Radimetric Dating
3 assumptions:
1) the rate of decay of parent into daughter has remained constant throughout the unobservable past;
2) the specimen being examined hasn’t been contaminated in any way (that is, no parent or daughter has been added or taken away at any point during the unobservable past); and
3) we can determine how much parent and daughter were present at the beginning of the decay process—not all of the Pb206 present today necessarily came from decaying U238; Pb206 may have been part of the original constitution of the specimen.
If any of these assumptions are wrong, the method cannot accurately determine the age of a specimen.
https://www.gotquestions.org/radiometric-dating.html
