What is k ar dating used for
The rock samples are crushed, in clean equipment, to a size that preserves whole grains of the mineral to be dated, then sieved to help concentrate these grains of the target mineral.
The selected size fraction is cleaned in ultrasound and acid baths, then gently oven-dried.
The target mineral is separated using heavy liquids, then hand-picked under the microscope for the purest possible sample.
Lava flows that lie above and below rock beds with ancient human fossils are a good—and true—example.
However, we cannot rely on all the argon being lost, and if it is not then when we apply K-Ar dating this will give us an essentially arbitrary date somewhere between the formation of the rock and the metamorphosis event.