As it decays, it disrupts the crystal and allows the lead atom to move.
This does not mean that all rock samples are unreliable, but it is possible to account for a process which throws off the data for metamorphic rocks.
For example, with Uranium-lead dating with the crystallization of magma, this remains a closed system until the uranium decays.
If it does not entirely contradict them, we put it in a footnote.
And if it is completely 'out of date', we just drop it."Few archaeologists who have concerned themselves with absolute chronology are innocent of having sometimes applied this method...
For these reasons, if a rock strata contains zircon, running a uranium-lead test on a zircon sample will produce a radiometric dating result that is less dependent on the initial quantity problem.