Lead 207, 208, and 204
207Pb is the end of the Actinium series from 235U.
208Pb is the end of the Thorium series from 232Th.
204Pb is entirely primordial, and is thus useful for estimating the fraction of the other lead isotopes in a given sample that are also primordial (since the relative fractions of the various primordial lead isotopes is constant everywhere). Any excess lead 206, 207, and 208 is thus assumed to be radiogenic in origin, allowing various uranium and thorium dating schemes to be used to estimate the age of rocks (time since their formation).
Read more about this topic: Isotopes Of Lead
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