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Any argon present in a mineral containing potassium-40 must have been formed as the result of radioactive decay.
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.
(Note that this does not mean that the ratios are the same everywhere on earth.
It merely means that the ratios are the same in the particular magma from which the test sample was later taken.) As strontium-87 forms, its ratio to strontium-86 will increase.
Radiometric dating is a means of determining the "age" of a mineral specimen by determining the relative amounts present of certain radioactive elements.
By "age" we mean the elapsed time from when the mineral specimen was formed.
The ratio of calcium formed to argon formed is fixed and known.
(Creationists claim that argon escape renders age determinations invalid.
However, any escaping argon gas would lead to a determined age younger, not older, than actual.
The creationist "argon escape" theory does not support their young earth model.) The argon age determination of the mineral can be confirmed by measuring the loss of potassium.
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.
Because of radioactivity, the fraction of rubidium-87 decreases from an initial value of 100% at the time of formation of the mineral, and approaches zero with increasing number of half lives.