Uranium 235 decays to lead 207, and Thorium 232 decays to lead 208.
In addition there is another stable isotope, lead 204, that is entirely primordial and does not form via radioactive decay at all.
With modern laser ablation methods, a detailed history can be derived for single zircon grains showing the ages of growth zones within the zircon. " Suppose you actually want to date uranium minerals themselves, say in a pegmatite or uranium deposit.
Fortunately, there is Lead 204, which we can be sure has no radiogenic component.
With a little luck some of the points will plot close to the true age, increasing the precision of the date.
U-Pb ages are accurate to within less than 1 per cent, highly resistant to metamorphism, and are considered the most precise ages now available.
Unfortunately, the breakdown of uranium creates radiation damage in the surrounding crystal, creating avenues for lead to escape.
The final point where the sample plots, will be some linear combination of the true concordia age and some final composition where lead diffusion stopped, either the present or some past metamorphic event.
This region is surrounded by many eruption centers (i.e.