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Sometimes, however, numerous discordant dates from the same rock will plot along a line representing a chord on the Concordia diagram. is then interpreted to be the date that the system became closed, and the younger date, t*, the age of an event (such as metamorphism) that was responsible for Pb leakage.

Pb leakage is the most likely cause of discordant dates, since Pb will be occupying a site in the crystal that has suffered radiation damage as a result of U decay.

After the passage of two half-lives only 0.25 gram will remain, and after 3 half lives only 0.125 will remain etc.

To see how we actually use this information to date rocks, consider the following: Usually, we know the amount, N, of an isotope present today, and the amount of a daughter element produced by decay, D*.

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The Concordia curve can be calculated by defining the following: ).

Zircon has a high hardness (7.5) which makes it resistant to mechanical weathering, and it is also very resistant to chemical weathering. Chemically, zircon usually contains high amounts of U and low amounts of Pb, so that large amounts of radiogenic Pb are produced.

Another possible scenario involves U leakage, again possibly as a result of a metamorphic event.

U leakage would cause discordant points to plot above the cocordia. Lunar rocks also lie on the Geochron, at least suggesting that the moon formed at the same time as meteorites. Pb separated from continents and thus from average crust also plots on the Geochron, and thus suggests that the Earth formed at the same time as the meteorites and moon.

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