Problems with radiometric dating of rocks
In many respects, igneous rocks are the easiest to date because the starting of the clocks are unambiguous. : Sedimentary rocks are, to a large degree, made from fragments of pre-existing rocks that have been broken, weathered, transported and ultimately deposited in ocean basins.These fragments (e.g., sand or mud) may become cemented together to form sedimentary rocks.As an example, consider Carbon: All atoms of Carbon consist of 6 protons and 6 electrons.The different isotopes, C-12, C-13 and C-14 differ in the number of neutrons in the nucleus, and consequently differ in atomic weight.Subsequent disintegration will produce daughter atoms replacing the original radioactive parents.
In other words, radioactive decay is a Constant Rate Process.
Radiometric dating of sedimentary rocks is, therefore, not common. : These rocks typically form in deep levels of the crust, and consist of minerals that have formed in response to increasing temperature and pressure.
If a new mineral grows in a metamorphic rock, and if that mineral incorporates radioactive isotopes in its crystal structure, then dating of that mineral can provide an estimate of the time of mineral growth (metamorphism).
Most metamorphic rocks are very complex, and many have undergone several episodes of metamorphism and/or mineral growth over protracted periods of time.
Radiometric dating of metamorphic rocks can be successful, but often the results are difficult to interpret, and in many cases are ambiguous.