Air consists roughly of $N_2$, $O_2$ and $Ar$.
Why is $Ar$ the relatively common noble gas compound and not for example the lighter ones $He$ or $Ne$?
Argon is produced when Potassium 40 decays. The majority of Argon in Earth's atmosphere comes from this source. We know this because K-40 decay produces Ar-40, and the majority of Ar in the atmosphere is Ar-40. In the sun the majority is Ar-36.