (0.7048 to 0.7057), relatively low and homogeneous Pb isotope ratios ( 206 Pb/ 204 Pb = 18.2948–18.6462, 207 Pb/ 204 Pb = 15.5587–15.6132, 208 Pb/ 204 Pb = 38.3843–38.8352) and positive ε Nd (t) values (3.0–4.9), similar to ophiolites from the Indus-Yarlung Tsangpo suture (IYTS). However, they are remarkably depleted in high field strength elements (HFSE; such as Nb and Ta) and slightly enriched in light rare earth elements (LREE) [(La/Yb) N = 2.8–4.4], both of which imply their mantle affinities with distinctive arc signatures. The δ 7 Li values of these rocks range from −2.0 to +2.0‰ (average = −0.7‰), with the majority of samples falling outside the depleted mantle range (+3.4 ± 1.4‰). They exhibit high Li contents (13.2 to 34.1 ppm) and Li/Y ratios (0.8 to 2.2), which are comparable with marine sediments. The negative correlation between Li/Y ratios and δ 7 Li values also suggests the addition of subducted marine sediments. The modeling of δ 7 Li values in combination with Pb-Sr-Nd isotopic ratios suggests that ~1 to 5% of marine sediments have been added into the mantle source of the Dagze mafic rocks. Combined with a previous study, we suggest that the Dagze mafic rocks were derived from the upwelling asthenospheric mantle metasomatized by subducted marine sediments. • Reporting the negative δ 7 Li values of the Dagze mafic rocks. • Demonstrating the lithium isotopic heterogeneities in the asthenospheric mantle. • Modeling ~1 to 5% of marine sediments to the mantle source of the Dagze mafic rocks.

A general ore formation model for metasediment-hosted Sb-(Au-W) mineralization of the Woxi and Banxi deposits in South China

Metasediment-hosted Sb-(Au-W) deposits occur widely in the Jiangnan Orogen (South China), but their complicated metallogenic histories are still under debate. Ore formation models such as syngenetic (SEDEX), granitoid-related, and orogenic scenarios have been proposed by previous studies, but their metal provenance and fluid evolutionary processes are still uncertain. In this study, two typical Sb-(Au-W) deposits (Woxi and Banxi) from this region were studied by in-situ geochemical and geochronological analyses of sulfides to establish a general ore formation model for this type of deposit. Two stages of gold-related pyrite (Py1 and Py2) and one stage of stibnite in the Woxi Sb-(Au-W) deposit, and two stages of arsenopyrite (Apy1 and Apy2) and stibnite (Stb1 and Stb2) in the Banxi Sb deposit were differentiated. Compared to Py1, Py2 is depleted in V but enriched in Au, As, Bi, Pb and Ag, with more negative δ34Sv-CDT and lower Pb isotopic values. Compared with stibnite at Woxi, stibnite at Banxi has higher but more variable contents of most trace elements (e.g., Li, Mg, Al, Si, Cu, Pb, Co, Ni, Fe, Sr and Mn, except for As) and lower Pb isotopic ratios. At Banxi, Stb2 has lower Fe, Cu, Pb, Co, Ni, Sr and Mn contents than Stb1, and Apy2 has higher Co, Ni, Zr, Ti, and Nb contents than Apy1. Analyses of Re-Os isotopes in four samples yielded an isochron age of 439 ± 3.4 Ma (2σ) for Apy1, with an initial 187Os/188Os of 0.3373 ± 0.0009. The above results indicate that early-stage Au mineralization at Woxi may have been orogeny-related, and that ore metals were derived mainly from local epimetamorphic sedimentary rocks with limited input from magmatic fluids. In contrast, the late-stage Sb mineralization at Banxi may have been influenced by deep granitic magmatism. We propose that the common temporal and spatial mineralization sequence of the metasediment-hosted Sb-(Au-W) ores is: orogenic-type gold (early stage; at deep levels of the ore field) → orogenic + magmatic-hydrothermal W (middle stage; at middle levels) → magmatic-hydrothermal + meteoric water Sb (late stage; at shallow levels). Both local metasedimentary rocks and granite magmatism provide metals and ligands for mineralization, with magmatic fluids playing an essential role in late-stage ore formation. Metasediment-hosted Sb-(Au-W) ores in this region exhibit similarities to both orogenic-type and intrusion-related gold systems.

Magmatism and related metamorphism as a response to mountain-root collapse of the Dabie orogen: Constraints from geochronology and petrogeochemistry of metadiorites

ABSTRACTPost-collisional mountain-root collapse and subsequent massive partial melting occurred in the high-temperature (HT) ultrahigh-pressure (UHP) metamorphic terrane of the North Dabie complex zone (NDZ), central China. The NDZ was deeply subducted in the Triassic, producing widespread migmatites and various magmatic intrusions in the Cretaceous. Post-collisional metadiorites with distinctive large K-feldspar augen porphyroblasts, locally reported but rarely exposed in the NDZ, underwent a complex evolutional history. In this contribution, integrated studies including field investigation, petrographic observation and mineral analysis, zircon U-Pb geochronological and Hf isotopic analyses, and whole-rock elemental and Sr-Nd-Pb isotopic analyses of the metadiorites were carried out. Our results provide new constraints on the mountain-root collapse in the Dabie orogen. The metadiorites are enriched in large ion lithophile elements and light rare earth elements, whereas they are depleted in high field strength elements and heavy rare earth elements with significant Ba positive anomalies, a composition consistent with the lower continental crust. All the studied samples have moderately enriched initial 87Sr/86Sr ratios (0.707582–0.708099), low εNd(t) values (−15.3 to −20.4), and low initial Pb isotopic ratios (16.0978–16.8452, 15.3167–15.4544, and 37.1778–37.8397 for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb, respectively). However, they have highly negative εHf(t) values and Paleoproterozoic two-stage Hf model ages, which are only partially consistent with data from the associated UHP metamorphic rocks. Such features suggest the metadiorites resulted from a magma produced by mixing of Triassic UHP mafic lithologies and minor amounts of mantle-derived materials. Zircon morphological analysis and U-Pb sensitive high-resolution ion microprobe dating combined with conventional thermobarometry indicate that these upwelling melts crystallized at pressure-temperature (P-T) conditions of 5.4–5.7 kbar and 750–768 °C at ca. 130 Ma and subsequently suffered HT metamorphism at ca. 125 Ma. We conclude that the metadiorites’ precursors were derived from partial melting of the Triassic subducted Neoproterozoic mafic lower-crustal rocks, with addition of minor amounts of mantle-derived materials in the Early Cretaceous, in response to mountain-root collapse of the orogen. Based on petrographic textures and mineral compositions, it is moreover inferred that formation of the distinctive K-feldspar porphyroblasts is likely related to a two-stage process, i.e., crystallization derived from biotite breakdown after the formation of the metadiorite at T = 640–703 °C and P &amp;lt; 4.5 kbar and coarsening related to shear deformation.

Ancient refractory asthenosphere revealed by mantle re-melting at the Arctic Mid Atlantic Ridge

The upper mantle is a heterogeneous mixture of refractory and recycled crustal domains. The recycled portions, more fertile and thus preferentially melted, dominate the composition of the basalts erupted on the surface, whereas the imprint of melting of the refractory counterparts is more difficult to discern from the basalt chemistry. Contrasting radiogenic isotopic signatures of mid-ocean ridge basalts and oceanic mantle, however, show that Hf isotope ratios may provide hints for melting of refractory source materials despite ubiquitous magma mixing during ascent and stalling in the crust. This property may allow identifying contributions from depleted mantle materials unseen in other isotope systematics in basalts. Here, we show that basalts from Mohns and Knipovich ridges, two >500-km long oblique super-segments in the Arctic Atlantic, have distinctly high Hf isotope ratios, not mirrored by comparatively high Nd and low Sr and Pb isotope ratios. These compositions can be explained if a highly depleted asthenospheric mantle melts beneath this section of the Arctic Mid Atlantic Ridge. We argue that this depleted source consists of high proportions of ancient (>1 Ga), ultra-depleted mantle, previously drained of enriched components before being re-melted in its current location following a recent ridge-jump, allowing the identification of ultra-depleted mantle components in the arctic subridge mantle.

Genesis of the Donggushan tungsten polymetallic deposit, central Anhui Province: A clue of diagenetic and metallogenic system of a crustal source

• The W mineralization in the Donggushan was caused by the Late Cretaceous magmatism . • Diagenetic and metallogenic system was likely to be originated from a crustal source dominated by the Northern Dabie Terrane . • The REEs of scheelite are mainly controlled by the substitution mechanism Ca 2+ + W 6+ = REE 3+ +Nb 5+ . • The ore-forming fluids were modified by the intense fluid-rock interaction. The newly discovered large-scale Donggushan tungsten polymetallic deposit with resources of 70,000 t WO 3 and 38,000 t Mo, located at the Lujiang county, center of Anhui Province, East China. The deposit is characterized by skarn W-Mo mineralization, which is developed along the external contact zone between the biotite granites and carbonate rocks. Zircon U-Pb dating yields a Late Cretaceous age of 98.3 ± 1.6 Ma for the Donggushan biotite granite. The hidden biotite granites of Donggushan is characterized by high silica and alkali but low Mg, Fe. It is enriched in light rare earth elements (LREEs) and large ion lithophile elements (LILEs: Rb, K, Th and U), but strongly depleted in heavy REEs, and high field strength elements (HFSEs: Nb, Ta and Ti), and Sr. This high-K granite has medium initial 87 Sr/ 86 Sr ratios (0.706518–0.707116) and low initial Pb isotopic ratios [( 206 Pb/ 204 Pb)i, 16.452–16.533; (207Pb/204Pb)i, 15.449–15.463; ( 208 Pb/ 204 Pb)i, 36.946.335–37.097], and is characterized by low εNd(t) and εHf(t) values (−22.1 to −16.1 and −21.3 to −15.1, respectively). All these data indicate that the Donggushan biotite granite is a high-K calc-alkaline fractionated I-type granitite. This granite may have derived from a crustal source, which is dominated by the Northern Dabie Complex (NDC) mixed with the basement materials of the Taihua Group. The magmatism in the Donggushan polymetallic deposit is quite different from those in the metallogenic belts of the Qinling-Dabie Mo belt (QDMB) and Middle-Lower Yangtze River Belt (MLYB). Based on the geochemical study of REEs of scheelite, it is proposed that the substitution mechanism of Ca2+ + W6+ = REE3+ +Nb5 + occurred during the formation of scheelite. Concentrations of Eu indicate that scheelite precipitated under the oxide conditions. The variations of the Mo concentrations might be caused by the changing of oxide reduction conditions. The Sr of the scheelite and S isotopes of pyrite in the Donggushan deposit reflect a complex magmatic-hydrothermal mineralization process within the intrusion and the country rocks.

Stagnant slab front within the mantle transition zone controls the formation of Cenozoic intracontinental high-Mg andesites in northeast Asia

AbstractThe geochemistry of Cenozoic intracontinental high-Mg andesites (HMAs) in northeast Asia, together with regional geophysical data, offers an opportunity to explore the genetic relationship between the formation of intracontinental HMAs and subduction of the Pacific plate. Compared with primary HMAs in arcs, Cenozoic intracontinental HMAs in northeast Asia have lower Mg# [100 × Mg/(Mg + Fe2+)] values (53–56) and CaO contents (5.8–6.6 wt%), higher alkali (Na2O + K2O) contents (5.15–6.45 wt%), and enriched Sr-Nd-Hf isotopic compositions (87Sr/86Sr = 0.7056–0.7059; εNd = −4.9 to −3.4; εHf = −4.7 to −2.6) as well as lower Pb isotope ratios (206Pb/204Pb = 16.76–19.19; 207Pb/204Pb = 15.42–15.45; 208Pb/204Pb = 36.71–37.11). These Cenozoic intracontinental HMAs are similar to Cenozoic potassic basalts in northeast China with respect to their Sr-Nd-Pb-Hf isotopic compositions but have higher SiO2 and Al2O3 contents and lower K2O, MgO, and light rare earth element contents. These features indicate that these Cenozoic intracontinental HMAs originated from the mantle, where recycled ancient sediments and water contributed to partial melting of peridotite. Combined with the presence of a large low-resistivity anomaly derived from the mantle transition zone (MTZ) near these intracontinental HMAs, and their occurrence above the stagnant slab front within the MTZ (at 600 km depth) in northeast Asia, we conclude that the stagnant slab front, with high contents of recycled ancient sediments and water, has controlled the formation of Cenozoic intracontinental HMAs in northeast Asia.

Appearance of an enigmatic Pb source in South America around 2000 BP: Anthropogenic vs natural origin

Neotropical wetlands in the paramo (a unique alpine-tundra ecosystem) region of South America have the potential to be natural archives for metal pollution by modern and past populations. An organic-rich sediment core from the El Triunfo mire, located in the paramo region, provides a record of natural and anthropogenic metal sources in the Northern Andes during the last four millennia. The Triunfo record is complex, as the mire is located in the Northern Volcanic Zone (NVZ) and receives direct input of volcanic material. Regardless of the volcanic input, calculated metal enrichment factors normalized to Sc show metal enrichment in the Northern Andes around 2000 years ago and again in recent industrial times. A number of samples show a shift to lower Pb isotope ratios indicating the appearance of a new, enigmatic Pb source around 2000 years ago. The topmost layer of the core shows the lowest Pb isotope ratios, reflecting input of modern anthropogenic Pb. In contrast to Pb, Nd isotopes do not show significant variations along the entire core, indicating mostly volcanic material input to the mire. The decoupling between Nd and Pb isotopes indicates that the enigmatic Pb source must be anthropogenic in origin.Based on the dominant atmospheric currents in the region, the El Triunfo mire can receive input from long-distance and local sources. Dispersion simulations validate the possibility of pollutant particle transport from Europe to the northern hemisphere Neotropics. As the first metal enrichment coincides with the Roman Empire times, the El Triunfo Pb isotopes are compared to contemporary peat records from Europe. All records show similar decrease in the Pb isotope ratios due to anthropogenic Pb input. Small Pb isotope differences between a record from Spain and El Triunfo indicate that the enigmatic Pb that appeared around 2000 years ago in the mire is unlikely to have originated from long-distance Roman Empire pollution.Instead, a group of deposits, namely San Lucas, San Martin de Loba, and El Bagre, located in north-central Colombia, show low Pb isotope ratios that can potentially explain the observed Pb signal in the El Triunfo sediments. The deposits are located up wind, along the predominant atmospheric currents in the region. Therefore, it is plausible that mining activities in the area of San-Lucas, San-Martin, and/or El Bagre released Pb in the atmosphere that was transported and deposited in the El Triunfo mire. These deposits are not associated with the known regions of influence of any of the early pre-Hispanic cultures in Colombia and there is no evidence for mining in this region around 2000 years ago. However, given that all other possibilities are unlikely, the appearance of lower Pb isotope ratios in the mire suggests the onset of mining in the region at least 400 years earlier than the available archaeological evidence at present. The El Triunfo mire record can be used as indirect evidence for significant metal exploitation by early pre-Hispanic cultures in the northern Andes as early as 2000 years ago.

Geochemistry and tectonic significance of late Paleoproterozoic A-type granites along the southern margin of the North China Craton

The Longwangzhuang pluton is a typical example of Paleoproterozoic A-type granite intrusions at the southern margin of the North China Craton. This pluton is composed of arfvedsonite granite and minor aegirine–augite granites. Samples from both granite types display similar zircon U-Pb ages with 207U-206Pb ages of 1612 ± 19 Ma [mean square weighted deviation (MSWD) = 0.66] and 1609 ± 24 Ma (MSWD = 0.5), respectively. The granites exhibit similar high silica (SiO2 = 71.1–73.4 wt.%), high alkaline (Na2O + K2O = 8.10–9.26 wt.%, K2O/Na2O > 1), and low Al2O3 (11.8–12.8 wt. %) contents and metaluminous to weakly peraluminous bulk chemistry. The chemical variations of the Longwangzhuang pluton suggest the effects of mineral fractionation. In addition, all samples show typical characteristics of A-type granites, such as high 10000Ga/Al ratios (4.10–7.28), high FeOtot/(FeOtot + MgO) ratios (0.88–0.99), and high Zr (484–1082 ppm), Ce (201–560 ppm), and Y (78–156 ppm) contents. The εNd(t) values and the (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t ratios of the arfvedsonite granite samples vary from −4.6 to –5.3, 15.021 to 17.349, 15.241 to 15.472, and 33.206 to 36.905, respectively, and those for the aegirine–augite granite sample amount at −0.2, 14.421, 15.175, and 33.706. The distinct and variable Nd and Pb isotope values indicate the presence of heterogeneous protoliths. Based on its geochemistry, its low initial Pb isotope ratios, and its enrichment in Nd isotopes, we infer that the Longwangzhuang A-type granite is the partial melting product of basement rocks such as the Taihua Group gneisses accompanied by some involvement of juvenile material from the mantle. Together with published data from other Paleoproterozoic A-type granite plutons exposed at the southern margin of the craton, our findings suggest that these granites had a similar origin. Furthermore, geochemically, they can be divided into two groups: A2-type, which formed earlier (~1.8–1.6 Ga), and A1-type, which formed later (~1.6–1.5 Ga). Combining this information with the variations in whole-rock Nd and zircon Hf isotopic composition at ca. 1.6 Ga, we propose that tectonic transformation from post-orogenic to anorogenic magmatism occurred at the southern margin of the North China Craton at that time.

Selective contamination of basaltic magmas and buchite genesis (on the example of Shufan plateau, Primorye)

The article discusses the problem of large-scale contamination of the Shufan volcanic plateaus basalts (Primorye) by selective melts from xenoliths. Silicic and alkaline liquids, selectively extracted from xenoliths, mix in a limited way with basaltic melts forming bands of granophyre that give lavas a taxitic structure. The article describes the unique compositions of buchites that were chemically modified by the diffusion interaction with basaltic magma and selectively molten politic xenoliths. Their mineral associations are represented by a high ferriferous cordierite (sekaninaite), ultra-ferriferous hercynite, (Al, Zr) Fe-armalcolite, Zr-ilmenite, mullite, sillimanite, high-lanthanum monazite, and barium-phosphate-aluminosilicate phase. The chemical and mineral compositions of buchites reflect the accumulation of refractory elements (Al, Fe, Ti, Zr, Ni, Cr) in the restite material of xenoliths. It is followed by the formation of ultra-alumina, ultra-ferruginous, initially immiscible metal-silicate composition. Low Pb isotopic ratios in Shufan basalts suggest a selective contamination by an ancient cratonic basements material. A consecutive increase of the isotopic ratios is observed the basalts contaminated by the upper crustal material, which is also shown by buchites isotopic composition.

Selective Contamination of Basaltic Magmas and Genesis of Buchites (on the Example of Shufan Plateau, Primorye)

The problems of large-scale contamination of the Shufan volcanic plateau (Primorye) basalts with selective melts from xenoliths are discussed. Silico-alkaline liquids that are selectively extracted from xenoliths mix only to a limited extent with the basalt melt, and form granophyre bands that give lavas a taxitic texture. The unique compositions of buchites (pelitic xenoliths chemically modified with a diffusion interaction with basaltic magma and selectively molten politic xenoliths) are characterized. The mineral associations of buchites are represented by highly ferriferous cordierite (sekaninaite), ultra-ferriferous hercynite, (Al, Zr) Fe-armalcolite, Zr-ilmenite, mullite, sillimanite, high-lanthanum monazite, and a barium–phosphate–aluminosilicate phase. The features of the chemical and mineral composition of buchites reflect the accumulation of refractory elements (Al, Fe, Ti, Zr, Ni, and Cr) in the restitic material of xenoliths, followed by the formation of ultra-aluminous, ultra-ferriferous, and initially immiscible metal-silicate composition. Low Pb isotopic ratios in the Shufan basalts indicate selective contamination with material of the ancient cratonic basement. A steady increase in these values is observed for basalts contaminated with the upper crustal material, which also reflects the isotopic composition of buchites.

Formation of rapakivi feldspar in the Deer Isle Granite Complex, coastal Maine: in situ lead isotope and trace-element analysis

Rapakivi and alkali feldspar phenocrysts from the Devonian Deer Isle Granite Complex, Maine were investigated to see if magma mixing or isothermal decompression was responsible for their formation. Pb-isotope and trace-element analyses of alkali feldspar ovoids and their plagioclase inclusions were obtained by LA-ICP-MS to determine the characteristics of the initial magma; these were compared with the plagioclase mantles. Rapakivi grains show a decrease in Pb-isotopic values and an increase in Ba, La, and Ce concentrations from the ovoids and inclusions to the innermost plagioclase mantles adjacent to the ovoids. These variations, along with CL imagery of quartz phenocrysts, indicate an open system with compositional changes in the magma chamber occurring during rapakivi feldspar growth. Repeated episodes of magma mixing/replenishment by Ba and LREE enriched magmas with lower Pb-isotopic ratios (along with hybridized variations with the host magma) created localized dissolution of alkali feldspar and quartz. Alkali feldspar phenocrysts proximal to these zones of thermal perturbation were first resorbed and then mantled by plagioclase that records the more primitive signature of the more mafic melt. Pb-isotopic values and Ba, La, and Ce concentrations trend back to the initial magma composition at the mantle rims as the effects of the mixing event dissipated. For grains that were somewhat more distal to these zones of mixing, resorption of the alkali feldspar crystals did not occur, yet the fresh supply of Ba resulted in Ba-rich alkali feldspar mantles over Ba-poor alkali feldspar cores. Other alkali feldspar crystals were too far from the site of mixing; hence, they lack any evidence of open system crystallization. As crystallization of the chamber continued along solidification fronts, batches of cooler crystal-rich magmas settled en masse to the floor. Disaggregation of these batches during settling, and subsequent accumulation on the reservoir floor, brought grains with disparate crystallization histories together. Filter pressing of the cumulate pile flushed highly evolved melts/fluids out from interstitial pores to the high silica, upper portions of the chamber. The crystallization, transportation, and juxtaposition of disparate types of feldspar phenocrysts suggest that a relatively large and active magma chamber, periodically recharged by localized batches of melt, must have existed. In this case, it is clear that rapakivi mantled feldspars are the result of magma mixing; such interpretations may apply to many other occurrences as well.

The tectonic domains of southern and western Madagascar

Southern and western Madagascar is comprised of five tectonic provinces that, from northeast to southwest, are defined by the: (i) Ikalamavony, (ii) Anosyen, (iii) Androyen, (iv) Graphite and (v) Vohibory Domains. The Ikalamavony, Graphite and Vohibory Domains all have intermediate and felsic igneous protoliths of tonalite-trondhjemite-granodiorite-granite composition, with positive εNd, and low Sr and Pb isotopic ratios. All three domains are interpreted to be the products of intra-oceanic island arc magmatism. The protoliths of the Ikalamavony and Graphite Domains formed repectively between c. 1080–980 Ma and 1000–920 Ma, whereas those of the Vohibory Domain are younger and date to between c. 670–630 Ma. Different post-formation geologic histories tie the Vohibory-Graphite and Ikalamavony Domains to opposite sides of the pre-Gondwana Mozambique Ocean.By contrast, the Androyen and Anosyen Domains record long crustal histories. Intermediate to felsic igneous protoliths in the Androyen Domain are of Palaeoproterozoic age (c. 2200–1800 Ma), of tonalite-trondhjemite-granodiorite-granite composition, and show negative εNd, moderate to high 87Sr/86Sr and variable Pb isotopic compositions. The felsic igneous protoliths of the Anosyen Domain are of granitic composition and, when compared to felsic gneisses of the Androyen Domain, show consistently lower Sr/Y and markedly higher Sr and Pb isotope ratios. Like the Vohibory and Graphite Domains, the Androyen Domain can be linked to the western side of the Mozambique Ocean, while the Anosyen Domain shares magmatic and detrital zircon commonalities with the Ikalamavony Domain. It is consequently linked to the opposing eastern side of this ocean. The first common event observed in all domains dates to c. 580–520 Ma and marks the closure of the Mozambique Ocean. The trace of this suture lies along the boundary between the Androyen and Anosyen Domains and is defined by the Beraketa high-strain zone.

Geochemical characteristics and tectonic setting of the bimodal volcanic rocks in Sepikou region of eastern Tianshan

The Sepikou region is located in the eastern Bogda Mountains of the east Tianshan. The Bogda belt is mainly composed of Carboniferous strata, including the Lower Carboniferous Qijiaojing Formation (C1q), Upper Carboniferous Liushugou Formation (C2l), and Qijiagou Formation (C2qj). Here, we report the petrology, geochemistry, and geochronology of bimodal volcanic rocks from the Upper Carboniferous Liushugou Formation, which is widely distributed in the Sepikou region. The basalts, basaltic andesite, and keratophyre have similar petrochemical characteristics and exhibit the high‐Na, low‐K characteristics belonging to calc‐alkalic rocks. These are characterized by the high abundance of K, Rb, Th, Ba, the depletion of HFSE (Nb, Ta, Zr, and Hf), and the slight depletion of Ti. Chondrite‐normalized rare‐earth elements (REE) patterns for basalts‐keratophyre show right‐leaning parallel curve clusters, together with a slight enrichment in light rare‐earth elements (LREE) contents and low heavy rare‐earth elements (HREE)/LREE ratios. The keratophyre exhibits slightly negative Eu anomalies. Compared with the basalts, the rhyolites have high alkali contents, slightly high ∑REE contents, low heavy rare‐earth elements/LREE ratios, negative εSr(t) (−9.1 to −12.0), low 87Sr/86Sr initial ratio, highly positive εNd (t) (+5.3 to +6.4), and low Pb isotope ratios. The fractionated REE patterns with prominent negative Eu anomalies suggest they were chiefly derived from the mantle magma underplated basaltic body remelting from the depleted mantle source. The LA‐ICP‐MS analysis on zircons from quartz keratophyre and rhyolite yield similar ages of 314.9 ± 1.2 and 314.0 ± 1.1 Ma, respectively, suggest that the major bimodal volcanism took place in the Late Carboniferous epoch. The quartz keratophyre zircons, with the 176Hf/177Hf ratios of 0.282897–0.283097, have highly positive εHf (10.99–19.97) and depleted mantle model ages between 180 and 628 Ma. The mafic rocks were chiefly derived from the depleted mantle with the partial incorporation of crustal components. The bimodal volcanic rocks were formed under a continental rift setting. The rift experienced extension in the early period of the Early Carboniferous and its final closure arose after a late period of the Late Carboniferous.

New isotope–geochemical and mineralogical data on the ultramafic xenoliths in the volcanic rocks of the Kamchatka–Koryak region: Two types of mantle protolith in the modern island-arc system

The paper presents new isotope–geochemical and mineralogical data on mantle xenoliths of the “island-arc” (Avacha, Shiveluch, and Kharchinsky volcanoes) and “within-plate” (Valovayam River, Cape Navarin, and Bakening Volcano areas) types. In terms of paragenesis and mineral composition, the “islandarc” xenoliths correspond to the olivine–plagioclase depth facies, while the “within-plate” xenoliths came from spinel lherzolite to wehrlite facies, which is transitional to the olivine–plagioclase equilibrium. The majority of the “within-plate” xenoliths are enriched in high-field-strength elements (Ti, Nb, Hf, Zr, Yb). The “island-arc” xenoliths in general are depleted in REE, while the “within-plate” xenoliths are enriched in all REE. The former have low Pb isotope ratios, being in isotope equilibrium with lower crustal basites, while most of the latter group are enriched in radiogenic Pb. The island-arc xenoliths are of magmatic origin and were derived from the sublithospheric crust–mantle mixture, while the “within-plate” xenoliths reflect the composition of the asthenospheric mantle source. The primary appearance of the xenoliths is obliterated by secondary recrystallization and metasomatic reworking.