One out of every 10, Potassium atoms is radioactive Potassium K For example lavas dated by K-Ar that are historic in age, usually show 1 to 2 my old ages due to trapped Ar. For every K atoms that decay, 11 become Ar But consider what happens if the argon came from deep within the Earth, where it was formed by 40K decay, and was then trapped in magma or transported into the rock by hydrothermal fluid.
Table 8. Effects of sample purity on discordant mineral ages found in K-Ar dating. Radiogenic helium and argon in ultramafic inclusions from Hawaii. This is determined by drawing a horizontal line from 0.