The potassium-argon dating method has been used to measure a wide variety of ages.
By converting potassium-39 to argon-39 then measuring the argon-39:argon-40 ratio, you can calculate the sample's potassium-40:argon-40 ratio, remembering potassium-40:potassium-39 is fixed.
But micas, plagioclase, hornblende, clays and other minerals can yield good data, as can whole-rock analyses.
Young rocks have low levels of 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.