Radiometric dating applied metamorphic rocks
Uranium-lead dating methods often use this approach because some of the minerals used in dating lose the lead decay products over time.It's amazing how often people fail to realize that you can't date materials if they don't have the necessary ingredients. You can't use carbon-14 to date an arrowhead with no carbon in it.
If the decay constant is very small, even tiny amounts of contamination by other radioactive materials can be very significant.Imagine we have an undiscovered element, Parentium, that has a radioactive isotope, Parentium-123, which decays to stable Daughterium-123.This is the only way Parentium-123 decays, and there is no other source of Daughterium-123.In other words there was originally 4 parts per million Parentium-123 and 0 parts per million Daughterium-123.
Since there is now only 1/4 of the original amount of Parentium-123, we know that two half-lives of Parentium-123 have elapsed.If you don't have minerals with those elements, you can't date the rock.