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1.
Stable Pb isotope profiles in dated lake sediment cores were used to gauge the relative amounts and possible sources of anthropogenic Pb deposited from the atmosphere in different regions of the Canadian Arctic. A distinct north-south difference was found. In four High Arctic lakes (i.e., north of 66°N) in this study, recent Pb isotopic shifts or concentration increases attributable to anthropogenic Pb were negligible. The maximum possible contribution from anthropogenic Pb was 0 to 19% of acid-leachable Pb in the 1980s or 1990s. In contrast, two lakes in the Hudson Bay region displayed significantly lower Pb isotope ratios and threefold to fivefold increases of Pb concentrations in modern sediments, corresponding to anthropogenic Pb inputs of at least 72 to 91% of leachable Pb. Eurasian urban and industrial Pb is known to dominate the High Arctic atmosphere. A possible explanation for its negligible influence on northern lake sediments is that atmospheric Pb deposition at northern latitudes is reduced compared with southern regions and is small compared to local geological inputs. 210Pb deposition declines with increasing latitude, apparently because of declining precipitation rates; stable Pb deposition may be similarly affected. Meteorological considerations and variations in the post-1900 Pb isotopic trajectories indicated that the predominant anthropogenic Pb source region in NW Hudson Bay was Eurasia, while in SE Hudson Bay, it was Canada and the United States, with a minor Eurasian component.  相似文献   

2.
Lead concentrations and stable lead isotopes (204Pb, 206Pb, 207Pb, 208Pb) were measured in forest moss samples (Pleurozium schreberi or Scleropodium purum) collected at 273 sites across the Czech Republic during 2010. Continuously decreasing median Pb concentrations in moss were documented over the last two decades: 1995: 11 mg/kg, 2000: 5.66 mg/kg, 2005: 4.94 mg/kg and 2010: 2.85 mg/kg. Several local anomalies have decreased in scale, the overall regional distribution patterns remained, however, the same. The regional Pb isotope ratio distributions show that the ratios show little variation for a large central part of the country and provide the large-scale background isotope ratios for the Czech Republic of about 204Pb/206Pb = 0.0550, 206Pb/207Pb = 1.167, 206Pb/208Pb = 0.478 and 207Pb/208Pb = 0.409 for 2010. This background Pb isotope ratio signal in moss has been locally (900–7500 km2) modified by specific Pb isotopic ratio signals caused by deposition of Pb emissions from known local anthropogenic Pb emission sources, such as industrial combustion of local coal, and a variety of industrial enterprises (metallurgical, engineering and glass works). At some sites where mining of uranium and polymetallic ores took place the moss samples show also a locally specific Pb isotope signal. The in terms of area affected largest deviations in the Pb-isotope ratios, e.g., in the Bohemian Massif, may be due to the input of geogenic dust.  相似文献   

3.
Isotopic analysis of nitrate and sulfate minerals from the nitrate ore fields of the Atacama Desert in northern Chile has shown anomalous 17O enrichments in both minerals. Δ17O values of 14-21 ‰ in nitrate and 0.4 to 4 ‰ in sulfate are the most positive found in terrestrial minerals to date. Modeling of atmospheric processes indicates that the Δ17O signatures are the result of photochemical reactions in the troposphere and stratosphere. We conclude that the bulk of the nitrate, sulfate and other soluble salts in some parts of the Atacama Desert must be the result of atmospheric deposition of particles produced by gas to particle conversion, with minor but varying amounts from sea spray and local terrestrial sources. Flux calculations indicate that the major salt deposits could have accumulated from atmospheric deposition in a period of 200,000 to 2.0 M years during hyper-arid conditions similar to those currently found in the Atacama Desert. Correlations between Δ17O and δ18O in nitrate salts from the Atacama Desert and Mojave Desert, California, indicate varying fractions of microbial and photochemical end-member sources. The photochemical nitrate isotope signature is well preserved in the driest surficial environments that are almost lifeless, whereas the microbial nitrate isotope signature becomes dominant rapidly with increasing moisture, biologic activity, and nitrogen cycling. These isotopic signatures have important implications for paleoclimate, astrobiology, and N cycling studies.  相似文献   

4.
The combination of the Sr, Nd and Pb isotope systems, recognized as tracers of sources, with the Ca isotope system, known to reveal biology-related fractionations, allowed us to test the reliability of spruce (Picea abies) growth rings as environmental archives through time (from 1916 to 1983) in a forest ecosystem affected by acid atmospheric deposition. Sr and Pb isotopes have already been applied in former tree-ring studies, whereas the suitability of Nd and Ca isotope systems is checked in the present article. Our Sr and Nd isotope data indicate an evolution in the cation origin with a geogenic origin for the oldest rings and an atmospheric origin for the youngest rings. Ca isotopes show, for their part, an isotopic homogeneity which could be linked to the very low weathering flux of Ca. Since this flux is weak the spruces’ root systems have pumped the Ca mainly from the organic matter-rich top-soil over the past century. In contrast, the annual growth rings studied are not reliable and suitable archives of past Pb pollution.  相似文献   

5.
Basanites and nephelinites from the Tertiary Rhön area (Germany), which are part of the Central European Volcanic Province (CEVP), have high MgO, Ni and Cr contents and prominent garnet signatures indicating that they represent near-primary magmas formed by melting of a CO2-bearing peridotitic mantle source at high pressure. The Pb and Hf isotope (and previously published Nd and Sr isotope) ratios of the Rhön lavas are rather uniform, whereas the Os isotope composition is highly variable. For the most primitive basanites, Pb, Os and Hf isotope compositions fall within the range of enriched MORB and some OIB. Other basanites and nephelinites with low Os concentrations have distinctly more radiogenic Os (187Os/188Os: 0.160–0.469) isotope compositions, which are inferred to originate from crustal contamination. The samples with the highest Os concentrations have the lowest Os isotope ratios (187Os/188Os(23 Ma): 0.132–0.135), and likely remain unaffected by crustal contamination. Together with their fairly depleted Sr, Nd and Hf isotope ratios, the isotopic composition of the Rhön lavas suggests derivation from an asthenospheric mantle source. Prominent negative K and Rb anomalies, however, argue for melting amphibole or phlogopite-bearing sources, which can only be stable in the cold lithosphere. We therefore propose that asthenospheric melts precipitated at the asthenosphere-lithosphere thermal boundary as veins in the lithospheric mantle and were remelted or incorporated after only short storage times (about 10–100 million years) by ascending asthenospheric melts. Due to the short residence time incorporation of the vein material imposes the prominent phlogopite/amphibole signature of the Rhön alkaline basalts but does not lead to a shift in the isotopic signatures. Melting of the lithospheric mantle cannot strictly be excluded, but has to be subordinate due to the lack of the respective isotope signatures, in good agreement with the fairly thin lithosphere observed in the Rhön area. The fairly radiogenic Pb isotope signatures are expected to originate from melting of enriched, low melting temperature portions incorporated in the depleted upper (asthenospheric) mantle and therefore do not require upwelling of deep-seated mantle sources for the Rhön or many other continental alkaline lavas with similar Pb isotope signatures.  相似文献   

6.
A recent survey by [Bollh?fer and Rosman 2000] and [Bollh?fer and Rosman 2001] has defined the extent to which Pb isotopic ratios in aerosols vary on a global scale. However, it is also important for some applications to know how stable these signatures are. Here we report time series from 38 sites distributed worldwide in which aerosols have been sampled for periods of between 4 months and 4 yr. Apart from a few sites that have atypical conditions, European sites exhibit variations of <0.6% in the 206Pb/207Pb ratio. There is, however, evidence of seasonal variations at sampling sites closer to Eastern Europe that probably reflect an enhanced westward transport of pollution in winter. The variability in Canada and the United States is now larger than before due to a decrease of airborne Pb levels coupled with an increase in the variety of industrial sources. The temporal changes observed in the United States do not exhibit a seasonal pattern. One site in Winnipeg, Canada, showed an extremely large variation, probably the result of seasonal changes influencing the direction of movement of local smelting emissions. Temporal variations in mainland Australia are comparatively small, with a typical range of 0.2% in the 206Pb/207Pb ratio and isotopic ratios that indicate leaded petrol was still a major source of atmospheric Pb over the sampling period.  相似文献   

7.
若尔盖高原牧场处于中国偏远洁净高海拔地区,大气沉降是污染物主要来源途径之一。由于季风的影响,污染源的辨析较为困难。本文通过多点大气气溶胶不同季节同时采样方式,利用热电离固体同位素质谱仪可有效校正质谱分析中同位素分馏效应的优点,对若尔盖地区土壤和大气气溶胶的铅同位素比值进行精确分析,并结合季风特征对该地区污染物的来源进行解析。结果表明:土壤的208Pb/204Pb比值变化范围为38.79059±0.00194~38.94461±0.00135,206Pb/207Pb为1.18551±0.00002~1.19362±0.00002;大气气溶胶的208Pb/204Pb比值变化范围为37.49571±0.00117~38.48980±0.00105,206Pb/207Pb为1.12894±0.00001~1.16734±0.00001。该地区土壤铅同位素的特征是放射成因铅高,来自于自身天然存在的岩石矿物,与大气污染关系不大;大气气溶胶的铅同位素组成与土壤差异较大,显示为多元混合模式,受到了天然物质和人类活动来源的混合影响,机动车尾气及来自北部(兰州)和西北部(青海、新疆、哈萨克斯坦、俄罗斯)的大气远程运移是若尔盖大气气溶胶及污染物质的主要来源。  相似文献   

8.
Soil O and C horizon samples (N = 752) were collected at a sample density of 1 site/36 km2 in Nord-Trøndelag and parts of Sør-Trøndelag (c. 25,000 km2), and analysed for Pb and three of the four naturally occurring Pb isotopes (206Pb, 207Pb and 208Pb) in a HNO3/HCl extraction. Soil O and C horizons are decoupled in terms of both Pb concentrations and Pb isotope ratios. In the soil C horizon the Grong-Olden Culmination, a continuous exposure of the Precambrian crystalline basement across the general grain of the Caledonian orogen, is marked by a distinct 206Pb/207Pb isotope ratio anomaly. No clear regional or even local patterns are detected when mapping the Pb isotope ratios in the soil O horizon samples. Variation in the isotope ratios declines significantly from the soil C to the O horizon. On average, Pb concentrations in the O horizon are four times higher and the 206Pb/207Pb isotope ratio is shifted towards a median of 1.15 in comparison to 1.27 in the C horizon. It is demonstrated that natural processes like weathering in combination with plant uptake need to be taken into account in order to distinguish anthropogenic input from natural influences on Pb concentration and the 206Pb/207Pb isotope ratio in the soil O horizon.  相似文献   

9.
Lead isotope signatures (207Pb/206Pb, 208Pb/206Pb, 208Pb/204Pb, 206Pb/204Pb), determined by magnetic sector ICP-MS in river channel sediment, metal ores and mine waste, have been used as geochemical tracers to quantify the delivery and dispersal of sediment-associated metals in the lower Danube River catchment. Due to a diverse geology and range of ore-body ages, Pb isotope signatures in ore-bodies within the lower Danube River catchment show considerable variation, even within individual metallogenic zones. It is also possible to discriminate between the Pb isotopic signatures in mine waste and river sediment within river systems draining individual ore bodies. Lead isotopic data, along with multi-element data; were used to establish the provenance of river sediments and quantify sedimentary contributions to mining-affected tributaries and to the Danube River. Data indicate that mining-affected tributaries in Serbia and Bulgaria contribute up to 30% of the river channel sediment load of the lower Danube River. Quantifying relative sediment contributions from mining-affected tributaries enables spatial patterns in sediment-associated metal and As concentrations to be interpreted in terms of key contaminant sources. Combining geochemical survey data with that regarding the provenance of contaminated sediments can therefore be used to identify foci for remediation and environmental management strategies.  相似文献   

10.
The Pb concentrations of atmospheric aerosol in the Chukchi Sea of the Arctic vary within the range of 0.167-0.962ng/m^2,with an average of 0.532ng/m^3,These concentration values are 200 times higher than the natural background values of snow samples there.Calculation of the Pb enrichment factor of aerosol indicates that the ocean-and continent-source lead account for 9.23% and 0.01%,respectively,but industrially released Pb accounts for more than 90% of the atmospheric Pb.The Pb isotopic composition of aerosol has revealed that the sources of lead from industrial lead that causes pollution include mainly the western part of North America,East Europe and the former Soviet union.The calculation of the total fallout flux of Pb indicates that the mean value of input flux into the Chukchi Sea is 0.02mgm^-2a^-1,equivalent to that of southern Pacific but slightly lower than that of northern Indian Sea and southern Atlantic.It is evidenced that the Pb input flux into the Chukchi Sea is far lower than that off the Baltic Sea,the North Sea and the Mediterranean Sea.  相似文献   

11.
Rock, soil, and plant (terrestrial moss, European mountain ash leaves, mountain birch leaves, bark and wood, and spruce needles and wood) samples, collected at 3 km intervals along a 120 km long transect (40 sites) cutting the city of Oslo, Norway, were analysed for their Pb concentration and Pb-isotope ratios. A general decrease in 206Pb/207Pb, 208Pb/207Pb and 206Pb/208Pb ratios, with a characteristic low variability in all plant materials and the plant-derived O-horizon of soil profiles, compared to rocks and mineral soils, is observed along the transect. It is demonstrated that minerogenic and biogenic sample materials belong to two different spheres, the lithosphere and biosphere, and that geochemical processes determining their chemical and isotopic compositions differ widely. Background variation for both sample materials needs to be established and documented at the continental and global scale before the anthropogenic influence on the geochemistry of the earth’s surface can be reliably estimated.  相似文献   

12.
《Applied Geochemistry》2006,21(4):563-579
Element concentrations, element ratios and Pb and Zn isotope data are reported for different geologic samples (barren and ore-bearing granites and host rocks), technogenic products (ore concentrates and tailings) and biologic samples (lichens and birch leaves) from the Orlovka–Spokoinoe mining district, Eastern Transbaikalia, Russia, with the aim to trace the sources of Pb and Zn at a local level within the mining site. Lichens and birch leaves were used as receptors of contamination within the mining site. Pb/Zr and Zn/Zr values indicated Pb and Zn enrichment relative to host rocks. Zn isotope data of 15 geologic and 11 lichen samples showed different Zn isotopic signatures with the total range for the geologic suite of −0.4‰ to +1.2‰ and for lichens of +0.4‰ to +1.4‰ in δ66Zn relative to Lyon JMC Zn standard. The source of isotopically heavy Zn within the Orlovka–Spokoinoe mining site could be potentially associated with long-range atmospheric aerosols that also contributed Pb to the studied mining site. Our results demonstrated that Zn isotopes might be used as new tools for Zn source assessment.  相似文献   

13.
The Pb, Sr and Nd isotopic compositions of biomonitors (lichen, moss, bark) and soil litter from different regions in the Rhine valley, as well as of <0.45 μm particles separated out of ice of the Rhône and Oberaar glaciers and lichens from the Swiss Central Alps, have been determined in order to deduce the natural baseline of the atmospheric isotopic compositions of these regions, which are suggested to be close to the isotopic compositions of the corresponding basement rocks or soils at the same sites. 206Pb/207Pb and 87Sr/86Sr isotope ratios are positively correlated. Most polluted samples from traffic-rich urban environments have the least radiogenic Pb and Sr isotopic compositions with 206Pb/207Pb and 87Sr/86Sr ratios of 1.11 and 0.7094, respectively. These ratios are very different from those of the atmospheric baseline for the Vosges mountains and the Rhine valley (206Pb/207Pb: 1.158–1.167; 87Sr/86Sr: 0.719–0.725; εNd: −7.5 to −10.1). However, this study indicates that the baseline of the atmospheric natural Pb and Sr isotopic compositions is affected by anthropogenic (traffic, industrial and urban) emissions even in remote areas. Lichen samples from below the Rhône and Oberaar glaciers reflect the baseline composition close to the Grimsel pass in the Central Swiss Alps (87Sr/86Sr: 0.714 − 0.716; εNd: −3.6 to −8.1). The 143Nd/144Nd isotope ratios are highly variable (8ε units) and it is suggested that the variation of the 143Nd/144Nd is controlled by wet deposition and aerosols originating from the regional natural and industrial urban environments and from more distant regions like the Sahara in North Africa. The least anthropogenetically affected samples collected in remote areas have isotopic compositions closest to those of the corresponding granitoid basement rocks.  相似文献   

14.
As part of the AIRMoN program, daily precipitation samples have been collected since the early 1990s throughout the eastern and central United States. Using precipitation stable isotope (δ18O and δD) and HYSPLIT back trajectory analysis of 591 samples collected by the AIRMoN program from Florida, Illinois, Pennsylvania, Tennessee, Vermont, and West Virginia, amount weighted seasonal average isotopic compositions were calculated for precipitation from Continental, Pacific, Gulf of Mexico, Arctic, North Atlantic, and Mid Atlantic sources. Our results suggest that these sources are isotopically distinct and variable at and among most sites during most seasons. However, in many instances, the isotopic differences of the sources require dramatic changes in precipitation amounts from these sources to modify the seasonal average isotopic composition at a particular site. The relative importance of each source type to the seasonal average isotopic composition is highly variable among and within sites. The largest differences in the isotopic compositions of different sources are in winter and spring precipitation from the high latitude sites. At the Vermont location, the seasonal average isotopic composition is potentially quite sensitive to the relative amounts of precipitation from Arctic and Gulf of Mexico sources.  相似文献   

15.
《Applied Geochemistry》2001,16(2):207-229
The environment surrounding Palmerton, Pennsylvania is contaminated with Pb arising from primary Zn smelting and a process involving Zn recovery from electric arc steel furnace dusts. Lead isotope systematics have been used to distinguish primary Zn smelting Pb (206Pb/204Pb∼18.4–18.5) from electric arc furnace dust lead (206Pb/204Pb∼19.0–19.1). Primary Zn smelting is the dominant source of Pb in O2 horizon soils from undisturbed near-Palmerton locations, which contain up to 3570 ppm Pb and 782 ppm Cd. Soils from undeveloped near-Palmerton locations also exhibit unusually elevated concentrations of other sphalerite-derived chalcophilic elements (Se, Ag, In, Sb, Te, Au, Hg, Tl and Bi); indium concentrations of up to 17.0 ppm are observed therein. Residential soils and dusts from Palmerton contain Pb which is largely explainable via mixing of Pb from primary Zn smelting and electric arc furnace dusts. Approximately 80% of the Pb in airborne particulate matter sampled at Palmerton in 1991 is derived from electric arc furnace dusts, and atmospheric enrichment factors for Cu, Sb, Pb, and Bi are observed which confirm this major source contribution. Residential samples from a control location contain Pb which is less radiogenic than is found in Palmerton, and exhibit no unusual elevation in sphalerite elements. Lead source discrimination in the Palmerton environment via Pb isotopic and elemental constituents approaches result in parallel conclusions.  相似文献   

16.
《Applied Geochemistry》2002,17(5):621-632
Sediments (568) and suspended particulate matter (SPM, 302 samples) of the southern German Bight and the adjacent tidal flat areas were analysed for selected major elements (Al, Fe, K), trace metals (Mn, Pb), and 206Pb/207Pb ratios using XRF, ICP–OES, ICP–MS. For selected samples a leaching procedure with 1 M HCl was used to estimate the Pb fraction associated with labile phases (e.g. Mn/Fe-oxihydroxide coatings) in contrast to the resistant mineral matrix. Enrichment factors versus average shale (EFS) reveal elevated Pb contents for all investigated sediments and SPM in the following order: Holocene tidal flat sediments (HTF, human-unaffected) <recent tidal flat sediments (RTF) <Helgoland Island mud hole sediments (MH) <nearshore SPM (SPM concentration>5 mg l−1) < offhore SPM (<5 mg l−1). Besides pollution, RTF contain elevated amounts of natural Pb-rich materials (K-feldspars and heavy minerals) due to a man-made high-energy environment (dike building) in comparison to HTF. 206Pb/207Pb ratios of RTF (1.192±0.019) are similar to the local geogenic background, determined from HTF (1.207±0.008). In contrast, Pb isotope ratios of nearshore SPM (1.172±0.007) and offshore SPM (1.166±0.012) show a distinct shift towards the anthropogenic/atmospheric signal of 1.11–1.14. This difference between RTF and SPM supports the assumption of low deposition rates of fine material in the intertidal systems. As the 206Pb/207Pb ratios of SPM do not reach the pure anthropogenic signal, the adsorbed Pb fraction was examined (leaching). However, the leachates also contained large amounts of geogenic Pb (SPM ≈40%, recent sediments ≈60%). The authors assume that the uptake of natural Pb occurs in nearshore waters, presumably in the turbid intertidal systems. Possible sources for dissolved Pb are mobilisation during weathering (geogenic signal) and dissolution of oxihydroxide coatings with subsequent release from porewaters, and unspecific riverine input. Comparatively small parts of SPM leave the coastal water mass and reach the open North Sea. This process therefore leads to a decontamination of the tidal flat sediments. Due to more pronounced atmospheric input, the offshore SPM becomes enriched in anthropogenic Pb as indicated by decreasing 206Pb/207Pb ratios with increasing distance from the coast.  相似文献   

17.
Analytical results for Pb-concentrations and isotopic ratios from ca. 150 samples of soil A horizon and ca. 145 samples of soil C horizon collected along a 4000-km east–west transect across the USA are presented. Lead concentrations along the transect show: (1) generally higher values in the soil A-horizon than the C-horizon (median 21 vs. 16.5 mg/kg), (2) an increase in the median value of the soil A-horizon for central to eastern USA (Missouri to Maryland) when compared to the western USA (California to Kansas) (median 26 vs. 20 mg/kg) and (3) a higher A/C ratio for the central to eastern USA (1.35 vs. 1.14). Lead isotopes show a distinct trend across the USA, with the highest 206Pb/207Pb ratios occurring in the centre (Missouri, median A-horizon: 1.245; C-horizon: 1.251) and the lowest at both coasts (e.g., California, median A-horizon: 1.195; C-horizon: 1.216). The soil C-horizon samples show generally higher 206Pb/207Pb ratios than the A-horizon (median C-horizon: 1.224; A-horizon: 1.219). The 206Pb/207Pb-isotope ratios in the soil A horizon show a correlation with the total feldspar content for the same 2500-km portion of the transect from east-central Colorado to the Atlantic coast that shows steadily increasing precipitation. No such correlation exists in the soil C horizon. The data demonstrate the importance of climate and weathering on both Pb-concentration and 206Pb/207Pb-isotope ratios in soil samples and natural shifts thereof in the soil profile during soil-forming processes.  相似文献   

18.
A peat core from an ombrotrophic bog documents the isotopic evolution of atmospheric Pb in central Ontario since AD 1804 ± 53 (210Pb dating). Despite the introduction of unleaded gasoline in the mid-1970’s, the ratio 206Pb/207Pb in atmospheric deposition has not increased as expected, but rather continues to decline. In fact, snowpack sampling (2005 and 2009) and rainwater samples (2008) show that the isotopic composition of atmospheric Pb today is often far less radiogenic than the gasoline lead that had been used in Canada in the past. The peat, snow, and rainwater data presented here are consistent with the Pb isotope data for aerosols collected in Dorset in 1984 and 1986 which were traced by Sturges and Barrie (1989) to emissions from the Noranda smelter in northern Quèbec, Canada’s largest single source of atmospheric Pb. Understanding atmospheric Pb deposition in central Ontario, therefore, requires not only consideration of natural sources and past contributions from leaded gasoline, but also emissions from metal smelting and refining.Lead in the streams which enter Kawagama Lake today (206Pb/207Pb = 1.16 − 1.19) represents a mixture between the natural values (1.191 − 1.201 estimated using pre-industrial lake sediments) and the values found in the humus layer of the surrounding forest soils (206Pb/207Pb = 1.15 − 1.19). In the lake itself, however, Pb is much less radiogenic (206Pb/207Pb as low as 1.09) than in the streams, with the dissolved fraction less radiogenic than particulate material. The evolution of Pb isotope ratios within the watershed apparently reflects preferential removal by sedimentation of comparatively dense, radiogenic, terrestrial particles (derived from the mineral fraction of soils) from the humus particles with lower ratios of 206Pb/207Pb (because of atmospheric Pb contamination). Despite the contemporary enrichments of Pb in rain and snow, concentrations of dissolved Pb in the lake are extremely low (sometimes below 10 ng/l), with Pb concentrations and Pb/Sc ratios approaching “natural” values because of efficient binding to particles, and their subsequent removal in the watershed.  相似文献   

19.
Lead isotope analyses were performed on 26 polymetallic massive sulphide deposits of the Iberian Pyrite Belt, as well as on overlying gossans and associated volcanic rocks. All the massive sulphide deposits (except for Neves-Corvo), and nearly all the volcanic rocks show very similar isotopic compositions grouped around 18.183 (206Pb/204Pb), 15.622 (207Pb/204Pb) and 38.191 (208Pb/204Pb), indicating that most of the ore deposit lead was derived from the same continental crust environment as the associated volcanic rocks. The isotopic compositions are representative of the average south Iberian crust during the Devonian to Early Carboniferous (Dinantian), and their constancy implies a homogenization of the mineralizing fluids before the deposition of the massive sulphides from hydrothermal fluids circulating through interconnected regional fracture systems. This isotopic constancy is incompatible with multiple, small, independent hydrothermal cells of the East Pacific Rise type, and fits much better with a model of hydrothermal convections driven by “magmatic floor heating”. Neves-Corvo is the only south Iberian massive sulphide deposit to have a heterogeneous isotopic composition with, in particular, a highly radiogenic stanniferous ore (206Pb/204Pb of the cassiterite is >18.40). A model of lead mixing with three components is proposed to explain these variations: (1) one derived from the Devonian to Early Carboniferous (Dinantian) continental crust that generated all the other massive ores; (2) an Eohercynian stanniferous mineralization partly remobilized during the formation of the massive sulphides, but independent of them; and (3) a Precambrian continental crust component. The juxtaposition of three different sources places Neves-Corvo in a specific paleogeographic situation that could also explain its mineralogical specificity. The geodynamic context that best explains all the obtained isotopic results is one of an accretionary prism. The fact that lead isotope signatures of the gossans are almost identical to those of the underlying massive sulphides means that this technique could be a useful exploration tool for the Iberian Pyrite Belt.  相似文献   

20.
In this study the effect of anthropogenic emissions on the lead isotopic composition of sediments from the Potengi-Jundiai river system near the fast growing city of Natal, NE-Brazil, is investigated. The lead isotope signatures of sediments from the region of Natal were measured by ICP QMS and can be discussed in terms of three different end members of lead. Two geogenic lead endmembers can be distinguished and also be spatially separated, as higher lead isotope ratios occur in the vicinity of the town of Macaiba while the lead isotope ratios decrease towards the city of Natal and the mouth of the estuary. Proterozoic rocks of different age are potential lead sources as Paleoproterozoic rocks occur in the catchment of Jundiai river and younger, Neoproterozoic rocks predominate towards the mouth of the river. The lead isotope signatures of the anthropogenically affected samples deviate from the signatures of the unaffected samples indicating the existence of a third, anthropogenic source of lead. This source represents the lead isotope signature of anthropogenic emitters like waste- and coal-combustion which is also revealed by other geochemical studies conducted in Brazil.  相似文献   

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