Glauconite radiometric dating
Each radioactive decay series takes a characteristic length of time known as the radioactive half-life, which is the time taken for half of the original (parent) isotope to decay to the new (daughter) isotope.
The decay series of most interest to geologists are those with half-lives of tens, hundreds or thousands of millions of years.
In this processes scientist compare the ratio of Carbon 12, a stable isotope of carbon, to that of Carbon 14, the radioactive isotope.
Both Carbon 12 and Carbon 14 occur naturally within our environment at a constant ratio (although carbon 14 is constantly decaying new carbon 14 is produced in the upper atmosphere as Nitrogen 14 is bombarded by cosmic radiation).
Radiometric dating of minerals in metamorphic rocks usually indicates the age of the metamorphism.
Since no part of the lithification process would reset the 'atomic clock' within individual minerals any date measured would represent the time the individual mineral grain crystallized not the time the sedimentary rock was lithified.
However, because the daughter isotopes are a different size they are excluded from the crystal structure.
This means that at the time of crystallization the mineral will only contain parent isotopes, in other words at the time of crystallization the mineral contains 100% parent isotope and 0% daughter isotope.
In order for any material to be radiometrically dated it must have incorporated radioactive isotopes within its crystal structure.
Because they are the right size some radioactive parent isotopes are capable of being incorporated into some mineral crystals.