Radiometric dating applied metamorphic rocks
One good example is granite, which normally has some potassium feldspar (Figure 8.4.2).Feldspar does not have any argon in it when it forms.
Isotopic dating of rocks, or the minerals in them, is based on the fact that we know the decay rates of certain unstable isotopes of elements and that these rates have been constant over geological time.
It is also based on the premise that when the atoms of an element decay within a mineral or a rock, they stay there and don’t escape to the surrounding rock, water, or air.
One of the isotope pairs widely used in geology is the decay of K is a radioactive isotope of potassium that is present in very small amounts in all minerals that have potassium in them.
It has a half-life of 1.3 billion years, meaning that over a period of 1.3 Ga one-half of the Figure 8.4.1 The decay of 40K over time.
Each half-life is 1.3 billion years, so after 3.9 billion years (three half-lives) 12.5% of the original 40K will remain.