Radiometric dating of rock materials
One good example is granite, which normally has some potassium feldspar (Figure 8.15).Feldspar does not have any argon in it when it forms.Over time, the Ar remain embedded within the crystal, unless the rock is subjected to high temperatures after it forms.The sample must be analyzed using a very sensitive mass-spectrometer, which can detect the differences between the masses of atoms, and can therefore distinguish between Ar in the case of the K-Ar method).
[SE] In order to use the K-Ar dating technique, we need to have an igneous or metamorphic rock that includes a potassium-bearing mineral.
A clastic sedimentary rock is made up of older rock and mineral fragments, and when the rock forms it is almost certain that all of the fragments already have daughter isotopes in them.
Furthermore, in almost all cases, the fragments have come from a range of source rocks that all formed at different times.
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So in order to date most older fossils, scientists look for layers of igneous rock or volcanic ash above and below the fossil.