The initial ratio has particular importance for studying the chemical evolution of the Earth's mantle and crust, as we discussed in the section on igneous rocks. First let me note that there is a lot more going on than just mixing. The rock or mineral must have remained closed to the addition or escape of parent and daughter atoms since the time that the rock or mineral system formed. As the magma chamber is depleted in daughter products, subsequent lava flows and ash beds would have younger dates.
What radiometric dating needs to do to show its reliability is to demonstrate that no such fractionation could take place. Using such techniques, scientists can very accurately date ancient volcanic events and can extrapolate from these dates to learn about the ages of other rock strata. In fact, more is true. I now describe this mixing in more detail.