Furthermore, Parentium and Daughterium are so different in chemical properties that they don't otherwise occur together.
Note that the mass number may vary for an element, because of a differing number of neutrons.
We could be sure that a mineral containing parentium originally had no daughterium.
If the mineral contained 1 part per million Parentium-123 and 3 parts per million Daughterium-123, we could be sure all the Daughterium-123 was originally Parentium-123.
For this purpose, isochron dating was developed, a process "that solves both of these problems (accurate date, assumptions) at once" (Stasson 1992). 2) The process must occur at a relatively uniform rate.
It has been established through extensive experimentation that radioactive decay occurs at a constant rate. In this case, the initial condition is the amount of daughter isotope in the rock when it was formed.