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).
It therefore seems worthwhile to point out that isotopic fractionation does occur during the mass spectrometric ratio measurement.Model ages subsequently re-appeared in more specialised aspects of Rb 1), yields the age of the minerals. If one of the minerals is very Rb-poor then this may yield the initial ratio directly. Results are presented for a variety of magmatic rocks with well-established thermal records: a sample each from the Klokken syenodiorite (Greenland; 1161 Ma), the Ulvö alkaligabbro (Sweden; 1256 Ma) and a pegmatite from the Bohus granite (Sweden; 920 Ma).This activity was selected for the On the Cutting Edge Reviewed Teaching Collection This activity has received positive reviews in a peer review process involving five review categories.