Lead Isotopes Geochemistry

Lead has four stable, naturally occurring isotopes: 204Pb (1.4%), 206Pb (24.1%), 207Pb (22.1%) and 208Pb (52.4%). Except for 204Pb, which is a primordial isotope, the lead isotopes (206Pb, 207Pb, and 208Pb) are radiogenic and are the end products of complex decay chains that begin at 238U, 235U, and 232Th, respectively. The corresponding half-lives of these decay schemes are 4.47109 yr for 238U-206Pb, 7.04108 yr for 235U-207Pb and 1.41010 yr for 232Th-208Pb. The ratio of these radiogenic isotopes with respect to 204Pb (the only non-radiogenic stable isotope) covers a variety of ranges in natural materials (206Pb/204Pb = 14.0 – 30.0, 207Pb/204Pb = 15.0 – 17.0, 208Pb/204Pb = 35.0 – 50.0).

Applications

As different materials in the earth system present specific ranges of lead isotope ratios, these parameters are extensively used in solid earth geochemistry, geochemical fingerprinting, contamination source tracking, forensic studies, and archaeology.

Solid Earth Geochemistry / Petrology

The lead isotopic composition of volcanic and plutonic rocks can be used to trace the sources of  diverse magma types originating from different tectonic settings.

Lead isotopes sediments
Pb isotope analyses of Pacific Ocean sediments and Mariana Active arc lavas, after Woodhead and Fraser, 1985.

Geochemical Fingerprinting

Two main factors controlling the stable isotope ratios (including Pb) of geologic materials are the age of the material and the ratio of parent to daughter elements. As different rocks present distinct parent/daughter ratios, this property has been extensively used for provenance study of weathered and eroded materials, namely dust.

Lead isotopes dust
Pb isotope composition of dust collected in Barbados compare to various potential sources in Africa, modified after Bozlaker et al., 2018.

Contaminant Source Tracing

Mining and industrial activities as well as intensive use of fossil fuels have resulted in the release of various forms of lead pollutants and other heavy metals to the environment. In recent years, there has been a growing interest in using lead isotopes to trace the source and origin (geogenic vs. anthropogenic) of contamination and to evaluate the persistence of these elements in the environment.

Lead isotopic signature gasoline
Pb isotope signature of gasoline produced in different continent (left panel, after Larsen set al., 2012) and changes in Pb isotope signature of US gasoline through time (right panel, after Dunlap et al., 2008)

Forensic Studies

The collection, examination and evaluation of physical evidence related to criminal cases are the focus of the forensic science. Lead, a common component in bullets, occurs naturally in many metallic and non-metallic ore deposits and different sources of lead present different isotopic signatures. However, in industry, lead from different sources may be combined as ore recycling has become a common practice. This variation in isotopic composition can be utilized to distinguish between lead bullets from different batches.

Isobar lead isotope analysis
Pb isotope signature of bullets from different manufacturers around the world (after Sjåstad et al., 2016).

Archaeology

A growing number of studies have included lead isotope ratios as an additional geochemical fingerprinting technique. Lead isotopes can be used to track the source(s) of archaeological artifacts such as ceramics, metal tools, coins, glazes, and stones. Lead isotopes are also used in human mobility studies and tracking the trade of faunal materials.

References

Bozlaker, A.; Prospero, J. M.; Price, J., and Chellam, S., 2018: Linking Barbados Mineral Dust Aerosols to North African Sources Using Elemental Composition and Radiogenic Sr, Nd, and Pb Isotope Signatures. Journal of Geophysical Research: Atmospheres, vol. 123, pp. 1384–1400. https://doi.org/10.1002/2017JD027505

Dunlap, C. E.; Alpers, C. N.; Bouse, R. M.; Taylor, H. E.; Unruh D. M. and Flegal, A. R., 2008: The persistence of lead from past gasoline emissions and mining drainage in a large riparian system: Evidence from lead isotopes in the Sacramento River, California. Geochimica et Cosmochimica Acta, vol. 72, pp. 5935–5948. DOI:10.1016/j.gca.2008.10.006.

Larsen, M. M.; Blusztajn, J. S.; Andersen, O. Dahllӧf, I., 2012: Lead isotopes in marine surface sediments reveal historical use of leaded fuel. Journal of Environmental Monitoring, vol. 14, pp.2893-2901. DOI: 10.1039/c2em30579h

Sjåstad, K.E; Lucy, D. and Andersen, T., 2016: Lead isotope ratios for bullets, forensic evaluation in a Bayesian paradigm. Talanta, vol. 146, pp.62–70.

Woodhead, J. D. and Fraser, D. G., 1985: Pb, Sr and 10Be isotopic studies of volcanic rocks from the Northern Mariana Islands. Implications for magma genesis and crustal recycling in the Western Pacific. Geochimica et Cosmochimica Acta, vol. 49. pp. 1925-1930