Non radiometric dating
For this purpose, isochron dating was developed, a process "that solves both of these problems (accurate date, assumptions) at once" (Stasson 1992). 2) The process must occur at a relatively uniform rate.
It has been established through extensive experimentation that radioactive decay occurs at a constant rate. In this case, the initial condition is the amount of daughter isotope in the rock when it was formed.
Isochron dating bypasses the necessity of knowing the quantity of initial daughter product in the rock by not using that value in the computation.
Instead of using the initial quantity of daughter isotope, the ratio of daughter isotope compared to another isotope of the same element (which is not the product of any decay process) is used as the comparison for isochron dating.
The process of evaluating the daughter product as a ratio against another isotope of the same element is a valid method because, when a mineral or rock forms from a homogenous state, the elements that are assimilated into crystalline formation are very restricted.