Periodic Table--Lead

Lead has four stable, naturally occurring isotopes: ²⁰⁴Pb (1.4%), ²⁰⁶Pb (24.1%), ²⁰⁷Pb (22.1%) and ²⁰⁸Pb (52.4%). ²⁰⁶Pb, ²⁰⁷Pb and ²⁰⁸Pb are all 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 vary markedly: 4.47 x 10⁹, 7.04 x 10⁸ and 1.4 x 10¹⁰ years, respectively. Each is reported relative to ²⁰⁴Pb, the only non-radiogenic stable isotope. The ranges of isotopic ratios for most natural materials are 14.0-30.0 for ²⁰⁶Pb/²⁰⁴Pb, 15.0-17.0 for ²⁰⁷Pb/²⁰⁴Pb and 35.0-50.0 for ²⁰⁸Pb/²⁰⁴Pb, although numerous examples outside these ranges are reported in the literature. Because U, Th and Pb have different geochemical behaviors, the fact that the Pb isotopic composition of any material is the composite of the three independent decay chains creates the potential for greater variability of isotopic values in minerals of a single rock relative to that for the Rb-Sr system. Also, atmospheric Pb tends to become concentrated in the uppermost, organic-rich soil horizons, while Pb in ground water should be dominated by that derived from rock weathering. Thus, the relative importance of the two sources in stream water can be determined.

Potential complications to the use of Pb isotopes, such as anthropogenic contamination, immobility of Pb in ground water, and the possibility for highly radiogenic Pb to exist in accessory minerals, can also be used to an advantage. Anthropogenic Pb (Veron et al., 1994) and accessory-phase Pb in minerals have distinct compositions, and the immobility of Pb produces concentration covariance (Nimz, 1998).

Another useful lead isotope for catchment research is ²¹⁰Pb, a radiogenic (via ²³⁸U and ²²²Rn) and radioactive isotope with a half-life of 22.1 years. ²¹⁰Pb can be used to age-date materials formed in the last 100 or so years, with typical precision of a few years under favorable conditions. The ²¹⁰Pb "age" reflects the time since the lead became incorporated in the material-- usually because of ²²²Rn or atmospheric fallout--or can be augmented by additional ²¹⁰Pb contributed by decay of uranium in the underlying geologic materials, thus complicating age determinations. A very powerful application is to use ²¹⁰Pb to age-date the deposition of layered materials and then to relate the changes in the chemical and isotopic compositions of the layers to changes in ambient environmental conditions.

Source of text: This review was assembled by Dan Snyder, Carol Kendall and Eric Caldwell, from the references below.