Lead isotope ratios

One of the most significant sources of change in isotope ratios is caused by the small mass differences between isotopes and their effects on the physical properties of elements and compounds. For example, ordinary water (mostly Ej O) has a lower density, lower boiling point, and higher vapor pressure than does heavy water (mostly H2 0). Other major changes can occur through exchange processes. Such physical and kinetic differences lead to natural local fractionation of isotopes. Artificial fractionation (enrichment or depletion) of uranium isotopes is the basis for construction of atomic bombs, nuclear power reactors, and depleted uranium weapons. 

Instability in the flame leads to varying efficiencies in ion formation within the plasma (varying plasma temperature) and, therefore, to variations in measured isotope ratios (lack of accuracy). 

In summary, pathways A, B, and D are the only pathways at room temperature that contribute to products. Pathways A and D both lead to the main product channel (H + CO + CO2), and are isotopicaUy distinguishable, whereas pathway B leads to a different product channel, but must be included in the analysis because it affects the isotope ratios. The observed spectra from the labeled reaction contain only C 0 and OC 0. Therefore, only upper limits can be placed on the isotope ratios. The three values needed to constrain the three pathways under consideration are the ratio limits [C 0]/[C 0] < 0.16 and [ 0C 0]/[ 0C 0] < 0.30, and the limit that the total OH + CO + CO production is <0.10 [45]. 

Data on mineral s arates in present day volcanic rocks. Since every dating method (including the K-Ar or C systems) can be affected by several geochemical perturbations which may lead to erroneous ages, the best test for the °Th- U mineral isochrons consists in the analysis of presently erupted lavas or historic lavas of well known eruption dates. Rather surprisingly the data obtained on such samples are not so numerous (some examples are illustrated in Fig. 10). Early data showed that, in some cases, there were interlaboratory analytical discrepancies, especially in Th isotope ratios measured on mineral separates extracted from the same lava flows (this was the case for the 1971 lava from Mt. Etna and 1944 lava from Mt. Vesuvius Capaldi and Pece 1981 Hemond and Condomines 1985 Capaldi et al. 1985). This emphasizes the fact that °Th- U mineral analyses... 

Many scientists thought that Earth must have formed as long as 3.3 billion years ago, but their evidence was confusing and inconsistent. They knew that some of the lead on Earth was primordial, i.e., it dated from the time the planet formed. But they also understood that some lead had formed later from the radioactive decay of uranium and thorium. Different isotopes of uranium decay at different rates into two distinctive forms or isotopes of lead lead-206 and lead-207. In addition, radioactive thorium decays into lead-208. Thus, far from being static, the isotopic composition of lead on Earth was dynamic and constantly changing, and the various proportions of lead isotopes over hundreds of millions of years in different regions of the planet were keys to dating Earth s past. A comparison of the ratio of various lead isotopes in Earth s crust today with the ratio of lead isotopes in meteorites formed at the same time as the solar system would establish Earth s age. Early twentieth century physicists had worked out the equation for the planet s age, but they could not solve it because they did not know the isotopic composition of Earth s primordial lead. Once that number was measured, it could be inserted into the equation and blip, as Patterson put it, out would come the age of the Earth. ... 

As a guide to the evolutionary history of the continents, Patterson decided to measure the lead isotope ratios of Earth s crust as a whole. As rocks erode, their minerals are collected and mixed in the oceans, where they eventually settle in layers of sediment. Patterson organized a formidable series of experiments to measure the lead isotopes on land, in various layers of ocean water, and in sediments on the sea floor. 

The four isotopes, as those of any element, have the same chemical properties. The four are not, however, uniformly distributed in the earth s crust the occurrence of three of them, in minerals and rocks, is associated with the radioactive decay of isotopes of thorium and uranium. In most minerals and rocks the relative amounts (or the isotopic ratios) of the isotopes of lead (often expressed relative to the amount of stable lead-204) are generally within well-known ranges, which are independent of the composition of the mineral or rock they are, however, directly related to the amounts of radioactive thorium and uranium isotope impurities in them. 

Gulson et al. (1998) used measured lead isotope ratios (207Pb/206Pb and 206Pb/204Pb) in mothers breast milk and in infants blood to establish that, for the first 60-90 days postpartum, the contribution from breast milk to blood lead in the infants varied from 36% to 80%. Maternal bone and diet appear to be the major sources of lead in breast milk. Mean lead concentration ( standard deviation) in breast milk for participants in the study was 0.73 0.70 pg/kg. 

Delves HT, Campbell MJ. 1988. Measurements of total lead concentrations and of lead isotope ratios in whole blood by use of inductively coupled plasma source mass spectrometry. J Analytical Atomic Spectrometry 3 343-348. 

Hirao Y, Mabuchi H, Fukuda E, et al. 1986. Lead isotope ratios in Tokyo Bay sediments and the implications in the lead consumption of Japanese industries. Geochemical Journal 20 1-15. 

Flegal and Stukas [406] described the special sampling and processing techniques necessary for the prevention of lead contamination of seawater samples, prior to stable lead isotopic ratio measurements by thermal ionisation mass spectrometry. Techniques are also required to compensate for the absence of an internal standard and the presence of refractory organic compounds. The precision of the analyses is 0.1 -0.4% and a detection limit of 0.02 ng/kg allows the tracing of lead inputs and biogeochemical cycles. 

A different perspective is provided in the analysis of pre-industrial and contemporary Alaskan Sea otter skeletons. The total concentrations of lead in the two groups of skeletons were similar, but their isotopic compositions were different. The pre-industrial skeletons contained lead with an isotopic ratio corresponding to natural... 

Furuta, N. (1991). Interlaboratory comparison study on lead isotope ratios determined by inductively coupled plasma mass spectrometry. Analytical Sciences 7 823-826. 

Hamester, M., Stechmann, H., Steiger, M. and Dannecker, W. (1994). The origin of lead in urban aerosols - a lead isotopic ratio study. Science of the Total Environment 146/7 321-323. 

Walder, A.J. and Furuta, N. (1993). High-precision lead isotope ratio measurements by inductively coupled plasma multiple collector mass spectrometry. Analytical Sciences 9 675-680. 

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