Since the two U isotopes should migrate together, and the three Pb isotopes should migrate together, the two U/Pb dates can be correlated, using the "concordia" method. Where the two isotope ratios give the same age is a "concordia line"; losses of Pb (or gains of U) move the ratios along a "discordia line".

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Ó 2010 Arthur V. Chadwick, Ph.D.