Excuse me for butting in but here is a succinct explanation for the phenomena you are discussing.
How is the Atomic Clock Set?
When rocks are heated to the melting point, any Ar-40 contained in them is released into the atmosphere. When the rock recrystallizes it becomes impermeable to gasses again. As the K-40 in the rock decays into Ar-40, the gas is trapped in the rock.
http://id-archserve.ucsb.edu/..._Potassium_Argon_Dating.html
Hope this helps,
lfen