The observed proportions of parent and daughter give the date: But that cannot be universally assumed, and there is a way to get around the initial presence of daughter element.
This uses the fact that the parent and daughter elements are likely to have different chemical affinities, thus preferring to crystallize in different minerals of a solidifying rock.
It naturally has different concentrations of Rb and Sr in different parts of the rock because these don’t necessarily mix perfectly. Once it cools all the way and crystallizes, it is considered "born" and atoms can no longer come in or out of the system.
At this point, its radiometric clock starts ticking.
Note that the mere existence of these assumptions do not render the simpler dating methods entirely useless.
In many cases, there are independent cues (such as geologic setting or the chemistry of the specimen) which can suggest that such assumptions are entirely reasonable.
So some of a rock's mineral crystals may have a lot of parent, and some a lot of daughter.
Radioisotope dating (also referred to as radiometric dating) is the process of estimating the age of rocks from the decay of their radioactive elements.This nuclide decays to Strontium-87 (Sr87) with a half-life of 48.8 billion years.Imagine going way back in time and looking at some lava that is cooling to become a rock.(For brevity's sake, hereafter I will refer to the parent isotope as ).In addition, it requires that these measurements be taken from several different objects which all formed at the same time from a common pool of materials.