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Lead

Lead has five stable isotopes. Three of these isotopes are radiogenic and are produced through the decay of uranium. Natural variations in lead isotope ratios are useful for determining the source of lead pollution in the environment.


Cost of Analysis (return to top)

Thermal Ionization/Mass Spectrometry (TIMS): $200-$250

Inductively Coupled Plasma-Mass Spectrometry (ICP-MS): price not available.

(See, for example West Coast Analytical Service)

(See also University of Rochester ICP-MS Laboratory)



Origin
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There are 32 lead isotopes in all; a complete listing is available at Resource-World.net. Five isotopes are significant for environmental studies: 204Pb, 206Pb, 207Pb, 208Pb (which are stable, the latter three are produced as the stable end product of uranium and thorium decay), and 210Pb (a radioactive intermediate of 238U decay).

The stable isotopes
204Pb occurs naturally and is not produced through radioactive decay; the other stable Pb isotopes are radiogenic and produced by the decay of other elements:

238U decays to 206Pb
235U decays to 207Pb
232Th decays to 208Pb

Lead isotope ratios are a function of the amount of uranium and thorium present. Geological processes affect the amount of U and Th present, thus, lead isotopes serve as a useful tool for understanding the nature and timing of these processes. Because the lead isotopic composition of geologic material is a function of three independent decay chains, there is a great potential for isotopic variability in minerals.

As an example, uranium and thorium concentrate in the liquid phase during melting and crystallization of magma, and are subsequently incorporated into acidic, silica-rich components. Thus, granites have high uranium and thorium content compared to basaltic rocks. Thorium is enriched compared to uranium in low-calcium granites. Sedimentary and igneous rocks have similar thorium/uranium ratios, but carbonate-rich rocks are strongly enriched in uranium.

In contrast to the stable isotopes of lead, 210Pb is a radioactive intermediate in the complex decay series of 238U. There are numerous intermittent daughter isotopes, among them is the radioactive gas, 222Rn (t1/2 = 3.8 days). As a gas, radon makes its way out of the ground or water and into the atmosphere. The flux of radion averages 42 atoms/min. cm2 of land surface. This radon then decays in the atmosphere to the relatively stable isotope 210Pb (t1/2 = 22.3 years). 210Pb is quickly removed from the atmosphere through precipitation and is deposited in lakes, glaciers, ice and snow, where it eventually decays to 206Pb.

See USGS' Periodic Table - Lead for more information on lead isotopes.



Measurement Techniques (return to top)

Lead isotopes are most commonly measured using thermal ionization mass spectrometry (TIMS). Since there is only one nonradiogenic isotope, instrument fractionation cannot be corrected (see Sr-Rb), thus, care must be taken when running a sample.

Sample preparation
Pb must be separated for most environmental work. Because it is very easily contaminated; clean lab techniques are required.

Isotopic ratio ranges
Pb isotopes are reported as a ratio with respect to the non-radiogenic isotope, 204Pb. Ranges for most natural materials are as follows:

Geologic dating
U-Pb, Th-Pb, and Pb-Pb isotopic ratios may be used in age dating and petrogenetic tracing of igneous, metamorphic, and hydrothermal rocks. Since there is a divergence in chemical behavior between uranium, thorium, and their daughter elements, many geological processes can lead to extensive fractionation of the various isotopes. This results in distinctive patterns that allow determination of rock histories.

Because Pb is produced through unique decay reactions, several methods can be used to determine the ages of rocks and the geologic processed that have affected them. Chief among these are:

1) common Pb methods, or single stage models (e.g., Holmes-Houtermans model; see Holmes 1946, Houtermans 1946) for lead minerals that have single-stage histories, i.e., that experienced no lead gain or loss. Such models plot trajectories of Pb growth after a primeval Pb state from the time of deposition in the earth's crust. The Holmes-Houtermans model provides an equation to date samples of common Pb. The model assumes 1) that at the time the Earth was forming, U, Th, and Pb were uniformly distributed; 2) that when the Earth became hardened small differences appeared in the U/Pb ratio, which changed as the result of U decay; and 3) that from the time common Pb minerals (such as galena) are formed, their isotopic composition remains constant;

2) two-stage models provide a refinement to the above models. These assume a primordial composition for Pb, which is then followed by a value for the date of the lead incorporation into a rock or an ore deposit; this may then be followed by the age of a subsequent metamorphic event or geochemical differentiation which results in lead gain or loss. Such models frequently use U-Pb Concordia Diagrams to plot 207Pb/235U against 206Pb/238U and compare the ratios with a concordia line that indicates the path that would be followed if the minerals had not suffered chemical disturbance. The amount of lead loss is shown as a chord connecting age of the material and the time lead was lost.

For more information on geologic dating methods, see Faure 1986 (pp. 309-340).



Hydrological Applications (return to top)


The most common applications of Pb isotopes in hydrological sciences are:

1) using the distinct isotopic composition of lead ratios in surface waters to identify pollution sources
(See, for example, Viers et al. 1999)

2) using 210Pb to date recent deposition of snow, lake sediments, etc. 210Pb has a half-life of 22.3 years, allowing dating within the past 100 years. It is also useful in determining changes in ambient environmental conditions.

3) determining the relative importance in stream water of atmospheric Pb (which concentrates in the upper soil layers) versus the Pb in groundwater that is derived from chemical weathering processes.


Other applications (return to top)

Aside from the geological applications identified in the "Measurement Techniques" section above, lead ratios can help to trace pollution in the atmosphere.
(See Keeler and Graney for more information)

Similarly, lead isotopes can be used in archaeology to date ores used in artifacts.
(See, the Smithsonian Center for Materials Research and Education for more information on use of lead isotope analysis for the dating of artifacts)


References and further reading (return to top)

  • Attendorn, H.-G., and R.N.C. Bowen, Radioactive and Stable Isotope Geology, Chapman and Hall, New York, 1997.

  • Bowen, R., Isotopes in the Earth Sciences, Elsevier, New York, 1988.

  • Faure, G., Principles of Isotope Geology, 2nd ed., John Wiley & Sons, New York, 1986.

  • Flegal, A.R., H. Maring, and S. Niemeyer, Anthropogenic lead in Antarctic sea water, Nature 365, Sept. 1993.

  • Holmes, A., An estimate of the age of the earth, Nature, 157, 680-684, 1946.

  • Houtermans, F.G., Die Isotopenhäufigkeiten im natürlichen Blei und das Alter des Urans. Naturwissenschaften, 33, 185-186, 219, 1946.

  • Stacey, J.S., and J.D. Kramers, Approximation of terrestrial lead isotope evolution by a two-stage model, Earth Planet. Sci. Let., 26, 207-221, 1975.

  • Sturges, W.T., and L.A. Barrie, Lead 206/207 isotope ratios in the atmosphere of North America as tracers of U.S. and Canadian emissions, Nature, 239, Sept. 1987.


Internet resources (return to top)

Archaeotrace, Lead Isotope Analysis

Bentor, Y., Lead, ChemicalElements.com

Boston College, Dept. of Chemistry, Web Elements - Lead

Encyclopedia.com, Lead - Properties and Isotopes

Georgia State University, Nuclear Physics Laboratory, Clocks in the Rocks

Keeler, G.J., and J.R. Graney, Final Report: Environmental Applications of Novel Instrumentation for Measurement of Lead Isotopes in Atmospheric Pollution Source Apportionment Studies, EPA Grant Number: R826177.

Kysar Mattietti, G., J. Lewis, and R. Wysoczanski, Lead Isotope Study of the Paleogene Igneous Rocks of the Sierra Maestra, Southeastern Cuba, Geological Society of America Annual Meeting, 2001.

Lawrence Berkeley Laboratory, Isotopes Project Home Page, Isotopes of Lead

Oak Ridge National Laboratory, Isotope Production and Distribution - Lead

Phoenix College, Chemlab Server, Periodic Table of Isotopes

Resource-World.net, Lead

Smithsonian Institution, Smithsonian Center for Materials Research and Education, Annual Report FY 1995, Lead Isotope Program

USGS, Periodic Table - Lead (http://wwwrcamnl.wr.usgs.gov/isoig/period/pb_iig.html)

Viers, J.H., M. McCoy, J.F. Quinn, and M.L. Johnson, Nonpoint source pollution modeling in the north coast of California within a GIS: a predictive screening tool for watershed management, paper presented at the 1999 ESRI User's Conference



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