Lead–lead dating

Lead–lead dating is a method for dating geological samples, normally based on 'whole-rock' samples of material such as granite. For most dating requirements it has been superseded by uranium–lead dating (U–Pb dating), but in certain specialized situations (such as dating meteorites and the age of the Earth) it is more important than U–Pb dating.

Decay equations for common Pb–Pb dating Three stable "daughter" Pb isotopes result from the radioactive decay of uranium and thorium in nature; they are 206Pb, 207Pb, and 208Pb. 204Pb is the only non-radiogenic lead isotope, therefore is not one of the daughter isotopes. These daughter isotopes are the final decay products of U and Th radioactive decay chains beginning from 238U (half-life 4.5 Gy), 235U (half-life 0.70 Gy) and 232Th (half-life 14 Gy) respectively. With the progress of time, the final decay product accumulates as the parent isotope decays at a constant rate. This shifts the ratio of radiogenic Pb versus non-radiogenic 204Pb (207Pb/204Pb or 206Pb/204Pb) in favor of radiogenic 207Pb or 206Pb. This can be expressed by the following decay equations:

( Pb 207 Pb 204 ) P = ( Pb 207 Pb 204 ) I + ( U 235 Pb 204 ) P ( e λ 235 t − 1 ) {\displaystyle {\left({\frac {{\ce {^{207}Pb}}}{{\ce {^{204}Pb}}}}\right)_{P}}={\left({\frac {{\ce {^{207}Pb}}}{{\ce {^{204}Pb}}}}\right)_{I}}+{\left({\frac {{\ce {^{235}U}}}{{\ce {^{204}Pb}}}}\right)_{P}}{\left({e^{\lambda _{235}t}-1}\right)}}

( Pb 206 Pb 204 ) P = ( Pb 206 Pb 204 ) I + ( U 238 Pb 204 ) P ( e λ 238 t − 1 ) {\displaystyle {\left({\frac {{\ce {^{206}Pb}}}{{\ce {^{204}Pb}}}}\right)_{P}}={\left({\frac {{\ce {^{206}Pb}}}{{\ce {^{204}Pb}}}}\right)_{I}}+{\left({\frac {{\ce {^{238}U}}}{{\ce {^{204}Pb}}}}\right)_{P}}{\left({e^{\lambda _{238}t}-1}\right)}}

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