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.(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.Any argon present in a mineral containing potassium-40 must have been formed as the result of radioactive decay.
By "age" we mean the elapsed time from when the mineral specimen was formed.
If we knew the fraction of a radioactive element still remaining in a mineral, it would be a simple matter to calculate its age by the formula To determine the fraction still remaining, we must know both the amount now present and also the amount present when the mineral was formed.
Contrary to creationist claims, it is possible to make that determination, as the following will explain: By way of background, all atoms of a given element have the same number of protons in the nucleus; however, the number of neutrons in the nucleus can vary.
The corresponding half lives for each plotted point are marked on the line and identified.
It can be readily seen from the plots that when this procedure is followed with different amounts of Rb87 in different minerals, if the plotted half life points are connected, a straight line going through the origin is produced. The steeper the slope of the isochron, the more half lives it represents.