# Radiometric dating moon rocks

An event like metamorphism could heat the crystal to the point where Pb will become mobile.

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

If a zircon crystal originally crystallizes from a magma and remains a closed system (no loss or gain of U or Pb) from the time of crystallization to the present, then the Discordant dates will not fall on the Concordia curve.

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.

The initial ratio has particular importance for studying the chemical evolution of the Earth's mantle and crust, as we discussed in the section on igneous rocks. If this happens, then the date obtained will be older than the date at which the magma erupted.

We can also construct a Concordia diagram, which shows the values of Pb isotopes that would give concordant dates.

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

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*.

By definition, D* = N-1) (2) Now we can calculate the age if we know the number of daughter atoms produced by decay, D* and the number of parent atoms now present, N.