A low-contamination method for hydrothermal decomposition of zircon and extraction of U and Pb for isotopic age determinations

A simple procedure for the decomposition of zircon and the extraction of U and Pb for isotopic age determinations has been developed and tested (Krogh, 1971a,b). Samples are decomposed at 220°C with 48 per cent HF in a Teflon® capsule confined by a self-sealing stainless steel jacket. Uranium and lead are isolated on a Teflon® anion exchange column using Dowex 1 resin. Measured lead contamination levels range from 0.5 to 5.0 ng. In contrast, lead blanks for the borax fusion technique used in all previous zircon analyses are typically 0.2 to 1.0 μg.Eight small samples weighing 0.3 to 7 mg contained 30 to 260 ng of Pb²⁰⁶. The average value for the total amount of common lead present in the lead isotopic composition analysis, contributed from both the sample and the chemical procedures, was 1.4 ng. The highest $\text{Pb}{}^{206}\text{Pb}{}^{204}$ ratio measured to date (126,000) was obtained on a 0.1-g sample that contained 50 ppm of Pb²⁰⁶. An exact determination of the amount of common lead in zircons is now possible. A maximum value of 0.3 ppm was found for fourteen nonmagnetic zircon fractions from granites and rhyolites. Higher values reported in the literature suggest that lead contamination levels are often underestimated in the analysis of zircons by the borax fusion technique.The silica-gel loading technique for lead provides stable emission for small samples as well as limited isotopic fractionation in the mass spectrometer. These features, combined with the low levels of lead contamination and the high precision of mass spectrometric analyses, make possible an average reproducibility (for duplicate decompositions of the same finely ground sample) of 0.3 m.y. for lead $\text{207}\text{206}$ ages of 2750 m.y. The new method requires fewer reagents and is much easier than the borax fusion technique.     

Determination of lead isotopic composition of a grain from the allende carbonaceous chondrite by LAMMA

The attempt of this work is to use the LAMMA, a newly developed modern micro-beam analytical technique, to determine the lead isotopic composition of some grains picked out from the white aggregate in the Allende (C3V) carbonaceous chondrite. The experimental results show that the Pb/Pb model age of the Allende calculated on the basis of the²⁰⁶Pb/²⁰⁷Pb ratio is 4.56 ±0.008 b. y., just in agreement with the values reported in literature. The LAMMA provides a new and powerful tool in the analytical arsenal dealing with trace elements and isotopic compositions, due to its high sensitivity, rapid performence and good space resolution.     

Stable isotope evidence for the atmospheric origin of CO2 involved in carbonation of MSWI bottom ash

Stable isotopes were used to constrain the origin of CO₂ involved in the ageing process of municipal solid waste incineration (MSWI) bottom ash under open-air conditions. The δ¹³C and δ¹⁸O values of CaCO₃ occurring in MSWI bottom ash samples of variable age and the δ¹³C of the residual organic matter content were measured, and laboratory assessments made of the isotopic fractionation accompanying CaCO₃ neo-formation during accelerated carbonation experiments of bottom ash or pure lime with atmospheric or industrial CO₂. The results indicate that stable isotopic compositions exhibited by fresh and aged bottom ash samples reflect non-equilibrium processes resembling those described in the carbonation of concrete and mortar. They also lead to conclusions on the prevalent involvement of atmospheric CO₂ in the open-air carbonation of MSWI bottom ash.     

Isotopic evolution of common lead

The isotopic composition of lead from galenas and feldspars of granitic rocks covering a time span of 3600 m.y. is reviewed. Hew data are combined with data from the literature, all ratios being normalized to a fixed value for the isotopic composition of lead in an inter-laboratory reference sample. Comparison of feldspar and galena results show that the isotopic composition of initial lead in granitic rocks may be identical to, or more radiogenic than, that in coeval stratiform (or ‘single stage’) ore deposits. The isotopic evolution of terrestrial lead over the past 3600 m.y. deduced from the stratiform ore data does not fit the closed system source. The data are best fitted to evolution models in which the value of μ (U²³⁸/Pb²⁰⁴ normalized to the present day) in the source has increased either over approximately the past 3.6 billion years or since the time of formation of the Earth. A limited number of high precision data for strontium likewise indicate open system isotopic evolution. In this system the Rb/Sr ratio appears to have decreased as a function of time. Physical models that explain the data are presented. Either mantle differentiation or crustal contamination processes could account for the change in μ in the source for the lead. The strontium data and some detailed studies of the isotopic composition of lead in oceanic volcanic rocks support mantle differentiation rather than crustal contamination. The exact nature of the mantle differentiation process cannot be specified, although empirical data favoring the existence of such a process are given. The best estimate of the age of the Earth, calculated with the data from 2750 and 3290 m.y.-old lead ores, is 4660 m.y.     

The Teutonic Bore deposit,Western Australia: a lead isotope study of an ore and its gossan

The Teutonic Bore deposit occurs in an Archaean greenstone belt within the Eastern Goldfields Province of the Yilgarn Block in Western Australia. The ore is hosted by basaltic rocks and consists of a conformable massive sulfide lens underlain by a thick zone of pyritic stringer ore. The zone of oxidation reaches a depth of 90–100 m with the development of secondary copper sulfides. The lead isotopic compositions of six samples of massive sulfide, three mineral separates from the ore and eight gossan samples collected from the open cut were determined by standard mass-spectrometric techniques. Four of the massive sulfide samples, all three mineral separates and seven of the eight gossan samples have lead isotopic compositions identical to each other, within experimental error. These results confirm the findings of earlier studies that the lead isotopic signature of a massive sulfide ore is transferred to its gossan, and provide additional data suggesting the usefulness of lead isotopic determinations in ore prospect evaluation. The Teutonic Bore leads plot below the average global lead evolution curves for the uranogenic isotopes ²⁰⁶Pb and ²⁰⁷Pb, suggesting that the lead in the ore contains a significant mantle component. This feature of the isotopic data is consistent with the idea of a mantle plume origin of the Eastern Goldfields greenstone belts.     

Lead and strontium isotopes in rocks of the Absaroka volcanic field,Wyoming

The Absaroka volcanic field is comprised of predominant andesitic volcaniclastic rocks and less abundant potassium-rich mafic lavas (shoshonites and absarokites). Strontium and lead isotopic variations preclude a simple derivation from an isotopically uniform source: Sr⁸⁷/Sr⁸⁶, 0.7042 to 0.7090; Pb²⁰⁶/Pb²⁰⁴, 16.31 to 17.30; Pb²⁰⁸/Pb²⁰⁴, 36.82 to 37.64. We postulate that these rocks were derived from a lower crust or upper mantle which underwent a preferential loss of uranium relative to lead approximately 2800±200 m.y. ago. Variations in lead and strontium isotopic compositions are thought to reflect small inhomogeneities in U/Pb and Rb/Sr ratios in the source.Publication authorized by the Director, U.S. Geological Survey.     

Evaluation of drilling priorities using lead isotopes at the Lady Loretta lead-zinc-silver deposit, Australia

The weathered surface expression of the Lady Loretta lead-zinc-silver deposit outcrops almost continuously over a distance of about 4 km. Lead isotope ratios have been determined for samples, some of which contain > 4000 ppm Pb, from geochemically anomalous areas in order to assign drilling priorities to the anomalies.Twelve of the 43 samples analyzed contain isotopically homogeneous lead with ratios (²⁰⁷Pb/²⁰⁶Pb 0.9532–0.9549) similar to the expected targets for major ore deposits in the Mount Isa-McArthur River metallogenic province (²⁰⁷Pb/²⁰⁶Pb 0.955–0.962). Three main zones of interest can be outlined on the basis of lead isotopic data. The top priority is assigned to a 100-m-long section of the ironstone which coincides with the known outcrop of the ore. Lower priority for drilling is given to the two other zones which outcrop over smaller areas and have more radiogenic lead in their immediate surroundings. Several samples with anomalously high lead contents (> 1000 ppm) contain more radiogenic lead (²⁰⁷Pb/²⁰⁶Pb < 0.950) which was most likely derived from the weathering country rocks by supergene leaching and redeposition. Of the 22 samples containing ≥ 380 ppm Pb, lead isotopic analyses would have reduced the number deserving further attention to 10. Lead isotopic analyses in the early stages of exploration could have assisted in minimizing exploration expenditure at Lady Loretta.     

On the age of the Onverwacht Group,Swaziland Sequence,South Africa

Some rocks of the Onverwacht Group, South Africa, have been analyzed for Rb and Sr concentrations and Sr isotopie composition. These rocks include volcanic rocks, layered ultramafic differentiates and cherty sediments. Whole rock data indicate that the Rb-Sr isotopie systems in many samples were open and yield no reasonable isochron relationships. However, the data of mineral separates from a basaltic komatiite define a good isochron of $\text{t = 3.50 ± 0.20}$ (2δ) b.y. with an initial Sr⁸⁷/Sr⁸⁶ ratio of $\text{0.70048 ± 5}$(2δ). The orthodox interpretation of this age is the time of the low grade metamorphism. Since the basaltic komatiite is stratigraphically lower than the Middle Marker Horizon (dated as $\text{3.36 ± 0.07}$ b.y. Hurley et al., 1972), and since it is commonly found that volcanism, sedimentary deposition, metamorphism and igneous intrusion in many Archean greenstone-granite terrain all took place in a relatively short time interval (less than 100 m.y.), it is reasonable to assume that the age of 3.50 b.y. might also represent the time of initial Onverwacht volcanism and deposition. The initial Sr⁸⁷/Sr⁸⁶ ratio obtained above is important to an understanding of the Sr isotopic composition of the Archean upper mantle. If the komatiite represents a large degree of partial melt (40–80 per cent) of the Archean upper mantle material, then the initial ratio obtained from the metamorphic komatiite should define an upper limit for the Sr isotopic composition of the upper mantle under the African crustal segment.     

Hydrogen and oxygen isotope ratios in nodular and bedded cherts

Hydrogen and oxygen isotopic compositions of cherts (δD for hydroxyl hydrogen in the chert, δ¹⁸O for the total oxygen) have been determined for a suite of samples from the central and western United States. When plotted on a δD-δ¹⁸O diagram, Phanerozoic cherts define domains parallel to the meteoric water line which are different for different periods of geologic time. The elongation parallel to the meteoric water line suggests that meteoric waters were involved in the formation of many cherts.The existence of different chert δ-values for different geologic times indicates that once the granular microcrystalline quartz of cherts crystallizes its isotopic composition is preserved with time. An explanation for the change with time of the isotopic composition of cherts involving large changes with time in the isotopic composition of ocean water is unlikely since δ¹⁸O of the ocean would have had to decrease by about 3‰between Carboniferous and Triassic time and then increase about 5%.` from Triassic to Cretaceous time. Such isotopic changes cannot be accounted for by extensive glaciation, sedimentation of hydrous minerals, or input of water from the mantle into the oceans.The variation with time of the chert δ-values can be satisfactorily explained in terms of past climatic temperature fluctuations if the chert-water isotope fractionation with temperature is approximated by 1000 lnα = 3.09 × 10⁶T^?2 – 3.29. Crystallization temperatures so inferred suggest that the average climatic temperatures for the central and western U.S. decreased from about 34 to 20°C through the Paleozoic, increased to 35–40°C in the Triassic, and then decreased through the Mesozoic to Tertiary values of about 17°C. A few data for the Precambrian suggest the possibility that Earth surface temperatures may have reached about 52°C at 1.3 b.y. and about 70°C at 3 b.y.     

LA-MC-ICPMS分析古老熔体包裹体Pb同位素组成中的误差评价

目前应用LA-MC-ICPMS分析熔体包裹体Pb同位素,由于没有同时测试U和Th的信号,导致熔体包裹体Pb同位素的研究仅局限于中生代以来的样品。本文应用LA-MC-ICPMS分析了玻璃样品以及存在显著U-Th衰变影响的古老熔体包裹体的Pb同位素组成,评价了U/Pb和Th/Pb分析误差对初始Pb同位素比值校正的影响。实验中以国际玻璃标样NKT-1G为外部标样,采用"标样-样品-标样法"进行仪器漂移和质量歧视校正,结果表明,国际玻璃标样BHVO-2G、TB-1G的208Pb/206Pb和207Pb/206Pb分析精度优于0.30%(2RSD),与推荐值的偏差小于0.30%,然而232Th/206Pb和238U/206Pb分析结果显示了较大分散性(外精度约5.0%)。根据误差传递计算,样品的年龄对初始铅的误差有很大影响。对于古生代以来的样品(年龄小于540 Ma),即使测试的232Th/206Pb和238U/206Pb与真值偏差达到10%,经过U-Th衰变校正后的Pb同位素比值与真值的偏差依然小于0.80%。因此本方法可以将熔体包裹体等地质样品的Pb同位素研究由新生代样品(年龄小于65 Ma)扩展到古生代样品。     

Lead isotopic systematics of major river sediments: a new estimate of the Pb isotopic composition of the Upper Continental Crust

The lead isotopic composition of river sediments is reported in the present work for the Earth's major river basins, from old cratonic to young orogenic areas and from subarctic to tropical climates. Sediment samples from these large river basins provide a useful tool to calculate the average upper crustal composition because they are large-scale integrated samples of the weathering products of the present-day Upper Continental Crust (UCC). Two different and complementary calculations were done to estimate the average lead isotopic composition of the UCC. The first, based on the flux weighted average of particulate lead delivered by the rivers, gave values of 19.07, 15.74 and 39.35 for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios, respectively. To avoid over-estimating the contribution of orogenic areas, which produces a bias (because the flux of particulate lead depends strongly on the physical erosion rate), a second calculation was done by averaging with drainage areas of each river basin. This gave values of 18.93, 15.71 and 39.03 for ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios, respectively. These direct calculations of the lead isotopic composition of the UCC are similar and are in agreement with previous estimates made using an indirect approach.     

U-Th-Pb measurements of Luna 20 soil

The concentrations of uranium, thorium and lead and the lead isotopic composition of Luna 20 soil were determined. The data indicate that the Luna 20 soil is mainly a mixture of highland anorthosites and low-K basalt, but little KREEP basalt. The U-Th-Pb systematics are discussed in comparison with other lunar soils, especially with Apollo 16 soils which were collected from a ‘typical’ highland region. The data fit well in the Apollo 16 soil array on a U-Pb evolution diagram, and they exhibit excess lead relative to uranium. This relationship appears to be a characteristic of highland localities. Considering the previous observations of lunar samples, we infer that lead enrichment in the soil relative to uranium occurred between 3.2 and 3.9 b.y. ago and that the soil was disturbed by ‘third events’ about 2.0 b.y. ago. A lunar evolution model is discussed.     

Geochronology of Archaean gneisses and tonalites from north of the Frederikshåbs isblink,S.W. Greenland

The main rock types in the area north of the Frederikshåbs isblink are streaky gneisses, massive tonalites and ‘supracrustals’. The gneisses are thought to be the parent rocks of the tonalite and can be seen to merge into tonalite across a narrow zone of nebulite. Rb-Sr whole rock points from samples of gneiss and tonalite fall on a common isochron with an age of 2662 ± 116 m.y. (2σ) and initial ratio of 0.7032 ± 0.0008 (2σ) (half-life of ⁸⁷Rb = 50 b.y.). The uncertainties in the isochron could mask small age and initial ratio differences between the gneiss and tonalite. However, our present interpretation is that the isochron reflects a homogenization of Sr isotopes within and between the two rock types. The presence of two out of four K-feldspar points on the whole rock isochron is interpreted as evidence that the K-feldspar became closed to Sr isotope migration at the same time as the whole rocks. Subsequent local isotopic disturbance has resulted in a minor loss of radiogenic strontium from two of the samples. The interpretation of the K-feldspar as a product of the epidoteamphibolite facies metamorphism allows the conclusion that the whole rock-K-feldspar isochron is recording a Sr isotopic homogenization during this event and is not related to the formation of the gneiss or the tonalite. Rb-Sr closure ages of ca. 2515 m.y. for muscovite and ca. 1950 m.y. for biotite could be recording separate isotopic disturbances or the cessation of strontium isotope migration as the minerals cooled through their characteristic blocking temperatures. Zircons from both the gneiss and the tonalite have igneous morphological features. Their U-Pb systems are complex, however, and suggest a multistage history of isotopic disturbance. Whereas the zircon U-Pb and whole rock Rb-Sr results suggest a maximum age of approximately 3000 m.y. for the parent rocks of the gneiss and tonalite they do not entirely exclude the possibility that the rocks represent older crust in which the isotopic systems have been almost completely reset ca. 2700 m.y. ago.     

Agpaitic magmatism at Norra Kärr? RbSr isotopic evidence

RbSr isotopic analyses of 10'whole-rock samples from the controversial peralkaline Norra Kärr complex of southern Sweden suggest an age (1580±62 m.y.) considerably older than had previously been anticipated, and indicate an initial ⁸⁷Sr/⁸⁶Sr ratio of 0.7072±0.0035 (errors at 2σ). The isotopic data are consistent with a primary magmatic origin for the Norra Kärr agpaites, but data from 8 mineral separates show that they have experienced at least one period of metamorphic disturbance since the original intrusion; the last episode of isotopic readjustment must have occurred after 1250 m.y. before present, and is attributed to the Sveconorwegian (Grenville) metamorphism.     

杭州市土壤铅污染的铅同位素示踪研究

根据杭州市40个土壤全铅和38个可溶相铅的统计分析,土壤中全铅平均含量为49.6×10^-6,可溶相铅平均为21.4×10^-6,城区表土的全铅高达76.1×10^-6,显著高于全国土壤平均值。分析结果还显示,从农村→远郊→近郊→公路旁,土壤可溶相铅含量逐渐增加,且土壤的可溶相铅含量与深度具明显的负相关关系。表明杭州市土壤受到了不同程度的铅污染,污染程度由农村→远郊→近郊→公路旁→城区有明显的增高趋势。通过对茶园土壤中可溶相铅、残渣态铅及城区表土全铅的同位素组成对比分析发现,从土壤残渣态(代表土壤背景)→土壤可溶相→城区表层土壤全铅²⁰⁶Pb/²⁰⁷Pb比值有明显的降低。²⁰⁸Pb/(²⁰⁶Pb+²⁰⁷Pb)也有类似的变化趋势。将土壤与杭州市的汽车尾气、大气等环境样品进行对比发现,随着土壤受污染程度的增加,铅同位素组成逐渐向汽车尾气铅漂移,表明汽车尾气排放的铅为其主要污染源。     

U-Th-Pb and Rb-Sr systematics of Allende and U-Th-Pb systematics of Orgueil

U-Th-Pb systematics study of Allende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The ²⁰⁶Pb/²⁰⁴Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the ²⁰⁶Pb/²⁰⁴Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁴Pb/²⁰⁴Pb plot. The slope of the best fit line is 0.6188 ± 0.0016, yielding an isochron age of 4553 ± 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-Al-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 ± 25 m.y. and 107 ± 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting that the Rb-Sr system was also disturbed by a later event, as suggested by the U-Pb concordia data. The lowest observed ⁸⁷Sr/⁸⁶Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (Papanastassiou, Thesis, 1970).The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb.     

Isotopic geochemical characterization of selected nepheline syenites and phonolites from the Poços de Caldas alkaline complex, Minas Gerais, Brazil

This paper presents and discusses the isotopic data from the hydrothermal studies of the Poços de Caldas Natural Analogue Project. The purpose of these studies was to elucidate the mass transport of relevant elements and isotopes associated with hydrothermal mineralization and alteration at the Osamu Utsumi uranium mine, as applicable to high-temperature radwaste isolation (particularly in the U.S. nuclear waste program). Research efforts were focused on studying the thermal, chemical and hydrologic nature of the palaeohydrothermal regime associated with a breccia pipe at the Osamu Utsumi mine, and related to the geochemical, geochronological and petrological characterization studies of unaltered regional nepheline syenite and phonolite.The regional rocks studies have a vertically elongated δD, δ¹⁸O pattern, which possibly indicates meteoric water/rock interaction. Regression of Rb---Sr whole-rock isotopic data for the regional nepheline syenite and phonolite samples did not produce isochrons. An internal, mineral-separate isochron regression from a nepheline syenite sample, considered representative of unaltered nepheline syenite of the Poços de Caldas plateau, yields an age of 78 Ma, and an initial ratio of approximately 0.7051. The initial ratios of the regional nepheline syenites are possibly indicative of a mantle source for the alkaline magmatism, with some incorporation of old, high Rb/Sr crustal material. The greater-than-mantle values of δ¹⁸O, if not due solely to surficial processes, also appear to require some assimilation of crustal material. Sm---Nd isotopic data for the regional rocks do not define any isochrons, although the nepheline syenite samples conform very well to a calculated reference isochron for 78 Ma and a fixed initial ¹⁴³Nd/¹⁴⁴Nd of 0.512359. The regional phonolite samples lie markedly off this isochron. This is probably due to the phonolite samples having different initial ¹⁴³Nd/¹⁴⁴Nd values. All regional samples lie within the “Mantle Array” trend. Their location within _Nd–_Sr space indicates as asthenospheric Mid Ocean Ridge Basalt (MORB)-type source magma also contaminated by continental igneous and metamorphic rocks (e.g. the Precambrian gneiss surrounding the Poços de Caldas plateau).The rocks studied at the Osamu Utsumi mine from the F4 drillcore have experienced varying degrees of hydrothermal mineralization and metasomatism, and deep weathering. The hydrothermally altered rocks have a quite pronounced δD shift, with only a slight δ¹⁸O shift. The δD-δ¹⁸O trend of the hydrothermally altered F4 samples most likely reflects the variability of temperature, hydrologic flow, mineralogical alteration and, therefore, water/rock interaction and isotopic exchange in the palaeohydrothermal regime.Regression of Rb---Sr whole-rock isotopic data for subsamples from a nepheline syenite xenolith sample yields an age of 76 Ma and an initial ratio of approximately 0.7053. Due to the marked hydrothermal alteration and metasomatism of this sample, the Rb---Sr isotopic system is interpreted as being re-equilibrated and thus the regressed age is the age of the hydrothermal event. Using a versus 1/Sr mixing diagram, distinct trends are seen for hydrothermal alteration, mineralization and weathering. Again, the F4 nepheline syenite samples do not define an Sm---Nd isochron, but conform very well to a calculated model isochron for 78 Ma and an initial ¹⁴³Nd/¹⁴⁴Nd of 0.512365. The Sm---Nd isotopic data also exhibit a possible disturbance by the hydrothermal, metasomatic alteration. A lamproite dyke which crosscuts the hydrothermal alteration in the Osamu Utsumi mine gives an age of 76 Ma, which is essentially the same as the Rb---Sr age of the hydrothermally altered nepheline syenite subsamples.     

Lead isotopic compositions of Tonga-Kermadec volcanics and their petrogenetic significance

Lead isotopic compositions, and lead and uranium concentrations have been determined for samples from the Tongan islands of Late, Fonualei, Hunga-Ha'apai and Eua and for the Kermadec islands of Raoul, Macauley, Esperance and Napier. The Kermadec samples form a linear array in a Pb²⁰⁶/Pb²⁰⁴ vs Pb²⁰⁷/Pb²⁰⁴ plot which if interpreted in terms of a two stage mantle model indicates a mantle fractionation of Pb from U at about 1.2×10⁹ yr. The Tongan volcanics, except Eua, have a very narrow range of isotopic compositions, only slightly exceeding the experimental error limits. This is interpreted to be the result of efficient mixing of the mantle in response to rapid extension behind the Tongan arc. The rate of extension appears to determine whether convection mixing can occur, since the sub-Kermadec mantle which experienced slower extension is not well mixed isotopically. The isotopic compositions show no evidence for a component in the magma derived from sediments dragged down the Benioff zone. The pre-Eocene Eua volcanics have lead which is substantially more radiogenic than the younger Tongan volcanics and are interpreted as a sample of the mantle before convective mixing occurred.     

The influence of air mass source on the seasonal isotopic composition of precipitation, eastern USA

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.     

Application of lead stable isotopes to the Guadiamar Aquifer study after the mine tailings spill in Aznalcóllar (SW Spain)

On 25 April 1998 the tailings dam of the Aznalcóllar mine burst, a great quantity of pyrite waste sludge and acid water was spilled reaching the vicinity of the Doñana National Park. In surface and ground water samples taken a week after dam breaking, metals, trace elements and Pb isotopic ratios (²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁶Pb) were analysed. In September 1998 a second sampling survey was carried out. The surface waters have a similar isotopic composition as the lead contained in the pyrite from the Aznalcóllar mine. The polluted groundwater of the Guadiamar aquifer also shows the influence of the mining origin of the lead. Lead isotope ratios (²⁰⁶Pb/²⁰⁷Pb and ²⁰⁸Pb/²⁰⁶Pb) in the groundwater of the Almonte-Marismas are very low and they differ clearly from the rest of groundwater samples. A further group of wells has a lead isotope composition intermediate between the Aznalcóllar mine and the atmospheric aerosols of the Iberian Peninsula.