Two different methods of geochronology allow geologists to date small quantities of minerals:
The 1st one uses the radioactive decay of the natural Potassium 40, obtained from the ratio K40/K39. The reactor neutrons are utilised to transform Potassium into Argon, and then the ratio Ar40/Ar39 is measured
The 2nd one is convenient to date minerals that contain Uranium. The ratio U235/U238 is representative of the age and it is obtained by irradiating the mineral and counting fissions of U235, compared to the number of spontaneus fissions of U238.