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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.At the same time, the fraction of strontium-87 increases from zero and approaches 100% with increasing number of half-lives.Rubidium-Strontium dating: The nuclide rubidium-87 decays, with a half life of 48.8 billion years, to strontium-87.Strontium-87 is a stable element; it does not undergo further radioactive decay.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.Carbon-14 dating: See Carbon 14 Dating in this web site.The corresponding half lives for each plotted point are marked on the line and identified.
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.
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.
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.
The formula for the fraction remaining is one-half raised to the power given by the number of years divided by the half-life (in other words raised to a power equal to the number of half-lives).
Therefore the amount of argon formed provides a direct measurement of the amount of potassium-40 present in the specimen when it was originally formed.