Pb Tracer Research Articles

U-Pb age of monazite from hypersthenic quartzite of the Yatran river basin (Yatran block of Ukrainian Shield)

Among the crystalline rocks of the Middle Bug region, which are mapped as the Dniester-Bug series in the valley of the Yatran river in the area of the village Rogove, quartzites were discovered, which, unlike the quartzites of the Bug series, are mainly composed of quartz with a small number of feldspars. At the same time, hypersthene plays a prominent role in their composition. To find out the time of crystallization of monazite and the primary nature of quartzite, a sample of hypersthenic quartzite (sample 24/16), which is common on the left bank of the Yatran River (Rogove village), was taken. Under binoculars and a polarizing microscope, the internal structure of zircon crystals was studied in artificial preparations, and the spatial relationship with rock-forming minerals was studied in transparent sections under a polarizing microscope. Age was determined based on the results of U-Pb isotopic dating of multigrain monazite samples. Chemical preparation of multigrain samples of monazite for isotopic dating was performed according to the standard method [5]. A mixed 235U+206Pb tracer was used to determine the content of uranium and lead in monazites. According to the results of U-Pb isotopic dating, the time of crystallization of monazites from hypersten-containing quartzite of the Yatran river basin occupies an intermediate position (2044,7±9,2 million years) between the age of monazites from quartzites common in Kosharo-Olexandrivska structures (2062,4±4,4 million years) and Shamrayivka structures (1857,5±1,1 million years).

Applications of Pb isotopes in granite K-feldspar and Pb evolution in the Yilgarn Craton

The isotopic composition of Pb in a mineral or rock at the moment it formed – often referred to as common Pb – provides an important tool to track geological processes through time and space. There is a wide range of applications of common Pb isotopes including understanding magma sources, melt production, fractionation, contamination, and crystallization in the crust. Pb but not U is incorporated into the structure of K-feldspar during crystal growth, which, together with its widespread occurrence as a framework mineral, makes it an excellent common Pb tracer. Consequently, common Pb isotopes in granite K-feldspar crystals provide a potential signature of source composition and a link to crustal growth processes in the mid to lower crust. Hence, combining common Pb isotopes with Sm-Nd (or Lu-Hf) isotopic signatures from the same dated rocks allows assessment of the degree of isotopic communication from deep fractionation systems to those higher in the crustal column. In this contribution, we analyze common Pb isotopic signatures in K-feldspar from a granite sample transect through the Archean Yilgarn Craton in Western Australia. This transect crosses the major crustal-scale Ida Fault that is apparent on Nd and Hf isotopic maps and interpreted as a fundamental lithospheric boundary across which magma sources change. Our results yield a difference in median values of the Pb isotope derivative parameters µ (238U/204Pb) and ω (232Th/204Pb) across the Ida Fault, with higher µ and ω associated with more evolved Nd and Hf isotopic signatures on the western side of the fault. Pb evolution in the Yilgarn Craton is distinct from the widely applied Stacey & Kramers (1975) model. New Yilgarn-specific Pb evolution models are developed with implication for common Pb correction. A correlation in the spatial trends of granite K-feldspar common Pb signatures with those of upper crustal Pb ores and also the Sm-Nd and Lu-Hf systems reveals geochemical communication all the way through the crustal column, implying a common source for the entire lithospheric section on each side of the Ida Fault. Pb isotopes in granite K-feldspar are not an independent geochronometer but may yield important source context on major phase silicate growth that helps refine U-Pb geochronology interpretations (e.g., distinguishing magmatic versus metamorphic zircon growth).

High-precision ID-TIMS cassiterite U–Pb systematics using a low-contamination hydrothermal decomposition: implications for LA-ICP-MS and ore deposit geochronology

Abstract. Cassiterite (SnO2) is the most common ore phase of Sn. Typically containing 1–100 µg g−1 of uranium and relatively low concentrations of common Pb, cassiterite has been increasingly targeted for U–Pb geochronology, principally via microbeam methods, to understand the timing and durations of granite-related magmatic–hydrothermal systems throughout geological time. However, due to the extreme resistance of cassiterite to most forms of acid digestion, there has been no published method permitting the complete, closed-system decomposition of cassiterite under conditions in which the basic necessities of measurement by isotope dilution can be met, leading to a paucity of reference and validation materials. To address this a new low blank (< 1 pg Pb) method for the complete acid decomposition of cassiterite utilising HBr in the presence of a mixed U–Pb tracer, U and Pb purification, and thermal ionisation mass spectrometry (TIMS) analyses has been developed. Decomposition rates have been experimentally evaluated under a range of conditions. A careful balance of time and temperature is required due to competing effects (e.g. HBr oxidation), yet the decomposition of 500 µm diameter fragments of cassiterite is readily achievable over periods comparable to zircon decomposition. Its acid-resistant nature can be turned into an advantage by leaching common Pb-bearing phases (e.g. sulfides, silicates) without disturbing the U–Pb systematics of the cassiterite lattice. The archetypal Sn–W greisen deposit of Cligga Head, SW England, is used to define accuracy relative to chemical abrasion–isotope dilution–thermal ionisation mass spectrometry (CA-ID-TIMS) zircon U–Pb ages and demonstrates the potential of this new method for resolving high-resolution timescales (<0.1 %) of magmatic–hydrothermal systems. However, data also indicate that the isotopic composition of initial common Pb varies significantly, both between crystals and within a single crystal. This is attributed to significant fluid–rock interactions and the highly F-rich acidic nature of the hydrothermal system. At microbeam precision levels, this issue is largely unresolvable and can result in significant inaccuracy in interpreted ages. The ID-TIMS U–Pb method described herein can, for the first time, be used to properly characterise suitable reference materials for microbeam cassiterite U–Pb analyses, thus improving the accuracy of the U–Pb cassiterite chronometer as a whole.

Geochronological constraint on the Cambrian Chengjiang biota, South China

The Cambrian Chengjiang biota of South China provides compelling fossil evidence for the rapid appearance of metazoan phyla in Earth history (the ‘Cambrian explosion’). However, the timing of the Chengjiang biota is poorly constrained owing to lack of datable rock materials within the Maotianshan Shale that yields the fossils. Here we integrate secondary ion mass spectrometry (SIMS) and chemical ablation isotope dilution thermal ionization mass spectrometry (CA-ID-TIMS) U–Pb analyses of detrital zircons from the Maotianshan Shale to provide high-precision geochronological constraint on the Chengjiang biota. The youngest group of SIMS U–Pb detrital zircon dates yields an age peak at 520 Ma. Six zircons from this group are further dated by the CA-ID-TIMS U–Pb technique, but suggesting that they were not formed from a single zircon growth event. Thereby neither the age peak nor the weighted mean age defined by the youngest SIMS U–Pb dates could represent the maximum depositional age of the Maotianshan Shale. Instead, the youngest CA-ID-TIMS U–Pb date, 518.03 ± 0.69/0.71 Ma (2 σ , analytical uncertainty/incorporates U–Pb tracer calibration uncertainty), provides the first robust maximum age of the Chengjiang biota. This new geochronological constraint on the Chengjiang biota indicates that the Cambrian explosion reached its major phase around 518.03 ± 0.69/0.71 Ma, demonstrating a protracted process (>22 myr) for the Cambrian explosion. Supplementary material: SIMS and CA-ID-TIMS zircon U–Pb data are available at https://doi.org/10.6084/m9.figshare.c.4024849

Metrology and traceability of U–Pb isotope dilution geochronology (EARTHTIME Tracer Calibration Part I)

Mixed 235U–233U–205Pb(–202Pb) tracers for U–Pb isotope-dilution isotope ratio mass spectrometry have been prepared under the auspices of the EARTHTIME Initiative. The methods and results for the preparation and calibration of the U/Pb ratio and isotopic abundances are given, and the various sources of uncertainty are discussed and quantified. The accuracy of the EARTHTIME U–Pb tracer isotopic composition can be traced back to SI units via a series of assay and isotopic composition reference materials combined with the experiments described herein. The parameters used in calculating U/Pb ratios (and inferentially U–Pb dates) have correlated uncertainties that result in a total uncertainty contribution to 206Pb/238U dates of ±<0.03% (95% confidence). For suitable terrestrial materials such as zircon, when other sources of uncertainty have been minimised (e.g., open-system behaviour, 238U/235U variation, intermediate daughter product disequilibrium, common Pb, etc.) the U–Pb tracer calibration uncertainty is a limiting factor in the accuracy of U–Pb geochronology – but less so than the uncertainty in the 238U and 235U decay constants (±0.11 and 0.14% 2σ). The calibration approach of the mixed EARTHTIME 235U–233U–205Pb(–202Pb) tracers, in addition to updated values for reference materials (e.g., mixed gravimetric reference solutions), and parameters (e.g., Pb reference material assay), can be applied to other laboratory-specific U–Pb tracers and will facilitate the generation of accurate and directly inter-comparable U–Pb data.

Evaluating uncertainties in the calibration of isotopic reference materials and multi-element isotopic tracers (EARTHTIME Tracer Calibration Part II)

A statistical approach to evaluating uncertainties in the calibration of multi-element isotopic tracers has been developed and applied to determining the isotopic composition of mixed U–Pb (202Pb–205Pb–233U–235U) tracers used for accurate isotope dilution U–Pb geochronology. Our experiment, part of the EARTHTIME initiative, directly links the tracer calibration to first-principles measurements of mass and purity that are all traceable to SI units, thereby quantifying the accuracy and precision of U–Pb dates in absolute time. The calibration incorporates new more accurate and precise purity measurements for a number of commonly used Pb and U reference materials, and requires inter-relating their isotopic compositions and uncertainties. Similar methods can be used for other isotope systems that utilize multiple isotopic standards for calibration purposes. We also detail the inter-calibration of three publicly available U–Pb gravimetric solutions, which can be used to bring the same first-principles traceability to in-house U–Pb tracers from other laboratories. Accounting for uncertainty correlations in the tracer isotope ratios yields a tracer calibration contribution to the relative uncertainty of a 206Pb/238U date that is only half of the relative uncertainty in the 235U/205Pb ratio of the tracer, which was historically used to approximate the tracer related uncertainty contribution to 206Pb/238U dates. The tracer uncertainty contribution to 206Pb/238U dates has in this way been reduced to <300ppm when using the EARTHTIME and similarly calibrated tracers.

Oral Succimer Decreases the Gastrointestinal Absorption of Lead in Juvenile Monkeys

Although succimer (Chemet, meso-2,3-dimercaptosuccinic acid, DMSA) is considered to be a safe and effective chelating agent for the treatment of lead poisoning in humans, there is concern that it may increase the gastrointestinal (GI) absorption and retention of Pb from exposures suffered concurrent with treatment. This concern is justified because the availability of Pb-safe housing during outpatient treatment with oral succimer is limited. We used a juvenile nonhuman primate model of moderate childhood Pb intoxication and a sensitive double stable Pb isotope tracer methodology to determine whether oral succimer chelation affects the GI absorption and whole-body retention of Pb. Infant rhesus monkeys (n = 17) were exposed to Pb daily for 1 year postpartum to reach and maintain a target blood lead (BPb) level of 35-40 microg/dL. Animals were administered succimer (n = 9) or vehicle (n = 8) over two successive 19 day succimer treatment regimens beginning at 53 and 65 weeks of age. The present study was conducted over the second chelation regimen only. Animals received a single intravenous (iv) dose of stable (204)Pb tracer (5 microg, 24.5 nmol) followed by a single oral dose of stable (206)Pb tracer (72.6 microg, 352 nmol) immediately before chelation, in order to specifically evaluate GI Pb absorption and whole-body Pb retention with treatment. We collected complete urine and fecal samples over the first 5 days and whole blood over the first 8 days of treatment for analyses of stable Pb isotopes using magnetic sector inductively-coupled plasma mass spectrometry. Results indicate that succimer significantly reduced the GI absorption of Pb (vehicle, 64.9% +/- 5.5; succimer, 37.0% +/- 5.8; mean +/- SEM). Succimer also significantly increased the urinary excretion of endogenous Pb by approximately 4-fold over the vehicle treatment, while endogenous fecal Pb excretion was decreased by approximately 33%. Finally, although succimer reduced the whole-body retention of endogenous Pb by approximately 10% compared to vehicle, the majority (77%) of the administered internal dose of Pb tracer was retained in the body when assessed after 5 days of treatment. These data do not support the concern that succimer treatment increases GI Pb absorption.

Oral succimer decreases the gastrointestinal absorption of lead in juvenile monkeys.

Although succimer (Chemet, meso-2,3-dimercaptosuccinic acid, DMSA) is considered to be a safe and effective chelating agent for the treatment of lead poisoning in humans, there is concern that it may increase the gastrointestinal (GI) absorption and retention of Pb from exposures suffered concurrent with treatment. This concern is justified because the availability of Pb-safe housing during outpatient treatment with oral succimer is limited. We used a juvenile nonhuman primate model of moderate childhood Pb intoxication and a sensitive double stable Pb isotope tracer methodology to determine whether oral succimer chelation affects the GI absorption and whole-body retention of Pb. Infant rhesus monkeys (n = 17) were exposed to Pb daily for 1 year postpartum to reach and maintain a target blood lead (BPb) level of 35-40 microg/dL. Animals were administered succimer (n = 9) or vehicle (n = 8) over two successive 19 day succimer treatment regimens beginning at 53 and 65 weeks of age. The present study was conducted over the second chelation regimen only. Animals received a single intravenous (iv) dose of stable (204)Pb tracer (5 microg, 24.5 nmol) followed by a single oral dose of stable (206)Pb tracer (72.6 microg, 352 nmol) immediately before chelation, in order to specifically evaluate GI Pb absorption and whole-body Pb retention with treatment. We collected complete urine and fecal samples over the first 5 days and whole blood over the first 8 days of treatment for analyses of stable Pb isotopes using magnetic sector inductively-coupled plasma mass spectrometry. Results indicate that succimer significantly reduced the GI absorption of Pb (vehicle, 64.9% +/- 5.5; succimer, 37.0% +/- 5.8; mean +/- SEM). Succimer also significantly increased the urinary excretion of endogenous Pb by approximately 4-fold over the vehicle treatment, while endogenous fecal Pb excretion was decreased by approximately 33%. Finally, although succimer reduced the whole-body retention of endogenous Pb by approximately 10% compared to vehicle, the majority (77%) of the administered internal dose of Pb tracer was retained in the body when assessed after 5 days of treatment. These data do not support the concern that succimer treatment increases GI Pb absorption.

Optimal double and triple spiking for high precision lead isotopic measurement

Conventional lead isotope ratio measurements by thermal ionization mass spectrometry (TIMS) are currently limited to external precisions of ∼0.1% (2 σ) due to variations in instrumental mass discrimination, the magnitude of which can only be assessed using external standard or duplicate measurements. The goal of the double-spike technique, by contrast, is to circumvent this fractionation problem entirely by determining the mass discrimination factor ϵ directly for each sample. The unknown factor can be obtained after performing a second measurement on a sample aliquot to which a double spike of known, calibrated composition has been added. Practical applications of the double-spike technique up until now have used a mixed 207 Pb– 204 Pb tracer with considerable success, but there appears to have been little or no attempt to optimize the composition of the tracer used. An optimal tracer composition must fulfil two criteria: (1) the ratio of the internal, measured errors to a posteriori errors—the error magnification factors—should be as small as possible; (2) the propagated errors should not be strongly influenced by the spike/sample ratio of the second, mixture run—the latter is important since sample size cannot, in general, be rigorously controlled in advance. Of equal importance to the tracer composition itself is the choice of the three mass-balance equation set used to solve for ϵ; geometrically, this is equivalent to defining a mixing line in a particular 3D isotope space. Potential spike compositions were examined in four such 3D isotope spaces, corresponding to cases where 208 Pb, 207 Pb, 206 Pb and 204 Pb in turn are common, denominator isotopes of the ratios on the three orthogonal axes. Within each isotope space there exist three possible double-spike families. Tracers composed of three stable isotopes of lead—so-called `triple spikes'—are also considered, of which there are three families per isotope space. These spike compositions were evaluated for a typical common lead composition, at first using geometric methods, and then refined by determining error magnification factors using an ion statistical error model. It is shown that the 207 Pb– 204 Pb double spike used in previous studies, where ϵ is determined in 208,207,206 Pb/ 204 Pb isotope space, is one of the least suitable to use. Optimal tracer compositions for determining ϵ appear to be restricted to two families: The first is a family of 207 Pb– 204 Pb– 206 Pb triple spikes with 207 Pb/ 204 Pb of around unity and the isotope space has 206 Pb as the denominator isotope; the second family consists of 204 Pb– 207 Pb double spikes, solved in 208,206,204 Pb/ 207 Pb isotope space. Both of these families yield error magnification factors on a posteriori, corrected 206 Pb/ 204 Pb ratios as low as ∼1.2, and are less than 2 for all but highly underspiked and overspiked mixture compositions.

Diffusivity of strontium, neodymium, and lead in natural rhyolite melt at 1.0 GPa

Chemical diffusivities approximating tracer diffusivities of Sr, Nd, and Pb have been measured in a natural rhyolite melt at 1.0 GPa and temperatures from 1000 to 1450 °C using the diffusion couple technique. Diffusivities for all three elements were determined under nominally anhydrous conditions. Lead diffusivities were also determined at ~1 and 2.5 wt% dissolved H 2O in the melt. The results are described by the following Arrhenius equations ( D in m 2/s, activation energy in J/mol, T in Kelvin): D (Sr,dry) = 1.58 × 10 −3 e −(264200/RT) D (Nd,dry) = 1.28 × 10 −2 e −(330200/RT) D (Pb,dry) = 1.92 × 10 −7 e −(148400/RT) D (Pb,~1%) = 1.18 × 10 −7 e −(131700/RT) D (Pb,~2.5%) = 2.18 × 10 −9 e −(72600/RT). The results are consistent to within an order of magnitude with previously reported Sr, Nd, and Pb tracer diffusivities. Under anhydrous conditions, the relative magnitudes of the diffusion coefficients are D Sr ≈ D Pb > D Nd. D Nd determined in this work is similar to previously reported D Si. The implication of these results is that Pb and Sr diffusion-controlled isotopic and chemical exchange can occur at a faster rate than major element chemical exchange, but that Nd exchange occurs at about the same rate as major element exchange. The consequences of this process are illustrated with a model of isotopic exchange in a basaltic-granitic, double-diffusive, convecting magma chamber. The diffusivity of Pb increases with increasing dissolved water more than that of Si increases suggesting that, at least for Pb, isotopic homogenization also occurs faster than chemical homogenization under hydrous conditions.

Effect of diesel vehicles on visibility in California

The effect of diesel vehicle elemental carbon on visibility in California is estimated for 1980 and projected to the early 1990's. An emissions budget model indicates that heavy-duty diesel trucks contributed nearly one-half of statewide elemental carbon emissions in 1980 and about 5–15% of statewide light extinction (visibility reduction). A lead tracer model indicates somewhat larger extinction contributions from heavy-duty diesel trucks, about 5 to 25% statewide in 1980. Even greater visibility impacts (too large to be reasonable) are suggested by a CO tracer model. Because of increased diesel usage — due to both overall traffic growth and partial conversion of the vehicle fleet to diesels (10% of light-duty, 20% of medium-duty, and 60% of currently gasoline heavy-duty) — visibility in California is projected to decrease significantly (about 9 to 35%) from 1980 to the early 1990's under a “no control” scenario. Diesel vehicle elemental carbon would then contribute 13 to 40% of statewide visibility reduction. Even in the early 1990's, heavy-duty trucks would account for about three-fourths of the emissions from the entire diesel fleet. The most uncertain aspect of the conclusions is the overall magnitude of the visibility effect predicted by the emission budget and Pb tracer models. The partition of visibility impacts among light-, medium-, and heavy-duty vehicles is more definite. The conclusions are rather insensitive to reasonable changes in the assumptions regarding emission factors, traffic growth, and dieselization percentages. The findings indicate that, in order to be effective in protecting visibility, the current California standards for light- and medium-duty vehicles would have to be extended to include regulations for heavy-duty trucks.

Grain-boundary diffusion of Sn in Pb

Grain-boundary diffusivities of a Sn119 radioactive tracer have been measured in polycrystalline Pb specimens in the temperature range 70–200 °C. The combined diffusion parameter δDb is expressed by the Arrhenius relation δDb = 7.3×10−9 exp(−0.41±0.02 eV/kT) (cm3/sec). The Sn impurity diffusivities are larger than Pb self-diffusivities in the grain boundaries, which has been confirmed by codiffusion measurements of the two species. The lattice diffusivities reported in the literature for the Sn and Pb tracers also show a similar trend. In both situations the effect appears to be related to the physical characteristics of the two diffusing species.