How reliable is potassium argon dating
Rubidium–strontium (Rb–Sr) dating was the first technique in which the whole rock isochron method was extensively employed.
Certain rocks that cooled quickly at the surface were found to give precisely defined linear isochrons, but many others did not.
If a simple mineral is widespread in the geologic record, it is more valuable for dating as more units can be measured for age and compared by the same method.
In this case, a valid age can still be obtained, provided that they remain within the rock.
Should a point plot below the line, it could indicate that a particular sample was open to migration of the dating elements or that the sample was contaminated and lay below the isochron when the rock solidified.
Minerals, too, are predictable chemical compounds that can be shown to form at specific temperatures and remain closed up to certain temperatures if a rock has been reheated or altered.
A rock, on the other hand, may contain minerals formed at more than one time under a variety of conditions.