Rubidium strontium dating meteorites
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.The two curves cross each other at half life = 1.00.At this point the fraction of Rb87 = Sr87 = 0.500; at half life = 2.00, Rb87 = 25% and Sr87 = 75%, and so on. 131, Strahler, Science and Earth History: Points are taken from these curves and a plot of fraction Sr-87/Sr-86 (as ordinate) vs. It turns out to be a straight line with a slope of -1.00.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.
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.
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).