Lead–lead dating
A radiometric method that uses the ratio of two lead isotopes produced by two uranium decay chains to obtain very precise ages.
Uranium has two long-lived isotopes. Uranium-238 decays through a chain to lead-206 with a half-life of about 4.47 billion years, while uranium-235 decays to lead-207 with a half-life of about 704 million years. Because both clocks run in the same sample at different speeds, the ratio of radiogenic ²⁰⁷Pb to ²⁰⁶Pb depends only on age and the present ²³⁸U/²³⁵U ratio. Lead–lead dating therefore needs no measurement of uranium concentration and is insensitive to recent uranium loss. Common (initial) lead is corrected using the non-radiogenic ²⁰⁴Pb, often via isochrons on leached fractions.
Applied to the oldest solar-system materials it gives precision of a few hundred thousand years over 4.5 billion. Work in the 2000s showed that ²³⁸U/²³⁵U is not perfectly constant, so modern studies measure it on each sample; older ages that assumed a fixed value of 137.88 can be off by around a million years, and by more in extreme cases.
CAIs from the CV chondrites Allende and Efremovka give ages of about 4,567.3 million years, the standard reference for the Solar System's age. Historically, Clair Patterson's 1956 result of about 4.55 billion years for Earth used lead from troilite in the Canyon Diablo iron meteorite.
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