With time each would then develop additional daughter abundances in proportion to the amount of parent present.
If a number of samples are analyzed and the results are shown to define a straight line within error, then a precise age is defined because this is only possible if each is a closed system and each has the same initial ratio and age.
In this case, it is a big advantage to present the data in a form in which the abundance of both the parent and daughter isotopes are given with respect to the abundance of the initial background daughter.The incremental additions of the daughter type can then be viewed in proportion to the abundance of parent atoms. It has already been shown— In practice, the isochron approach has many inherent advantages.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.By careful selection, certain minerals that contain little or no daughter element but abundant parent element can be analyzed.In this case, the slope of the line in potassium–argon (K–Ar) dating, for example, because most minerals do not take argon into their structures initially.
For example, the mineral those composed primarily of one or more ferromagnesian, dark-coloured minerals).