Thus, the lead isotopic composition of any mineral or rock depends upon its age and the environment from which it was formed; that is, it would depend upon the ratio of uranium plus thorium to lead in the parent material.The Earth can be assumed to be a very large sample containing lead evolving from primordial lead by radiogenic increments.
Of the four isotopes of lead, two are formed from the uranium isotopes and one is formed from the thorium isotope; only lead-204 is not known to have any long-lived radioactive progenitor.
These elements are usually present in constant (equilibrium) amounts in a sample, as long as the system remains closed.
A closed system is one in which there is no free exchange of elements in a material with the outside environment.
To me it has been a real eye opener to see all the processes that are taking place and their potential influence on radiometric dating.
Radiometric dating is largely done on rock that has formed from solidified lava.