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1.
The hafnium isotope composition of Pacific Ocean water   总被引：1，自引：0，他引：1  
The first Hf isotope data for seawater are reported for a series of stations in the Northwestern Pacific and define a range from εHf = 3.5 ± 1.4 to 8.6 ± 1.6. Most samples have values within error of the average of εHf = 5.9, but significant variations are found in intermediate waters at a depth of 600 m, as well as in deep waters. The Nd and Hf isotope compositions of the deep waters fall within the range of values found for surfaces of hydrogenetic ferromanganese crusts in the region, confirming that Hf in the Fe-Mn crusts has been derived from the overlying water column, which thus provide an archive of past seawater compositions. Although the seawater samples are generally close to the global εNd-εHf correlation obtained from ferromanganese crusts, there are significant deviations from this correlation indicating that there is some additional decoupling between Nd and Hf isotope signals, most likely caused by local water mass mixing and differences in residence times. This is not resolved in the crust samples, which integrate seawater signals over 104 years. The combined use of these two isotope systems in seawater therefore provides an additional dimension for tracing water masses in the oceans. Studies of the distribution of oceanic Hf isotope compositions that have been confined to deep water and boundary waters, as recorded in seafloor ferromanganese crusts, can now be extended and aimed at characterising the entire present-day water column. Average Hf concentrations measured in this study are somewhat lower than previously reported, suggesting a shorter residence time for Hf in the global oceans, although the uncertainty in the extent of Hf removal from the water column during estuarine mixing as well as a lack of data on hydrothermal and dust inputs remains a limit on how well the residence time can be defined.  相似文献   

2.
The covariant behavior of Lu-Hf and Sm-Nd isotopes during most magmatic processes has long been recognized, but the details of this behavior in the depleted mantle reservoir have not been adequately examined. We report new whole-rock Hf and Nd isotope data for 1) juvenile, mantle-derived rocks, mid-Archean to Mesozoic in age, and 2) early Archean gneisses from West Greenland. Hf and Nd isotopic compositions of the juvenile rocks are well correlated, with the best fit corresponding to the equation εHf = 1.40 εNd + 2.1, and is similar to the collective Hf-Nd correlation for terrestrial samples of εHf = 1.36 εNd + 3.0. The early Archean Greenland gneisses, in contrast, have an extreme range in εNd values (−4.4 to +4.2; Bennett et al., 1993) that is not mirrored by the Hf isotopic system. The εHf values for these rocks are consistently positive and have much less variation (0 to +3.4) than their εNd counterparts.The information from the Hf isotopic compositions of the West Greenland gneisses portrays an early Archean mantle that is relatively isotopically homogeneous at 3.8 to 3.6 Ga and moderately depleted in incompatible elements. There is no evidence that any of these gneisses have been derived from an enriched reservoir. The Hf isotopic data are in stark contrast to the Nd isotopic record and strongly imply that the picture of extreme initial isotopic heterogeneity indicated by Nd isotopes is not a real feature of the West Greenland gneisses but is rather an artifact produced by disturbances in the Sm-Nd isotope system of these rocks.Although Hf and Nd isotopic data do not uniquely constrain either the nature of the earliest crust or the timing of crustal growth, the most probable candidate for the enriched reservoir complementary to the depleted mantle in the pre-4.0 Ga Earth is a mafic, oceanic-type crust. In order to explain the predominantly positive εHf and εNd values for the early Archean rocks, this crust must have had a short residence time at the surface of the Earth before returning to the mantle where it was isolated from mixing with the depleted mantle for several hundred million years. The following period from 3.5 to 2.7 Ga may mark a transition during which this early formed mafic crust was mixed progressively back into the depleted mantle reservoir. While a present-day volume of continental crust at 4.0 Ga cannot be excluded on isotopic grounds, we find such a scenario unlikely based on the lack of direct isotopic and physical evidence for its existence. An important aspect of crustal growth and evolution, therefore, may be the transformation of the enriched reservoir from being predominantly mafic in the early Earth to becoming progressively more sialic through time.  相似文献   

3.
The nature of the lower crust and tectonic setting of the Chinese Altai in the early to middle Paleozoic are still hotly debated. Decoupling between zircon Hf and whole-rock Nd isotopic systems for granites results in different interpretations for the above issues. In order to solve the problem, whole-rock Nd–Hf isotopic analyses were conducted on representative early to middle Paleozoic I-type granite and strongly peraluminous granites and rhyolites from the Chinese Altai. The I-type granites show metaluminous to weakly peraluminous feature and have εNd(t) values ranging from − 2.2 to + 0.8 and εHf(t) from + 3.9 to + 12.9, respectively. The strongly peraluminous granites and rhyolites have similar εNd(t) and εHf(t) values ranging from − 3.0 to + 1.7 and from + 2.1 to + 10.4, respectively. All samples plot above the Terrestrial Array on Nd–Hf isotopic diagram, indicating significant Nd–Hf isotopic decoupling in the magma sources. These samples show flatten HREE pattern and have Lu/Hf ratios similar to the average crust, suggesting that Nd–Hf isotopic decoupling was not originated from an ancient basement with elevated Lu/Hf ratios. The observed isotopic decoupling is similar to those modern island arcs, such as the Lesser Antilles and Sunda, where Nd selectively enriched over Hf due to metasomatism in the mantle wedge and consequently resulted in decoupling between the Sm–Nd and Lu–Hf isotopic systems. Our results, combined with the available data, show that prolonged subduction and crust–mantle interaction caused the Nd–Hf isotopic decoupling in the lithospheric mantle beneath the Chinese Altai. The crust of the Chinese Altai was extracted from the lithospheric mantle and inherited the Nd–Hf isotopic decoupling feature. Therefore, the Hf, rather than Nd, isotopic data more faithfully reflect the nature of the lower crust that was quite juvenile in the Paleozoic, and the Chinese Altai represents an early Paleozoic magmatic arc possibly built near western Mongolia.  相似文献   

4.
Hafnium isotopes in Arctic Ocean water   总被引：1，自引：0，他引：1  
The first isotopic compositions of dissolved hafnium in seawater from across the Arctic Ocean are reported. Most samples from the four sub-basins of the Arctic Ocean have values within error of an average of εHf = +0.8. Combined Hf-Nd isotope compositions do not fall on the well-established positive correlation for mantle and crustal rocks. Instead, Arctic waters have Hf that is more radiogenic than that typically found in rocks with similar Nd isotope compositions, a feature previously found in ferromanganese crusts and waters from the Pacific Ocean. Arctic seawater samples generally fall on the lower part of the ferromanganese crust array, reflecting influences of inputs from Arctic rivers and interactions of shelf waters with underlying sediments. Arctic rivers have much higher Hf concentrations (7-30 pM) than Arctic seawater (0.36-4.2 pM). Water from the Mackenzie River has the least radiogenic Hf, with εHf = −7.1 ± 1.7, and plots furthest away from the ferromanganese crust array, while waters from the Ob, Yenisey, and Lena Rivers have values that are indistinguishable from most Arctic waters. In the Amundsen, Makarov, and Canada basins, Hf concentrations are highest at the surface and lowest in the deeper waters, reflecting the influences of riverine inputs and of waters that have flowed over the extensive Siberian continental shelves and have Nd and Hf characteristics that reflect water-sediment interactions. This is in contrast to the relatively low near surface Hf concentrations reported for locations elsewhere. The Pacific water layer in the Canada Basin exhibits the highest value of εHf = +6.8 ± 1.8, reflecting the Hf isotopic composition of waters entering the Arctic from the Pacific Ocean. Mixing relationships indicate that a substantial fraction of the Hf in the Mackenzie River is lost during estuarine mixing; the behaviour of Hf from other rivers is less constrained.  相似文献   

5.
Neodymium isotopic compositions (εNd) have been largely used for the last fifty years as a tracer of past ocean circulation, and more intensively during the last decade to investigate ocean circulation during the Cretaceous period. Despite a growing set of data, circulation patterns still remain unclear during this period. In particular, the identification of the deep-water masses and their spatial extension within the different oceanic basins are poorly constrained. In this study we present new deep-water εNd data inferred from the Nd isotope composition of fish remains and Fe–Mn oxyhydroxide coatings on foraminifera tests, along with new εNd data of residual (partly detrital) fraction recovered from DSDP Sites 152 (Nicaraguan Rise), 258 (Naturaliste Plateau), 323 (Bellinghausen Abyssal Plain), and ODP Sites 690 (Maud Rise) and 700 (East Georgia Basin, South Atlantic). The presence of abundant authigenic minerals in the sediments at Sites 152 and 690 detected by XRD analyses may explain both middle rare earth element enrichments in the spectra of the residual fraction and the evolution of residual fraction εNd that mirror that of the bottom waters at the two sites. The results point towards a close correspondence between the bottom water εNd values of Sites 258 and 700 from the late Turonian to the Santonian. Since the deep-water Nd isotope values at these two sites are also similar to those at other proto-Indian sites, we propose the existence of a common intermediate to deep-water mass as early as the mid-Cretaceous. The water mass would have extended from the central part of the South Atlantic to the eastern part of proto-Indian ocean sites, beyond the Kerguelen Plateau. Furthermore, data from south and north of the Rio Grande Rise–Walvis Ridge complex (Sites 700 and 530) are indistinguishable from the Turonian to Campanian, suggesting a common water mass since the Turonian at least. This view is supported by a reconstruction of the Rio Grande Rise–Walvis Ridge complex during the Turonian, highlighting the likely existence of a deep breach between the Rio Grande Rise and the proto-Walvis Ridge at that time. Thus deep-water circulation may have been possible between the different austral basins as early as the Turonian, despite the presence of potential oceanic barriers. Comparison of new seawater and residue εNd data on Nicaraguan Rise suggests a westward circulation of intermediate waters through the Caribbean Seaway during the Maastrichtian and Palaeocene from the North Atlantic to the Pacific. This westward circulation reduced the Pacific water influence in the Atlantic, and was likely responsible for more uniform, less radiogenic εNd values in the North Atlantic after 80 Ma. Additionally, our data document an increasing trend observed in several oceanic basins during the Maastrichtian and the Palaeocene, which is more pronounced in the North Pacific. Although the origin of this increase still remains unclear, it might be explained by an increase in the contribution of radiogenic material to upper ocean waters in the northern Pacific. By sinking to depth, these waters may have redistributed to some extent more radiogenic signatures to other ocean basins through deep-water exchanges.  相似文献   

6.
海洋Nd同位素演化及古洋流循环示踪研究   总被引：2，自引：0，他引：2       下载免费PDF全文
海洋Nd同位素演化已经成为示踪陆源风化输入和洋流循环改变的最重要的手段之一,得到了越来越多的应用,并取得了许多重要的成果。海水的Nd同位素组成主要受陆源输入物质控制,热液输入几乎可以忽略。由于Nd在海洋中的停留时间(约500～1000a)略小于海水的平均混合时间(约1500a),且各洋盆有不同的Nd同位素风化输入,因此现代各大洋海水具有不同的Nd同位素组成。在陆源输入稳定的情况下,可以利用海水的Nd同位素组成和演化来示踪水体的混合或洋流循环的改变。目前主要依靠对海洋中水成铁锰结壳、海洋钙质有孔虫壳体、磷酸质鱼骨头或鱼牙齿化石以及沉积物中铁锰氧化物组分等的研究来恢复和反演古海水的Nd同位素组成和演化。4种分析材料各有其优缺点。其中,通过对水成铁锰结壳的Nd同位素分析,基本建立了各大洋新生代以来的主要洋流的Nd同位素组成的长尺度演化。通过有孔虫壳体、鱼化石碎片和沉积物中Fe-Mn氧化物组分可以进行高时间分辨率的古海水Nd同位素演化示踪。利用海水Nd同位素演化可以示踪古洋流通道的开启或闭合,以及获得水体交换的直接信息,为研究构造运动与气候变化之间的关系提供指示。同时,将海水Nd同位素演化与气候变化的指标结合起来,可以用于示踪各种气候条件下洋流循环的改变,将洋流循环的改变与气候变化联系起来,研究两者之间的成因关系。对表层水体的Nd同位素组成的研究则可以示踪不同气候条件下大陆陆源风化输入的改变。  相似文献   

7.
We present hafnium (Hf) and neodymium (Nd) isotopic compositions and concentrations in surface waters of the eastern Atlantic Ocean between the coast of Spain and South-Africa. These data are complemented by Hf and Nd isotopic and concentration data, as well as rare earth element (REE) concentrations, in Saharan dust.Hafnium concentrations range between a maximum of 0.52 pmol/kg in the area of the Canary Islands and a minimum value of 0.08 pmol/kg in the southern Angola Basin. Neodymium concentrations also show a local maximum in the area of the Canary Islands (26 pmol/kg) but are even higher between ∼20°N and ∼4°N reaching maximum concentrations of 35 pmol/kg. These elevated concentrations provide evidence of inputs from weathering of the Canary Islands and from the partial dissolution of dust from the Sahara/Sahel region. The inputs from ocean island weathering are also reflected in radiogenic Hf and Nd isotopes.The Hf isotopic compositions of dust samples themselves are highly variable, ranging between εHf = −20 and −0.6. The combined Hf and Nd isotopic compositions of dust plot close to the “terrestrial array” during periods of appreciable dust load in the atmosphere. During low atmospheric dust loading combined Hf and Nd isotopic compositions similar to seawater are observed. Most of the variability can be explained in terms of variable degrees of zircon loss from the dust samples, which in turn is linked to sorting during atmospheric transport to the eastern Atlantic Ocean and possibly presorting by sedimentary redistribution on the continent. In addition, increasing relative proportions of radiogenic clay minerals with decreasing grain size may contribute to the radiogenic Hf isotopic compositions observed.While the Nd isotopic composition in the surface ocean reflects the Nd isotopic composition of the Saharan dust adjacent to the Sahara/Sahel region, the release of Hf from that dust appears to be incongruent and results in surface ocean Hf isotopic compositions which are ∼10 εHf more radiogenic than the bulk dust. Radiogenic Hf appears to be released from clays and possibly from trace apatite. Rare earth element patterns of dust samples indicate the presence of apatite but provide no evidence for ferromanganese grain coatings, suggesting that such coatings are insignificant in the release of Hf and Nd from Saharan dust to the surface ocean.The Nd isotopic composition of the surface waters becomes less radiogenic south of the equator, most likely reflecting the release of Nd from Congo river sediments. The release of Hf from Saharan dust and the Congo river sediments, however, does not produce distinct Hf isotopic signatures in the surface ocean, implying that the mobile fraction of Hf integrated over large continental areas is isotopically uniform. The Hf isotopic uniformity in the surface ocean means that the limited variability in deep water isotopic compositions is consistent with a short deep water residence time and reflects homogenous continental inputs rather than efficient deep water homogenization.  相似文献   

8.
http://www.sciencedirect.com/science/article/pii/S1674987113000820   总被引：1，自引：1，他引：0  
The combined use of Hf,Nd and Sr isotopes is more useful in understanding the supercontinent cycle than the use of only Hf isotopic data from detrital zircons.Sr and Nd seawater isotopes,although not as precise as εNd and εHf distributions,also record input from ocean ridge systems.Unlike detrital zircons where sources cannot be precisely located because of crustal recycling,both the location and tectonic setting often can be constrained for whole-rock Nd isotopic data.Furthermore,primary zircon sources may not reside on the same continent as derivative detrital zircons due to supercontinent breakup and assembly.Common to all of the isotopic studies are geographic sampling biases reflecting outcrop distributions,river system sampling,or geologists,and these may be responsible for most of the decorrelation observed between isotopic systems.Distributions between 3.5 and 2 Ga based on εHf median values of four detrital zircon databases as well as our compiled εNd database are noisy but uniformly distributed in time,whereas data between 2 and 1 Ga data are more tightly clustered with smaller variations.Grouped age peaks suggest that both isotopic systems are sampling similar types of orogens.Only after 1 Ga and before 3.5 Ga do we see wide variations and significant disagreement between databases,which may partially reflect variations in both the number of sample locations and the number of samples per location.External and internal orogens show similar patterns in εNd and εHfwith age suggesting that both juvenile and reworked crustal components are produced in both types of orogens with similar proportions.However,both types of orogens clearly produce more juvenile isotopic signatures in retreating mode than in advancing mode.Many secular changes in εHf and εNd distributions correlate with the supercontinent cycle.Although supercontinent breakup is correlated with short-lived decreasing εHf and εNd （≤ 100 Myr） for most supercontinents,there is no isotopic evidence for the breakup of the Paleoproterozoic supercontinent Nuna.Assembly of supercontinents by extroversion is recorded by decreasing εNd in granitoids and metasediments and decreasing εHf in zircons,attesting to the role of crustal reworking in external orogens in advancing mode.As expected,seawater Sr isotopes increase and seawater Nd isotopes decrease during supercontinent assembly by extroversion.Pangea is the only supercontinent that has a clear isotopic record of introversion assembly,during which median εNd and εHf rise rapidly for ≤ 100 Myr.Although expected to increase,radiogenic seawater Sr decreases （and seawater Nd increases） during assembly of Pangea,a feature that may be caused by juvenile input into the oceans from new ocean ridges and external orogens in retreating mode.The fact that a probable onset of plate tectonics around 3 Ga is not recorded in isotopic distributions may be due the existence of widespread felsic crust formed prior to the onset of plate tectonics in a stagnant lid tectonic regime,as supported by Nd and Hf model ages.  相似文献   

9.
《Geochimica et cosmochimica acta》1999,63(11-12):1689-1708
We compare the time series of major element geochemical and Pb- and Nd-isotopic composition obtained for seven hydrogenous ferromanganese crusts from the Atlantic, Indian, and Pacific Oceans which cover the last 60 Myr.Average crust growth rates and age–depth relationships were determined directly for the last about 10 Myr using 10Be/9Be profiles. In the absence of other information these were extrapolated to the base of the crusts assuming constant growth rates and constant initial 10Be/9Be ratios due to the lack of additional information. Co contents have also been used previously to estimate growth rates in Co-rich Pacific and Atlantic seamount crusts (Puteanus and Halbach, 1988). A comparison of 10Be/9Be- and Co-based dating of three Co-rich crusts supports the validity of this approach and confirms the earlier chronologies derived from extrapolated 10Be/9Be-based growth rates back to 60 Ma. Our data show that the flux of Co into Co-poor crusts has been considerably lower. The relationship between growth rate and Co content for the Co-poor crusts developed from these data is in good agreement with a previous study of a wider range of marine deposits (Manheim, 1986). The results suggest that the Co content provides detailed information on the growth history of ferromanganese crusts, particularly prior to 10–12 Ma where the 10Be-based method is not applicable.The distributions of Pb and Nd isotopes in the deep oceans over the last 60 Myr are expected to be controlled by two main factors: (a) variations of oceanic mixing patterns and flow paths of water masses with distinct isotopic signatures related to major paleogeographic changes and (b) variability of supply rates or provenance of detrital material delivered to the ocean, linked to climate change (glaciations) or major tectonic uplift. The major element profiles of crusts in this study show neither systematic features which are common to crusts with similar isotope records nor do they generally show coherent relationships to the isotope records within a single crust. Consequently, any interpretation of time series of major element concentrations of a single crust in terms of paleoceanographic variations must be considered with caution. This is because local processes appear to have dominated over more basin-wide paleoceanographic effects. In this study Co is the only element which shows a relationship to Pb and Nd isotopes in Pacific crusts. A possible link to changes of Pacific deep water properties associated with an enhanced northward advection of Antarctic bottom water from about 14 Ma is consistent with the Pb but not with the Nd isotopic results. The self-consistent profiles of the Pb and Nd isotopes suggest that postdepositional diagenetic processes in hydrogenous crusts, including phosphatization events, have been insignificant for particle reactive elements such as Pb, Be, and Nd. Isotope time series of Pb and Nd show no systematic relationships with major element contents of the crusts, which supports their use as tracers of paleo-seawater isotopic composition.  相似文献   

10.
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 εNd values at depths of 800-1000 m (εNd = −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 εNd 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 εNd 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.  相似文献   

11.
The Weihai migmatite in the Sulu ultra-high-pressure (UHP) metamorphic terrane, eastern China, underwent partial melting in the Late Triassic during its exhumation. The primary partial melts experienced a decompressional fractional crystallization (DFC) process to produce plagioclase (Pl)-rich leucosome crystallized under eclogite to granulite facies conditions and K-feldspar (Kfs)-rich pegmatitic veins crystallized under amphibolite-facies conditions. In this study, our results demonstrate that the DFC process can cause decoupling between whole-rock Sr and Nd isotopes. The Pl-rich leucosome has εNd(t) values (–10.4 to ?15.0) and initial (87Sr/86Sr) ratios (0.708173–0.712476) very similar to those of the melanosome, but the Kfs-rich pegmatitic veins have homogeneous εNd(t) values (?14.8 to ?15.2) and significantly high initial (87Sr/86Sr) ratios (0.713882–0.716284). Our results also suggest that the DFC process can change zircon 176Yb/177Hf and 176Lu/177Hf isotopic ratios, with no effect on 176Hf/177Hf ratios or εHf (t) values. Zircon 176Yb/177Hf and 176Lu/177Hf ratios increase dramatically from the Pl-rich leucosome to the Kfs-rich pegmatitic veins, but zircon 176Hf/177Hf ratios (Pl-rich leucosomes = 0.282330 ± 0.000017; Kfs-rich pegmatitic veins = 0.282321 ± 0.000026) and εHf (t) values (Pl-rich leucosomes = ?10.9 ± 0.6; Kfs-rich pegmatitic veins = ?11.6 ± 0.8) remain almost unchanged. We propose that the isotopic decoupling between the Pl-rich leucosome and Kfs-rich pegmatitic vein might be caused by melt fractional crystallization occurring too rapidly to allow complete equilibrium between them.  相似文献   

12.
The subduction factories in convergent plate margins exert crucial control on recycling terrestrial components and returning to the overlying crust. The Nd and Hf isotopic systems provide potential tracers to evaluate these processes. Here we present a case where these isotopic systems are decoupled in a suite of granites from the Chinese Altai, showing a wide range of εHf(t) values(from -4.7 to +10.8) in contrast to a limited range of εNd(t) values(from -5.8 to -1.9). The zircon xenocrysts occurring frequently in these rocks show markedly negative εHf(t) values(from -34.3 to -6.5) and positive d7 Li values(from +12.5 to +18.2). We propose a model to explain the observed relationship between residual zircon and Nde Hf isotope decoupling. We suggest that the Altai granites originated from partial melting of subducted slab components under relatively low temperature conditions which aided the residual zircon from oceanic sediments to inherit and retain a significant amount of177 Hf in the source, thereby elevating the176 Hf/177 Hf ratio of the melt, and decoupling from the143 Nd/144 Nd ratio during the subsequent magmatic processes. Our study illustrates a case where sediment recycling in subduction zone contributes to decoupling of Nd and Hf isotopic systems, with former providing a more reliable estimate of the source characteristics of granitic magmas.  相似文献   

13.
The Santa Catalina Mountains, SE Arizona, was one of the first metamorphic core complexes to be described. Despite its status as a type example, relatively little is known about precise ages and origins of the intrusive rocks that make up most of the crystalline core. U–Pb and Hf isotopic data by laser ablation–inductively coupled plasma–mass spectrometry from zircons and Nd isotopic results from whole rocks were obtained for 12 granitoids ranging from 1,440 to 26 Ma. Results confirm that the 1.44-Ga Oracle Granite extends through the Catalina Range as variably mylonitic granite and banded gneiss. Laramide intrusions (67–73 Ma) display initial εNd values ?5 to ?8 and εHf from ?7.5 to ?9. Magmatic ages for the prominent white granite sills of the Wilderness suite are 46–57 Ma, in agreement with Terrien (2012), and these granites have initial εNd values ?8 to ?10 and εHf from ?7 to ?14. Lastly, the undeformed Catalina Granite has an age of 26 Ma, with an initial εNd and εHf of ?6 and ?8, respectively. Our Nd results agree with limited results from Farmer and DePaolo (89:10141–10160, 1984). Although the Catalina Granite seems to have a significant juvenile component based on Nd and Hf, most of the Laramide and Wilderness intrusions contain Nd and Hf compositions lying close to the evolution of 1.44-Ga Oracle Granites, a fact that is confirmed by the U–Pb data, which show both 1.7- and 1.4-Ga zircon cores in these samples, with 1.4 Ga as the dominant core age. In order to become the dominant source of most of the 72–45-Ma magmas, the Oracle pluton must not only extend across the whole Catalina region, but also have abundant deep-seated equivalents to provide magma sources.  相似文献   

14.
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 226Raex/Ba dating reaches from recent growth back to ∼4300 and 1000 yr BP, respectively. Over this time range, the Nd isotope composition varies from εNd (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 εNd 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 (εNd about −22) to the Baltic Sea or a decrease of the input of more radiogenic Nd from continental European sources (εNd about −12) and/or North Sea water (εNd 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 εNd 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.  相似文献   

15.
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 εSr 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 εSr 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 εNd and εHf of the saprolites drift only 0–2.6 and 0–3.7 away from the parent rocks, respectively. The negative drifts of the εNd and εHf are coupled with Nd and Hf losses in the saprolites; i.e., larger proportions of Nd and Hf loss correspond to lower εNd and εHf. 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 εNd and εHf 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 εNd and εHf 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 εHf values at a given εNd value, and the regression line between these εNd and εHf 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 εNd and εHf correlation. Our results also indicate that caution is needed when using εSr, εNd, and εHf to trace provenances for sediments and soils.  相似文献   

16.
The Mesozoic tectonic architecture of the Korean peninsula is largely governed by the continental collision between the North and South China blocks. Zircon Hf isotopic compositions presented in this study and whole‐rock geochemical and Sr‐Nd isotope data in the literature collectively suggest that the lithophile‐elements‐enriched signature of the Late Triassic post‐collisional plutons from the Gyeonggi massif in central Korea is a primary feature inherited from the metasomatized mantle lithosphere. Highly negative zircon εHf (t) values (?23 to ?19) of plutons from the middle and eastern parts of the massif indicate an ancient metasomatism of their mantle source. Distinctly higher zircon εHf (t) values (?15 to ?12) from the southwestern plutons are ascribed to a contribution from an accreted component of the South China‐like block. The involvement of asthenospheric mantle is not recognized in zircons from either group. The implications of these isotopic features are discussed in the context of Mesozoic collisional tectonics.  相似文献   

17.
Four vertical profiles of the concentration and isotopic composition of Nd in seawater were obtained in the western North Pacific. Two profiles from the Kuroshio Current regime showed congruently that although the Nd concentration increases gradually with depth, its isotopic composition varies significantly with depth depending upon the water mass occupying the water column. The high-salinity Kuroshio waters originating from the North Pacific Tropical Water (NPTW) carry the least radiogenic Nd (?Nd = −7.4 to −8.7) to this region at ∼250 m from the western margin continental shelves, most likely from the East China Sea. The Nd isotopic compositions in the North Pacific Intermediate Water (NPIW) that occurs at 600 to 1000 m in the subtropical region are fairly uniform at ?Nd = −3.7. The profile data from the ∼38° to 40°N Kuroshio/Oyashio mixed water region off Sanriku of Honshu, Japan, also suggest that the newest NPIW with ?Nd = −3.2 is formed there by the mixing of various source waters, and the radiogenic component of Nd is derived mainly from the Oyashio waters.In the Pacific Deep Water (PDW) below ∼1000 m, the Nd isotopic composition is neither vertically nor horizontally homogeneous, suggesting that it serves as a useful tracer for sluggish deep water circulation as well. Two profiles from the Izu-Ogasawara Trench showed a minimum ?Nd value at ∼2000 m, suggesting that there exists a horizontal advective flow in the vicinity of Honshu, Japan. There is some evidence from other chemical properties to support this observation. The waters below 4000 m including those within the trench in the subtropical region have ?Nd values of around −5, suggesting that the deep waters are fed from the south along the western boundary, ultimately from the Antarctic Bottom Water (AABW) in the South Pacific. This extends up to ∼40°N along the Japanese Islands. In the subarctic region (>∼42°N), the waters have more radiogenic Nd with ?Nd > −4.0 throughout the water column, presumably due to the supply of Nd by weathering in such igneous provinces as the Kuril-Kamchatska-Aleutian Island chain. The lateral inhomogeneity of the Nd isotopic composition in PDW suggests that there may be different circulation and mixing regimes in the North Pacific Basin.  相似文献   

18.
Growing evidence from the accessible geological record reveals that crust-mantle differentiation on Earth started as early as 4.4 Ga. In order to assess the extent of early Archean mantle depletion, we obtained 176Lu-176Hf, 147Sm-143Nd, and high field strength element (HFSE) concentration data for the least altered, well characterized boninite-like metabasalts and associated metasedimentary rocks from the Isua supracrustal belt (southern West Greenland). The metasediments exhibit initial εHf(3720) values from −0.7 to +1.5 and initial εNd(3720) values from +1.6 to +2.1. Initial εHf(3720) values of the least altered boninite-like metabasalts span a range from +3.5 to +12.9 and initial εNd(3720) values from −0.3 to +3.2. These initial Hf-isotope ratios display coherent trends with SiO2, Al2O3/TiO2 and other relatively immobile elements, indicating contamination via assimilation of enriched components, most likely sediments derived from the earliest crust in the region. This model is also consistent with previously reported initial γOs(3720) values for some of the samples. In addition to the positive εHf(3720) values, the least disturbed samples exhibit positive εNd(3720) values and a co-variation of εHf(3720) and εΝd(3720) values. Based on these observations, it is argued, that the most depleted samples with initial εHf(3720) values of up to +12.9 and high 176Lu/177Hf of ∼0.05 to ∼0.09 tap a highly depleted mantle source with a long term depletion history in the garnet stability field. High precision high field strength element (HFSE) data obtained for the Isua samples confirm the contamination trend. Even the most primitive samples display negative Nb-Ta anomalies and elevated Nb/Ta, indicating a subduction zone setting and overprint of the depleted mantle sources by felsic melts generated by partial melting of eclogite. Collectively, the data for boninite-like metabasalts support the presence of strongly depleted mantle reservoirs as previously inferred from Hf isotope data for Hadean zircons and combined 142Nd-143Nd isotope data for early Archean rocks.  相似文献   

19.
Nd–Hf isotopic decoupling has frequently been observed in the continental and oceanic mantle, but its origin remains controversial. Here we present combined elemental and Sr–Nd–Hf isotopic study on peridotite xenoliths entrained in Cenozoic basalts from Shuangliao and Jiaohe in Northeast China, which provides insight into this issue. The data reveal a heterogeneous lithospheric mantle beneath Northeastern China, consisting of fertile (type I) to strongly refractory (type II) peridotites. Type I peridotites are largely shielded from late metasomatism, thus preserving information of depletion events. Nd model age suggests a Proterozoic lithospheric mantle beneath NE China. Type II peridotites are mostly refractory harzburgites and show ubiquitous enrichment of incompatible elements. They are further divided into two sub-groups. Clinopyroxenes from type IIa samples have high and wide Lu/Hf (0.34–1.3) and very radiogenic Hf isotopic ratios (εHf = 44.4–63.8). Hf concentration is generally low (0.12–0.43 ppm) and plots along or slightly above the modeled partial melting depletion trend. In contrast, Nd content in type IIa clinopyroxenes is significantly higher than the modeled concentrations in residues at a given degree of melt depletion. The difference in enrichment of Hf and Nd translates to decoupling of Lu/Hf–Sm/Nd ratios and of Nd–Hf isotopes (εNd = −1.3 to 8.4). Clinopyroxenes from most of type IIb peridotites have relatively low Lu/Hf ratios (0.04–0.24) and coupled Nd–Hf isotopes. Both Hf and Nd plot significantly above the depletion trend; their concentrations are governed by the equilibrium partitioning between percolating melt and peridotites. The distinct geochemical characteristics of type IIa and type IIb clinopyroxenes may have resulted from chromatographic percolation of small volumes of silicate melts, in which percolation fronts of incompatible elements are dependent on their relative incompatibilities.  相似文献   

20.
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 ԐHf vs. ԐNd 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 TDM 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.  相似文献   

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