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Computer plot of results of U-Pb analyses of zircons from a granite in N. Labrador. Each ellipse represents one analysis with its associated errors.

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.

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