K ar dating method
Argon can mobilized into or out of a rock or mineral through alteration Ar and potassium, there is not a reliable way to determine if the assumptions are valid.
Therefore, unlike the conventional K/Ar technique, absolute abundances need not be measured.
If we dated a number of individual grains in the sedimentary rock, we would likely get a range of different dates, all older than the age of the rock.
It might be possible to date some chemical sedimentary rocks isotopically, but there are no useful isotopes that can be used on old chemical sedimentary rocks.
Potassium can be mobilized into or out of a rock or mineral through alteration processes.
Due to the relatively heavy atomic weight of potassium, insignificant fractionation of the different potassium isotopes occurs.