Neodymium isotopes in the Mediterranean Sea: comparison between seawater and sediment signals

Nine depth-profiles of dissolved Nd concentrations and isotopic ratios (ε_Nd) were obtained in the Levantine Basin, the Ionian, the Aegean, the Alboran Seas and the Strait of Gibraltar. Thirteen core-top sediments and Nile River particle samples were also analyzed (leached with 1 N HCl, acetic acid or hydroxylamine hydrochloride). The seawater ε_Nd values become more radiogenic during the eastward circulation in the Mediterranean Sea. The relationship between salinity and the seawater ε_Nd shows that the Nd isotopic signature is more conservative than salinity in the Mediterranean Sea. The water mass with the highest ε_Nd (−4.8) is found at about 200 m in the easternmost Levantine basin. The average ε_Nd value for deep waters is −7.0 in the eastern basin, 2.5 ε-units higher than in the western basin. By examining the sensitivity of seawater ε_Nd to Nd inputs from the Nile, we conclude that the most significant radiogenic Nd source is partially dissolved Nile River particles. The Nd flux from the Nile River water has a minor influence on the Mediterranean seawater ε_Nd. Except for the easternmost Levantine Basin, the leachate ε_Nd values are consistent with the seawater values. In the easternmost Levantine Basin, the leachate ε_Nd values obtained with HCl leaching are systematically higher than the seawater values. The relationship between leachate and residual ε_Nd values indicates that the HCl leaching partially dissolves lithogenic Nd, so the dissolution of Nile River particles is the cause of the observed shift. Some ε_Nd values obtained with hydroxylamine hydrochloride leaching are higher than those obtained with HCl leaching. Although the reason for this shift is not clear, ⁸⁷Sr/⁸⁶Sr successfully detects the presence of a nonmarine component in the leachate. Our results suggest that leaching performance may vary with the mineralogy of marine sediments, at least in the case of the Mediterranean Sea.     

Nd/Nd and Sm/Nd stratigraphy of Upper Ordovician epeiric sea carbonates

Stratigraphic changes in the ε_Nd of epeiric sea carbonates from central North America track the submergence history of the interior craton during the Late Ordovician. Fluctuations in sea level changed the Nd isotope balance of the epeiric sea by modifying the flux of Nd weathered from the highlands of the Taconic Orogen (ε_Nd = −6 to −9) and from the low relief Precambrian basement (ε_Nd = −22 to −15) of the Transcontinental Arch and Canadian Shield. Transgressions over the Arch and Shield, which diminished the weathering flux of Nd from the Precambrian basement, are recorded as positive shifts in the ε_Nd profiles of carbonates. Negative ε_Nd shifts reflect regression and reexposure of the Precambrian basement to erosion. Correlation of Upper Ordovician carbonates by use of the ε_Nd profiles demonstrates the potential for Nd isotope stratigraphy. Comparison of stratigraphic variations in carbonate Sm/Nd ratios with sea level curves, conodont paleoecology, and the ε_Nd profiles suggest that our observed variations in Sm/Nd ratios are related to changes in depth. Increasing Sm/Nd ratios correlate with increasing depth, whereas decreasing Sm/Nd ratios correlate with decreasing depth. This relationship between Sm/Nd ratios and depth suggests Sm/Nd profiles have potentially wide applications in understanding the paleoceanography of ancient epeiric seas.     

Homogeneous impact melts produced by a heterogeneous target?: Sr-Nd isotopic evidence from the Popigai crater, Russia

The 35.7 ± 0.2 Ma old Popigai crater, Siberia, with a diameter of about 100 km is one of the best preserved large terrestrial impact structures. The heterogeneous target at the impact site consists of Archean to Lower Proterozoic metamorphic rocks of the crystalline basement, Upper Proterozoic quartzites and other clastic deposits, as well as Cambrian to Cretaceous clastic sediments and sedimentary rocks, including carbonate rocks. Moreover, Proterozoic and Permo-Triassic dolerite dykes are found in the target area. We report major element, Sr and Nd isotope data for 13 of these target rocks and for various types of impactites. The 15 analysed impactite samples include tagamites (impact melt rocks), suevites and impact glass from small veins. Furthermore, two impact breccias and two impact glass-coated gneiss bombs were analysed. We discuss the relation of these impactites to the target lithologies, and evaluate on the basis of literature data the relation of microkrystites (and associated microtektites) in Upper Eocene sediments to the Popigai event.The impactites have SiO₂ abundances ranging from 59 to 66 wt.% and show significant variations in the content of Fe, Ca, and Ti. They have present day ⁸⁷Sr/⁸⁶Sr ratios between 0.7191 and 0.7369. Their Sr model ages T^Sr_UR range from 1.9 to 2.3 Ga. The ¹⁴³Nd/¹⁴⁴Nd ratios for the impactite samples cluster between 0.5113 and 0.5115. The Nd model ages T^Nd_CHUR range from 1.9 to 2.1 Ga.In an ε_CHUR(Nd)-ε_UR(Sr) diagram, the impactites and Upper Eocene microkrystites (and associated microtektites) plot in a field delimited by Popigai target lithologies. The impactites are restricted to the field of crystalline basement rocks and Upper Proterozoic quartzites, but they show different isotopic signatures in different crater sectors. Impactites and Upper Eocene microkrystites plot in different, only partly overlapping clusters. The leucocratic microkrystites and microtektites have a higher affinity to the post-Proterozoic rocks in the target area than the impactites. Seemingly, the melanocratic microkrystites originated mostly from crystalline basement. This data alignment supports the assumption that Popigai is the source crater for all three types of ejecta. For the first time, clear relations are established of the geochemically variable Upper Eocene microkrystites and associated microtektites to specific target lithologies at Popigai crater. Finally, the observed range in Sr and Nd isotope parameters determined for impact melt lithologies that originated during the Popigai event show a much higher variability than known from other craters. This result indicates that mixing of impact melt which later formed tagamite sheets and glass particles in different impact breccias, was incomplete at the time of ejecta dispersal.     

Variations of Sr–Nd–Hf isotopic systematics in basalt during intensive weathering

The Sr–Nd–Hf isotopic compositions of both saprolites and parent rocks of a profile of intensively weathered Neogene basalt in Hainan, South China are reported in this paper to investigate changes of isotopic systematics with high masses. The results indicate that all these isotopic systematics show significant changes in saprolites compared to those in corresponding parent rocks. The ⁸⁷Sr/⁸⁶Sr system was more seriously affected by weathering processes than other isotope systems, with ε_Sr drifts 30 to 70 away from those of the parent rocks. In the upper profile (> 2.2 m), the Sr isotopes of the saprolites show an upward increasing trend with ε_Sr changing from ~ 50 at 2.2 m to ~ 70 at 0.5 m, accompanying a upward increasing of Sr concentrations, from ~ 10 μg/g to ~ 25 μg/g. As nearly all the Sr of the parent rock has been removed during intensive weathering in this profile, the upward increasing of Sr concentrations in the upper profile suggests import of extraneous Sr. Rainwater in this region, which enriches in Sr (up to 139 μg/L) from seawater, may be the important extraneous source. Thus, the Sr isotopes of the saprolites in the upper profile may be mainly influenced by import of extraneous materials, and the Sr isotopic characteristics may not be retained. In contrast, the ε_Nd and ε_Hf of the saprolites drift only 0–2.6 and 0–3.7 away from the parent rocks, respectively. The negative drifts of the ε_Nd and ε_Hf are coupled with Nd and Hf losses in the saprolites; i.e., larger proportions of Nd and Hf loss correspond to lower ε_Nd and ε_Hf. Compared with the relative high Nd and Hf concentrations of the saprolites, the contributions of extraneous Nd and Hf both from wet and dry deposits of aeolian input are negligible. Thus, the ε_Nd and ε_Hf changes in the profile are mainly resulted from consecutive removal of the Nd and Hf. Calculation indicates that the ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf ratios in saprolites are all significantly lower than their initial values in the parent rock. Simply removing part of the Nd and Hf by incongruent decomposing some of the minerals may not account for this. Fractionation should be happen, which ¹⁴³Nd and ¹⁷⁶Hf may be preferentially removed from the profile relative to ¹⁴⁴Nd and ¹⁷⁷Hf during intensive chemical weathering, resulting in lower ¹⁴³Nd/¹⁴⁴Nd and ¹⁷⁶Hf/¹⁷⁷Hf ratios in saprolites relative to the parent rock, even though details for this process is not known. A positive correlation is observed between the ε_Nd and ε_Hf of the saprolites. Interestingly, the saprolites with a net loss of Nd and Hf in the upper profile show good positive correlation, and the regression line parallels the terrestrial array. By contrast, saprolites with a net gain of Nd and Hf in the lower profile generally show higher ε_Hf values at a given ε_Nd value, and the regression line between these ε_Nd and ε_Hf appears to parallel the seawater array. This supports the hypothesis that the contribution of continental Hf from chemical weathering release is the key to the obliquity of the seawater array away from the terrestrial array of the global ε_Nd and ε_Hf correlation. Our results also indicate that caution is needed when using ε_Sr, ε_Nd, and ε_Hf to trace provenances for sediments and soils.     

浙江省中生代火成岩的Nd-Sr同位素研究

本文报道了浙江省21个中生代火成岩的Nd-Sr同位素组成,其中火山岩的ε_Nd值为-12.6——4.9,I_Sr值为0.70613-0.71079,t_DM年龄为1945-1296Ma;花岗岩类的ε_Nd值为-12.9——5.8,I_Sr值为0.70533-0.71208,t_DM年龄为1900-1230Ma,表明两者具有相似的同位素组成。这种相似性在同一火山-侵入杂岩体中表现更为明显,意味着两者在时、空、源方面具有同一性。与扬子地块的相比,华夏地块的中生代火成岩具有较低的ε_Nd值,较高的I_Sr值和较古老的Nd模式年龄,这种差异可能主要同这两个区域内基底变质岩在形成时代和成分上的差异有关。通过Sm-Nd同位素组成的对比研究,笔者认为,浙江境内的中生代火成岩可能主要是由基底变质沉积岩衍生的。原始岩浆的形成可能同中、下地壳岩石的熔融有关。     

Evidence for crustal components in the mantle and constraints on crustal recycling mechanisms: pyroxenite xenoliths from Hannuoba,North China

Spinel and garnet pyroxenite xenoliths in Cenozoic basalts from Hannuoba, North China show extremely heterogeneous chemical and isotopic compositions (ε_Nd=−27 to +34). Most of these pyroxenites are relatively young, probably late Mesozoic in age, although a few Al-pyroxenites could be very old (∼2 Ga). While their texture and major element compositions suggest an origin of high pressure cumulates, the trace element and isotopic compositions of the Hannuoba pyroxenites require multiple segregation processes from different parental magmas. Strong LREE enrichment, ubiquitous HFSE depletion and some Eu anomalies of the Al- and Cr-pyroxenites indicate the involvement of crust components in their source. Their Sr–Nd isotopic ratios are negatively correlated and plot below the MORB–OIB–IAB–sediment trend, suggesting that the parental melts of the Cr- and Al-pyroxenites may have been derived from a mixture of asthenospheric melts and a long-term evolved continental crust. The garnet pyroxenites significantly deviate from the isotopic array defined by the Al-pyroxenites, due to their relatively high ⁸⁷Sr/⁸⁶Sr at given ε_Nd. They thus more likely represent segregates from melts derived from partial melting of hydrothermally altered oceanic crust (basalts+marine sediment). If the crustal component involved in the Al-pyroxenites is subducted terrigenous sediments or other continental materials from the Archean Sino-Korean Craton, the Al-pyroxenites and garnet pyroxenites may have formed contemporaneously at a palaeo-convergent plate margin. This may be related to the subduction of the Mongol–Okhotsk plate beneath North China during the late Jurassic. Alternatively, if the delaminated lower crust was involved, it implies that most of the Al-pyroxenites are younger than the garnet pyroxenites, and their formation may be temporally correlated with lithospheric thinning during the Cretaceous. This model is attractive because the inferred tectonic evolution from a convergent setting to an extensional environment is consistent with the geologic record in the area.     

New data on genesis of the crust in the eastern segment of the Middle Urals: Sr–Nd isotopic constraints

The analysis of the Sr and Nd isotopic composition in different granitoids of the Verkhisetsk, Shartash, Krasnopolsk, Petrokamensk, and Shabry massifs, which were successively formed in the island arc, continental marginal, and collisional geodynamic settings during the period from the Middle Devonian to the early Permian, revealed that ⁸⁷Sr/⁸⁶Sr₀ values in them vary from 0.70331 to 0.70431 and ε_Nd(t), from +1.9 to +6.2. The two-stage model Nd age of granitoids (938–629 Ma) indicates that their magma originates from material at least Neoproterozoic in age, not younger. The observed variations in the Nd model ages of granitoids and ⁸⁷Sr/⁸⁶Sr₀ values provide grounds for assuming the primary heterogeneity of the source of granitoid melts.     

Neodymium isotopes in seawater from the Barents Sea and Fram Strait Arctic-Atlantic gateways

The neodymium concentration, C_Nd, and isotopic composition, ε_Nd, in seawater have been determined in the water column at five sites in the Barents Sea-Fram Strait area where most of the water exchange between the Arctic Ocean and the North Atlantic takes place. In the main Arctic Ocean inflow branch across the Barents Sea the concentration and isotopic composition (C_Nd = 15.5 pmol/kg and ε_Nd = −10.8) are similar to those reported for the northeastern Nordic Seas, which is consistent with this region being a source area for the Arctic inflow. Due to the addition of Nd from Svalbard shelf sediments, the C_Nd in the surface waters above 150 m, in the Fram Strait inflow branch is higher by a factor of 2 and the ε_Nd is shifted to lower values (−11.8).In the stratified Nansen Basin, where cold low salinity water overlies warmer Atlantic water the C_Nd and ε_Nd do not vary with the vertical temperature-salinity structure but are essentially constant and similar to those of the Atlantic inflow throughout the entire water column, down to 3700 m depth, which indicates that the Nd is to a large extent of Atlantic origin.Compared to the Atlantic inflow water, the Nd in the major Arctic Ocean outflow, the Fram Strait, show higher C_Nd in the surface waters above 150 m, and a higher ε_Nd (−9.8) throughout the entire water column down to 1300 m depth. Sources for the more radiogenic Nd isotopic composition in deep water of the Fram Strait outflow most likely involve boundary exchange with sediments on the shelf and slope as the water passes along the Canadian archipelago. River water is a possible source in the surface water but it also seems likely that Pacific water Nd, modified by interactions on the shelf, is an important component in the Fram Strait surface outflow. Changes in the relative proportions of inflow of river water and flow of Pacific water through the Arctic Ocean could thus influence the isotopic composition of Nd in the North Atlantic.     

Geochemical and Nd‐Sr‐Pb isotopic evidence on the origin and geodynamic evolution of mid‐Cretaceous continental arc volcanic rocks of the Spences Bridge Group,south‐central British Columbia

The mid‐Cretaceous Spences Bridge Group (SBG) comprises a series of basaltic to rhyolitic lavas and related volcaniclastic rocks (Pimainus Formation) overlain by a succession of mainly amygdaloidal andesites (Spius Formation) related to the closure of the Methow–Tyaughton basin and accretion of the Insular terrane in the North American Cordillera. Geochemical variation in the SBG is related primarily to metasomatic processes in the mantle wedge. Pimainus lavas include low‐ to high‐K, tholeiitic and calc‐alkaline types, and have isotopic compositions (ε_Nd(100Ma) = + 5.2 to + 7.0, ε_Sr(100Ma) = − 10 to − 20, ²⁰⁶Pb/²⁰⁴Pb = 18.82 to 18.91, ²⁰⁷Pb/²⁰⁴Pb = 15.55 to 15.60, ²⁰⁸Pb/²⁰⁴Pb = 38.24 to 38.43) between the ranges for primitive arcs and accreted terrane crust. Crustal sources are identified only for some low–medium K dacite and rhyolite compositions. The occurrence of intermediate compositions with high MgO contents (up to 6 wt%) and the presence of adakitic trace element features in medium–high K felsic lavas attests to metasomatism of the mantle wedge by slab melts during Pimainus volcanism. Spius lavas have comparable K₂O and Pb isotopic compositions to the Pimainus, even higher MgO (up to 9.2 wt%), and display a mild intraplate character in having up to 0.6 wt% P₂O₅, 15 ppm Nb, and 240 ppm Zr. Spius Nd−Sr isotopic compositions (ε_Nd(100Ma) = + 5.3 to + 6.9, ε_Sr(100Ma) = − 14 to − 25) define an array extending from Pimainus to alkaline seamount compositions. The low ε_Sr values, elevated high field strength element contents, and moderate silica contents suggest Spius volcanism was related to the introduction of small melt fractions from the asthenosphere into the mantle wedge which had previously generated Pimainus melts. The range of compositional types in the Pimainus Formation constrains tectonic scenarios to include an elevated slab thermal regime, likely from approach of an ocean ridge system toward the continental margin. Spius volcanism may have been generated by asthenospheric upwelling triggered by slab window development or slab‐hinge roll‐back on closure of the Methow–Tyaughton basin. Copyright © 2002 John Wiley & Sons, Ltd.     

Origin of Middle Cambrian and Late Silurian potassic granitoids from the western Kunlun orogen,northwest China: a magmatic response to the Proto-Tethys evolution

This paper presents new SHRIMP zircon U–Pb chronology, major and trace element, and Sr–Nd–Hf isotopic data of two Early Paleozoic granitic plutons (Yierba and North Kudi) from the western Kunlun orogen, in attempt to further constrain the Proto-Tethys evolution. SHRIMP zircon U–Pb dating shows that the Yierba pluton was emplaced in the Middle Cambrian (513?±?7 Ma) and the North Kudi pluton was emplaced in the Late Silurian (420.6?±?6.3 Ma). The Yierba pluton consists of quartz monzodiorite, quartz monzonite and granodiorite. These granitoids are metaluminous and potassic, with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7072–0.7096, ε_Nd (T) of ?0.2 to ?1.6 and ε_Hf (T) (in-situ zircon) of ?1.2. Elemental and isotopic data suggest that they were formed by partial melting of subducted sediments, with subsequent melts interacting with the overlying mantle wedge in an oceanic island arc setting in response to the intra-oceanic subduction of Proto-Tethys. The North Kudi pluton consists of syenogranite and alkali-feldspar granite. These granites are metaluminous to weakly peraluminous and potassic. They show an affinity of A₁ subtype granite, with initial ⁸⁷Sr/⁸⁶Sr ratios of 0.7077–0.7101, ε_Nd (T) of ?3.5 to ?4.0 and ε_Hf (T) (in-situ zircon) of ?3.9. Elemental and isotopic data suggest that they were formed by partial melting of the Precambrian metamorphic basement at a shallow depth (<30 km) during the post-orogenic regime caused by Proto-Tethyan oceanic slab break-off. Our new data suggest that the subduction of the Proto-Tethyan oceanic crust was as early as Middle Cambrian (～513 Ma) and the final closure of Proto-Tethys was not later than Late Silurian (～421 Ma), most probably in Middle Silurian.     

Transantarctic Mountain microtektites: Geochemical affinity with Australasian microtektites

We extended the petrographic and geochemical dataset for the recently discovered Transantarctic Mountain microtektites in order to check our previous claim that they are related to the Australasian strewn field. Based on color and composition, the 465 microtektites so far identified include two groups of transparent glass spheres less than ca. 800 μm in diameter: the most abundant pale-yellow, or normal, microtektites, and the rare pale-green, or high-Mg, microtektites. The major element composition of normal microtektites determined through electron microprobe analysis is characterized by high contents of silica (SiO₂ = 71.5 ± 3.6 (1σ) wt%) and alumina (Al₂O₃ = 15.5 ± 2.2 (1σ) wt%), low total alkali element contents (0.50-1.85 wt%), and MgO abundances <6 wt%. The high-Mg microtektites have a distinctly higher MgO content >10 wt%. Transantarctic Mountain microtektites contain rare silica-rich (up to 93 wt% SiO₂) glassy inclusions similar to those found in two Australasian microtektites analyzed here for comparison. These inclusions are interpreted as partially digested, lechatelierite-like inclusions typically found in tektites and microtektites. The major and trace element (by laser ablation - inductively coupled plasma - mass spectrometry) abundance pattern of the Transantarctic Mountain microtektites matches the average upper continental crust composition for most elements. Major deviations include a strong to moderate depletion in volatile elements including Pb, Zn, Na, K, Rb, Sr and Cs, as a likely result of severe volatile loss during the high temperature melting and vaporization of crustal target rocks. The normal and high-Mg Transantarctic Mountain microtektites have compositions similar to the most volatile-poor normal and high-Mg Australasian microtektites reported in the literature. Their very low H₂O and B contents (by secondary ion mass spectrometry) of 85 ± 58 (1σ) μg/g and 0.53 ± 0.21 μg/g, respectively, evidence the extreme volatile loss characteristically observed in tektites. The Sr and Nd isotopic compositions of multigrain samples of Transantarctic Mountain microtektites are ⁸⁷Sr/⁸⁶Sr ≈ 0.71629 and ¹⁴³Nd/¹⁴⁴Nd ≈ 0.51209, and fall into the Australasian tektite compositional field. The Nd model age calculated with respect to the chondritic uniform reservoir (CHUR) is T^Nd_CHUR ≈ 1.1 Ga, indicating a Meso-Proterozoic crustal source rock, as was derived for Australasian tektites as well.Coupled with the Quaternary age from the literature, the extended dataset presented in this work strengthens our previous conclusion that Transantarctic Mountain microtektites represent a major southward extension of the Australasian tektite/microtektite strewn field. Furthermore, the significant depletion in volatile elements (i.e., Pb, B, Na, K, Zn, Rb, Sr and Cs) of both normal and high-Mg Transantarctic Mountain microtektites relative to the Australasian ones provide us with further confirmation of a possible relationship between high temperature-time regimes in the microtektite-forming process and ejection distance.     

Trace Elements and Sr,Nd Isotope Geochemistry of the Lajimiao Norite-Gabbro from the North Qinling Belt1

Abstract The Lajimiao norite-gabbro complex, as a part of the ophiolites on the southern side of the North Qinling belt, consists of gabbro and norite-gabbro. They were derived from different magma series: the gabbro was derived from tholeiitic magma series with higher TiO₂, REE abundance and Fe³⁺ / Fe²⁺ ratio; norite-gabbro was derived from calc-alkali magma series with lower TiO₂, Fe³⁺ / Fe²⁺ ratio and REE abundance and much lower HREE abundance, which suggests that the source of the norite-gabbro magma was deeper and controlled by eclogite facies. Geochemical characteristics of both plutonic rocks are similar to those of island-arc basalts, such as relatively high contents of Ba, Pb and Sr and relatively low contents of Nb, Zr and Ni. The Sr, Nd isotopic characteristics of the Lajimiao norite-gabbro complex are similar to those of ophiolites. Its 8_Nd values are constant, about +2; whereas 8_Sr values have wide variation from — 6.4 to +31.2 and positively correlate with Na₂O, H₂O⁺ and CO₂ contents and the Fe³⁺ / Fe²⁺ ratio. The ?_Nd—Nd / Th, ?_Nd—La/Nb and ?_Nd—Ba/Nb diagrams clearly show that there were significant components of terrigenous sediments in the mantle source of the Lajimiao norite-gabbro complex. It suggests that large amount of sediments had been carried into the mantle by the subducted ancient Qinling sea plate during the Palaeozoic.     

Nd isotopic composition in the central North Pacific

Three vertical profiles of seawater concentration and isotopic composition of Nd were determined for the western to central North Pacific Ocean.In the subarctic oceanic region, at depths greater than 500 m, one vertical profile of Nd isotopic composition was indistinguishable from most previously reported profiles from here. The data indicate a rather homogeneous Nd isotopic composition in the subarctic oceanic region at middle to deep depths (>500 m). Two stations in the subtropical oceanic region exhibited similar Nd isotopic composition profiles to those previously reported. The maxima ε_Nd values at depths of 800-1000 m (ε_Nd = −3.4 to −2.7), which correspond to the North Pacific Intermediate Water (NPIW), are found at both subtropical stations. This implies a ubiquitous distribution of NPIW showing a radiogenic ε_Nd value in the North Pacific. The subsurface minimum at a depth of ∼200 m, which indicates the penetration of the North Pacific Tropical Water (NPTW) with an unradiogenic Nd isotopic signal, was observed at one station in the western Pacific. This station had much lower ε_Nd than the central station at depths around 5000 m, suggesting the greater prominence of Antarctic Bottom Water (AABW) in the western subtropical Pacific than in the central to eastern subtropical Pacific.Results of a model calculation assuming boundary exchange indicate that the Hawaiian Islands play an important role in supplying radiogenic Nd to the central Pacific, similar to some continental margins.We show that Nd isotopic composition is a versatile tracer for ocean circulation and the geochemical cycle of Nd in the North Pacific. Further studies on the distribution of Nd isotopic composition in the Pacific Ocean, including the Southern Pacific, will better elucidate the circulation and geochemical cycle of Nd in the Pacific.     

Association of Late Paleozoic Adakitic Rocks and Shoshonitic Volcanic Rocks in the Western Tianshan, China

The late Paleozoic adakitic rocks are closely associated with the shoshonitic volcanic rocks in the western Tianshan Mountains, China, both spatially and temporally. The magmatic rocks were formed during the period from the middle to the late Permian with isotopic ages of 248-268 Ma. The 87Sr/86Sr initial ratios of the rocks are low in a narrow variation range (-0.7050). The 143Nd/144Nd initial ratios are high (-0.51240) with positive εND(t) values (+1.28-+4.92). In the εNd(t)-(87Sr/86Sr)i diagram they fall in the first quadrant. The association of the shoshonitic and adakitic rocks can be interpreted by a two-stage model: the shoshonitic volcanic rocks were formed through long-term fractional crystallization of underplated basaltic magma, while the following partial melting of the residual phases formed the adakitic rocks.     

安徽伏川蛇绿岩套的Nd-Sr-O同位素研究

安徽歙县伏川的蛇绿岩套形成于中-晚元古宙,其Nd、Sr和O同位素组成是:ε_Nd(T)=+0.7-+3.8,ε_Sr(T)=+30.7-+53.9,δ¹⁸O=3.2-11.0‰。据地质学和同位素地球化学特征,该岩套位于杨子板块南缘、江南古岛弧的弧后小洋盆地轴部。ε_Nd(T)值的变化是由于蛇绿岩形成过程中受到下伏不成熟硅铝质基底地壳的混染引起的;ε_Sr(T)和δ¹⁸O的变化,是在蛇绿岩形成时或形成后不久遭受海水热液蚀变的结果。     

Isotopic compositions and geological significance of sediments from the Heihe River,North Qilian Orogen,NW China

Strontium (Sr), neodymium (Nd), and hafnium (Hf) isotopic analyses of different size-fractions of sediments collected from the Heihe River in the North Qilian Orogen (NQO) were carried out to trace the sediment sources and to determine the relationships between the Sr–Nd–Hf isotopic behaviors, and the grain-size and hydraulic sorting effects during fluvial transport. Our results demonstrate that the sand and suspended load samples collected from the same site have different Nd isotopic compositions, while their ⁸⁷Sr/⁸⁶Sr ratios are only slightly different. These features indicate that contributions to the sediments from different sources with different grain sizes vary greatly. Coarse-grained sand may more intuitively reflect the variations in local sources than the suspended load. The suspended load samples are distributed along the Clay Array on the Ԑ_Hf vs. Ԑ_Nd diagram due to zircon sorting. A similar NdHf decoupling phenomenon is widespread in the global oceans and large river systems, indicating that the NdHf isotopic behavior depends on the hydrodynamic sorting of minerals during fluvial transport. Moreover, the crustal accretion of the NQO from the depleted mantle occurred in the Proterozoic based on the constraints provided by the T_DM values of the river sediments. The Nd isotopic compositional characteristics of the river sediments indicate that the NQO has an affinity with the Yangtze Craton. The determination of the basement tectonic affinity of the NQO significantly contributes to our understanding of the Neoproterozoic evolution of the Gondwana continental margin.     

Nd isotope signature of Holocene Baltic Mn/Fe precipitates as monitor of climate change during the Little Ice Age

Neodymium (Nd) isotope profiles were analyzed on two Baltic Mn/Fe precipitates (99/2 and TL1) from shallow water (20 m) of the Mecklenburg Bay. The age range of these Mn/Fe precipitates determined by ²²⁶Ra_ex/Ba dating reaches from recent growth back to ∼4300 and 1000 yr BP, respectively. Over this time range, the Nd isotope composition varies from ε_Nd (0) = −13.1 to −17.5 in the selected Baltic precipitates indicating substantial changes in the Nd isotope composition of the Baltic Sea. The lowest ε_Nd values were recorded during the time interval of the Little Ice Age (LIA, AD ∼1350 to 1850). These minimum values indicate either an increase of the input of less radiogenic Nd from Scandinavian Archean-Proterozoic sources (ε_Nd about −22) to the Baltic Sea or a decrease of the input of more radiogenic Nd from continental European sources (ε_Nd about −12) and/or North Sea water (ε_Nd about −10). Variations of both, erosive continental input and North Sea inflow may indicate a direct response of the Nd isotope signal in the Baltic Sea to climate changes during the LIA and be related to cyclic shifts in the atmospheric circulation triggered by the North Atlantic Oscillation (NAO). Another aspect that possibly influenced the input of trace elements and Nd isotopes into the Baltic Sea is the population development in the circum Baltic area during the LIA. The lowest ε_Nd values also correspond to the medieval demographic crises that led to a significant decrease of agricultural activity and farmland. The reduction of soil erosion and enhanced regrowth of natural vegetation may have changed the amount and proportions of dissolved and suspended particulate matter transported into the Baltic Sea by rivers which in turn may have resulted in a change of the Nd isotope composition of Baltic Sea water.     

Age of metamorphic rock masses in Laba Basin,North Caucasus

A Neoarchaean sanukitoid pluton that was intruded into the base of the Guyang greenstone belt in the Yinshan Block of the North China Craton hosts a number of hornblendite enclaves. Geochemically, the pluton is characterized by high MgO, Mg#, Cr, Ni, large-ion lithophile element (LILE), and heavy rare earth element (HREE) contents and low TiO₂ and high-field strength element (HFSE) contents, and has relatively low Sr/Y ratios and negative Eu anomalies. Whole-rock Sr–Nd isotopic analyses indicate that it has ε_Nd(t) values of +1.4 to +2.0. These geochemical and isotopic characteristic suggest that the sanukitoid was formed under a low-pressure and high-temperature environment by slab melting and assimilation of hornblendite enclaves. The hornblendite enclaves show high MgO, Mg#, Cr, and Ni contents, high and variable K₂O, LREE, and Th contents, enrichment in LILEs and LREEs, and depletion in HFSEs. They have high Y contents and relatively low Sr/Y values, strongly negative Eu anomalies, and whole-rock ε_Nd(t) values of +1.0 to +1.9. The geochemical and isotopic characteristics indicate that these enclaves might have been derived from a mixed source of an enriched mantle, metasomatized by melts expelled from subducted sediments in a high-temperature, low-pressure environment. To explain these characteristics, a ridge subduction model is proposed for the formation of the sanukitoid and hornblendite in the Yinshan Block in the Neoarchaean.     

Geochemical and isotopic studies of aeolian sediments in China

The Sr and Nd isotopic, rare earth element (REE) and major element compositions, together with mineral and grain‐size proportions, are reported for aeolian loess deposits and desert sands from several Chinese localities. The study was carried out in order to examine regional variations in the isotopic and geochemical features of these aeolian sediments, and to constrain the provenance of Chinese loess. Samples include loesses from the Tarim and Junggar basins and desert sands from the Taklimakan desert in north‐west China, loess from the Ordos area and desert sands from the Tengger and Mu‐us deserts in north‐central China, as well as loess and desert sands from the Naiman area, north‐east China. REE distributions show minimal variation among the Chinese loess deposits, whereas those for the desert sands show regional variations. New isotopic data document a latitudinal variation in Sr and Nd isotopic features for the loesses and desert sands. The Naiman and Junggar loesses have distinctly lower ⁸⁷Sr/⁸⁶Sr ratios and higher ε_Nd(0) values than the loesses from the Tarim Basin, the Ordos area and the Loess Plateau. Among the desert sands, the Naiman samples have higher ε_Nd(0) values than the Taklimakan, Tengger and Mu‐us samples. Isotopic data suggest that loesses of the Loess Plateau were supplied from the Tarim Basin loesses and Taklimakan Desert sand, and that the Naiman loesses were supplied from the Junggar Basin loesses. The latitudinal variation in the loesses and desert sands may be partly explained by isotopic variations reported previously for moraines from the Tianshan and west Kunlun Mountains, which are possible sources for the loesses and desert sands. These inferences on the provenance of the loesses and desert sands are consistent with the dust transport pattern over East Asia.     

Nd isotope composition of conodonts: An accurate proxy of sea-level fluctuations

Analyses of the Nd isotope composition of conodonts from the Anti-Atlas (Morocco), the Montagne Noire (France) and the Rhenohercynian domain (Germany) provide insight into the temporal and lateral variations in seawater geochemistry in the western part of the Variscan Sea during the Late Devonian. Most of the isotopic excursions accurately record changes in sea level. The ε_Nd values decreased during regression phases when erosion accelerated supply of unradiogenic Nd from old continental sources. A rise in sea level generated a positive shift in ε_Nd values due to input of more radiogenic oceanic water into the seawater reservoir of the shelf areas. The method has great potential for sea-level research, paleoceanography and correlation because sea-level changes can be precisely recognized even in successions with uniform, monotonous lithologies. However, it has a significant limitation being valid only in epeiric seas and shelf areas. We have used the Nd isotope record to reconstruct the eustatic sea-level changes in late Frasnian–early Famennian time. The onset of the organic-rich Kellwasser units coincides with negative trends in ε_Nd values. This accords with oxygen deficiency at the sea bottom being initiated by short-term regressive events, which in turn would be consistent with cooling periods of glacial origin, as previous authors have suggested.