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).
The slope of this line is a function of the age of the rock. The reason scientists normalize with another stable isotope of the same element as the daughter is because most chemical or physical processes that occurs normally in nature does not differentiate between different isotopes of the same element when the difference in mass is as small as it is between isotopes of the same element that is used in radiometric dating.
Let us critically examine each of these claims and see if they hold up against the science.
Not only that, different radioactive isotopes decay differently and it is enormously improbable that a postulated difference in decay rates would affect all of them in the same way, yet as we have seen, different radiometric dating methods converge on the same date (within margins of error).
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).
Measuring this ratio gives us an idea of how long ago the rock formed. Doesn’t this assume that the rocks are closed systems?
Both of these are divided or normalized by a stable isotope of the same elements as the daughter element.
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).