The graphs shows two ages of formation for different
components of the granite. Note the errors of ±3 and ±5
Ma for a rock that is more than 2,500 Ma old. The graph on the lower
right is an analysis of an additional component of the same rock
indicating that its “clock” was reset by high temperature
metamorphism at 1311 ±8 Ma.
A second application of radiogenic isotopes is the use of samarium-neodymium
(Sm-Nd) ratios in granites to probe the lower crust. Granites are
produced mainly by the melting of source rock in the deep crust.
They travel upward, bringing with them a Sm-Nd ratio and Nd isotopic
composition inherited from their source.