Lead atoms created by uranium decay are trapped in the crystal and build up in concentration with time.

If nothing disturbs the grain to release any of this radiogenic lead, dating it is straightforward in concept.

In a 704-million-year-old rock, 235U is at its half-life and there will be an equal number of 235U and 207Pb atoms (the Pb/U ratio is 1).

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Second, zircon has a high trapping temperature of 900°C.

Its clock is not easily disturbed by geologic events—not erosion or consolidation into sedimentary rocks, not even moderate metamorphism.

With 238U the Pb/U ratio grows much more slowly with age, but the idea is the same.

If you took rocks of all ages and plotted their two Pb/U ratios from their two isotope pairs against each other on a graph, the points would form a beautiful line called a concordia (see the example in the right column). First, its chemical structure likes uranium and hates lead.

However, zircon is so overwhelming a favorite that geologists often just refer to "zircon dating."But even the best geologic methods are imperfect.

Dating a rock involves uranium-lead measurements on many zircons, then assessing the quality of the data.Third, zircon is widespread in igneous rocks as a primary mineral.This makes it especially valuable for dating these rocks, which have no fossils to indicate their age.Fourth, zircon is physically tough and easily separated from crushed rock samples because of its high density.Other minerals sometimes used for uranium-lead dating include monazite, titanite and two other zirconium minerals, baddeleyite and zirconolite.Consider the concordia: as zircons age, they move outward along the curve.