In some cases the discovery of a rare trace mineral results in a major breakthrough as it allows precise ages to be determined in formerly undatable units.For example, the mineral those composed primarily of one or more ferromagnesian, dark-coloured minerals).Should a point plot below the line, it could indicate that a particular sample was open to migration of the dating elements or that the sample was contaminated and lay below the isochron when the rock solidified.
If a simple mineral is widespread in the geologic record, it is more valuable for dating as more units can be measured for age and compared by the same method.
However, if a single parent–daughter pair that is amenable to precise analysis can be measured in a variety of minerals, the ages of a wide variety of rock types can be determined by a single method without the need for intercalibration.
This is true because uranium is continuously creating more lead.
A lead-rich mineral formed and isolated early in Earth history would have a low lead-206 to lead-204 ratio because it did not receive subsequent additions by the radioactive decay of uranium.
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.
With time each would then develop additional daughter abundances in proportion to the amount of parent present.
If the Earth’s interior were a simple and homogeneous reservoir with respect to the ratio of uranium to lead, a single sample extracted by a volcano would provide the time of extraction. No parent–daughter value for a closed system is involved, rather just a single isotopic measurement of lead viewed with respect to the expected evolution of lead in the Earth.