Potassium occurs in two stable isotopes (Ar atoms trapped inside minerals.
What simplifies things is that potassium is a reactive metal and argon is an inert gas: Potassium is always tightly locked up in minerals whereas argon is not part of any minerals. So assuming that no air gets into a mineral grain when it first forms, it has zero argon content.
Therefore, mafic rocks and minerals often contain less potassium than an equal amount of silicic rock or mineral.
Potassium can be mobilized into or out of a rock or mineral through alteration processes.
That is, a fresh mineral grain has its K-Ar "clock" set at zero.
The method relies on satisfying some important assumptions: Given careful work in the field and in the lab, these assumptions can be met.
The quantity of potassium in a rock or mineral is variable proportional to the amount of silica present.
The potassium-argon (K-Ar) isotopic dating method is especially useful for determining the age of lavas.
Developed in the 1950s, it was important in developing the theory of plate tectonics and in calibrating the geologic time scale.
We strongly prefer to carry out final sample preparation and loading for irradiation here at Lehigh, and will either do this ourselves or guide you in these final steps.
Any costs you have been quoted for analyses do not include the cost of mineral-separation supplies or technician time, and in general we will do this work for you only under special circumstances.