206Pb207Pb

206Pb and 207Pb are two stable isotopes of lead. They are the end products of natural radioactive decay chains starting from uranium isotopes: 238U decays to 206Pb and 235U decays to 207Pb. Because these decay processes occur at well-known rates, the relative amounts of radiogenic lead increase over time in a closed system.

The decay of 238U to 206Pb has a half-life of about 4.47 billion years, while 235U to 207Pb has a half-life of about 703.8 million years. As uranium in minerals slowly decays, radiogenic 206Pb and 207Pb accumulate. The present-day 206Pb/207Pb ratio thus encodes the elapsed time since the system closed to uranium loss and gain.

In uranium–lead dating, measurements of lead isotopes are combined with uranium isotopic ratios to determine ages. The two decay channels provide two independent clocks, enabling cross-checks and more robust ages. Methods include the concordia diagram, which plots 206Pb/238U against 207Pb/235U to yield a concordant age for closed samples, and Pb–Pb dating schemes that use ratios involving nonradiogenic lead (common lead) to construct isochrons and derive ages even when initial lead is present.

Applications span geology, planetary science, and archaeology, with U–Pb dating among the most widely used for dating the oldest terrestrial rocks and meteorites. Limitations arise from lead loss, isotopic resetting, or excess nonradiogenic lead, which can complicate interpretations and require concordia or isochron approaches to constrain ages.