However, it is easy to imagine scenarios that could cause isochrons to yield false daates, and geologists recognize that isochrons can yield bad dates.
The idea of isochrons is this: Suppose X is a parent element that decays in to Y and Z is another isotope of Y not produced by radioactive decay. Since Y and Z are isotopes, we would assume they have similar chemical properties.
Suppose A is very old (or appears very old) and B is very young. Their perceived radiometric age would then be between that of A and B.
For purposes of illustration, consider the rubidium-strontium decay system.
It follows that the ratios x/z and y/z have a linear relationship whose slope yields the age of the sample.
If these ratios are observed to obey such a linear relationship in a series of rocks, then an age can be computed from them.
When a single body of liquid rock crystallizes, parent and daughter elements may separate so that, once solid, the isotopic data would define a series of points, such as those shown as open circles designated R.
With time each would then develop additional daughter abundances in proportion to the amount of parent present.