This assumption is in many cases not valid, as daughter atoms certainly existed in the mineral or rock at the time the radiometric clock started.
If a new mineral grows in a metamorphic rock, and if that mineral incorporates radioactive isotopes in its crystal structure, then dating of that mineral can provide an estimate of the time of mineral growth (metamorphism).
Accurate measurement of either the absolute or relative abundance of trace quantities of radioactive isotopes requires sophisticated instruments, known as mass spectrometers, and instrument operators who really know what they are doing.
The technique appears to be simple and straightforward, but is actually very difficult and time-consuming.
I use the term "appropriate" in the sense that the specimen to be dated must obviously contain isotopes of a well known radioactive decay series, and be suitable for precise chemical analysis.
In the simplest ideal situation,the decay equation is utilized by making the following substitutions: P = N (# of parent atoms currently present as measured inthe lab) Pand can accurately measure D and P, in principle, we can determine the absolute age.