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U-235 decays to Pb-207 with a half-life of 704 million years.
(Creationists claim that argon escape renders age determinations invalid.
By comparing this ratio to the half-life logarithmic scale of the parent isotope, they are able to find the age of the rock or fossil in question.
There are several common radioactive isotopes that are used for dating rocks, artifacts and fossils. U-235 is found in many igneous rocks, soil and sediment.
Because argon is an inert gas, it is not possible that it might have been in the mineral when it was first formed from molten magma.
Any argon present in a mineral containing potassium-40 must have been formed as the result of radioactive decay.