Rubidium strontium dating example
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.
Radioactive elements "decay" (that is, change into other elements) by "half lives." If a half life is equal to one year, then one half of the radioactive element will have decayed in the first year after the mineral was formed; one half of the remainder will decay in the next year (leaving one-fourth remaining), and so forth.Strontium-87 is a stable element; it does not undergo further radioactive decay.(Do not confuse with the highly radioactive isotope, strontium-90.) Strontium occurs naturally as a mixture of several nuclides, including the stable isotope strontium-86.The number of protons in the nucleus of an atom is called its atomic number.The sum of protons plus neutrons is the mass number.Carbon-14 dating: See Carbon 14 Dating in this web site.
Rubidium-Strontium dating: The nuclide rubidium-87 decays, with a half life of 48.8 billion years, to strontium-87.
F, the fraction of K40 remaining, is equal to the amount of potassium-40 in the sample, divided by the sum of potassium-40 in the sample plus the calculated amount of potassium required to produce the amount of argon found. In spite of the fact that it is a gas, the argon is trapped in the mineral and can't escape.
(Creationists claim that argon escape renders age determinations invalid.
In old rocks, there will be less potassium present than was required to form the mineral, because some of it has been transmuted to argon.
The decrease in the amount of potassium required to form the original mineral has consistently confirmed the age as determined by the amount of argon formed.
At the same time, the fraction of strontium-87 increases from zero and approaches 100% with increasing number of half-lives.