This means that the while different rocks contain different absolute amounts of the two isotopes, the is same.
Furthermore, by studying supernovas far away, scientist have determined that decay rates have been constant in the ancient past as well.
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).
But what about assuming that initial amounts are known?
A second property of isochron diagrams is that it actually gives the initial amount of daughter isotope as a result of the method. At this intersect, the ratio of parent/(another stable isotope of the same element as the daughter) is by definition 0 and so no amount of the daughter here is produced by decay of the parent in the rock.
Surely, if some daughter nuclei left the rock or parent nuclei entered the rock, the dates would come out all wrong! Follow Debunking Denialism on Facebook or Twitter for new updates.
While this is technically true, there are several mini-industries dedicated developing methods and techniques to make sure that there is no contamination and check to see if the rocks where disturbed between forming and being tested by scientists. On of the great things about many forms of radiometric dating is that they are self-checking.
If so, the data will fall on an isochron line, but will be all over the place.
This tells scientists that the sample has been disturbed and cannot be dated with this particular method.
A very important tool in radiometric dating is the so called isochron diagram and it holds the key to refuting the central creationist claims about radiometric dating.
One of the most beneficial things about it is that it can check itself for accuracy; the method tells you how well the rocks have been closed systems.
Those that did the decaying are called parent nuclei.