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To date rocks or other objects, scientists typically use radiometric dating. In short, the ratio of radioactive and stable isotopes in the sample are determined and the measured rate at which the isotopes decay is used as an indicator of the age of the sample. However, it is typically unknown and simply assumed whether these ratios of elements are the result of radioactive decay over time or other processes that have taken place in the rock. Sometimes different methods used on the same rock produce different ages. Furthermore, the same method can produce different ages on different parts of the same rock. Sometimes these are close but other times they are very different. Isotopic Fractionation is a physical separation of isotopes and a non-radioactive source of isotope ratios. It can be caused by heating and cooling, water flow, contact between high and low concentration magma and just normal molecular motion. Evidence for Isotopic Fractionation does show up in isotopic data so it is a factor that needs to be considered. Water flow through rocks is important because all parent substances and many daughter substances are water soluble.
The following radioactive decay processes have proven particularly useful in radioactive dating for geologic processes:. Note that uranium and uranium give rise to two of the natural radioactive series , but rubidium and potassium do not give rise to series. They each stop with a single daughter product which is stable. Some of the decays which are useful for dating, with their half-lives and decay constants are:.
Radiometric dating , radioactive dating or radioisotope dating is a technique which is used to date materials such as rocks or carbon , in which trace radioactive impurities were selectively incorporated when they were formed. The method compares the abundance of a naturally occurring radioactive isotope within the material to the abundance of its decay products, which form at a known constant rate of decay. Together with stratigraphic principles , radiometric dating methods are used in geochronology to establish the geologic time scale. By allowing the establishment of geological timescales, it provides a significant source of information about the ages of fossils and the deduced rates of evolutionary change. Radiometric dating is also used to date archaeological materials, including ancient artifacts.