If a rock is heated during its lifetime, the system gets disturbed and some of the parent and/or daughter isotopes may move in or out of the rock.
If you have a rock that contains radioactive isotopes, these will decay over time.As time goes on, the ratio of the parent to daughter nuclei will change and decrease (as more parent nuclei decay into daughter nuclei, the former decreases and the latter increases).As a scientific skeptics, we ask ourselves: is this really the case?Let us critically examine each of these claims and see if they hold up against the science.These claims generally land in three different categories: (1) radiometric dating assumes that initial conditions (concentrations of mother and daughter nuclei) are known, (2) radiometric dating assumes that rocks are closed systems and (3) radiometric dating assumes that decay rates are constant.
Most young earth creationists reject all of these points.
At the time of formation for a rock, the isotopes for an element are homogenized and so the composition of a certain isotope is the same in all the minerals in the rock.
But what happens when the rocks have been disturbed?
So on the x-axis, we have parent/(another stable isotope of the same element as the daughter) and on the y-axis we have daughter/(another stable isotope of the same element as the daughter).
If the samples have been undisturbed closed systems since formation, the data will fall on the same line (the isochron from which the diagram is named).
Those isotopes that are not stable decay into daughter nuclei.