Parent radiometric dating
Note that the mere existence of these assumptions do not render the simpler dating methods entirely useless.
Rb-Sr is a good example for explaining the process, however.Simply counting the number of rings will give one a fairly good idea of the age of the tree.Periods of heavy rain and lots of sunshine will make larger gaps of growth in the rings, while periods of drought might make it difficult to count individual rings. When a given quantity of an isotope is created (in a supernovae, for example), after the half-life has expired, 50% of the parent isotope will have decomposed into daughter isotopes.Isochron methods avoid the problems which can potentially result from both of the above assumptions.
Isochron dating requires a fourth measurement to be taken, which is the amount of a different isotope of the same element as the daughter product of radioactive decay.Meteorites are among the oldest objects we know about - formed about 4.5 billion years ago. This article describes the principles and methods used to make that determination.