Nd ratios on several minerals with a mass spectrometer and then from the slope determine the age of the rock. If a magma cools quickly on the surface of the Earth, some of the Ar may be trapped.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.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.
Later called Ötzi the Iceman, small samples from his body were carbon dated by scientists.
The results showed that Ötzi died over 5000 years ago, sometime between 33 BC. Uranium has a very long half-life and so by measuring how much uranium is left in a rock its approximate age can be worked out.
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: ).
For example lavas dated by K-Ar that are historic in age, usually show 1 to 2 my old ages due to trapped Ar.