How are scientists able to assume that igneous rock contains no 40Ar when it first forms?
Argon is a very mobile element and is also completely incompatible in mineral structures. This means that it will never get initially locked in to a mineral during formation and therefore any quantity found within a rock must have been produced by radioactive decay. As argon is also very mobile, any produced by radioactive decay while the minerals are above their respective closure temperatures will be lost meaning that what we find now must have been produced since the rock was last heated significantly. This mobility is also the biggest weakness of using argon to date rocks. As it is so mobile it is prone to being lost quite easily and thus has potential to produce ages which are too young when compared to other products such as lead.
