Uranium-Lead Dating

Ages determined by radioactive decay are always subject to assumptions about original concentrations of the isotopes. The natural radioactive series which involve lead as a daughter element do offer a mechanism to test the assumptions. Common lead contains a mixture of four isotopes. Lead 204, which is not produced by radioactive decay provides a measure of what was "original" lead. It is observed that for most minerals, the proportions of the lead isotopes is very nearly constant, so the lead-204 can be used to project the original quantities of lead-206 and lead-207. The two uranium-lead dates obtained from U-235 and U-238 have different half-lives, so if the date obtained from the two decays are in agreement, this adds confidence to the date. They are not always the same, so some uncertainties arise in these processes.

There are powerful rationales for using lead isotopes as indicative of concentrations at the point when the lead-containing mineral was in the molten state. Since the isotopes of lead are chemically identical, any processes that brought lead into the mineral would be completely indiscriminate about which isotope was brought in. The forming mineral will incorporate lead-204, lead-206 and lead-207 at the ratio at which they are found at that location at the time of formation. Any departure from the original relative concentrations of lead-206 and lead-207 relative to lead-204 could then be attributed to radioactive decay.

Making use of the decay constants of both ²³⁸U and ²³⁵U, plus the fact that the consistent isotopic ratio of ²³⁸U/²³⁵U = 137.88 is found, Holmes and Houtermans developed a system to use the ratios of the lead isotopes to produce Pb-Pb isochrons for dating minerals. This approach is generally considered to be the most precise for determining the age of the Earth.

Potassium-Argon Method