What is "rubidium-strontium dating"

n. geological dating based on the proportions of radioactive rubidium into its decay product strontium; radioactive rubidium has a half-life of 47,000,000,000 years

The rubidium-strontium dating method is a radiometric dating technique used by scientists to determine the age of rocks and minerals from the quantities they contain of specific isotopes of rubidium (Rb) and strontium (Sr, Sr).

Development of this process was aided by German chemists Otto Hahn and Fritz Strassmann, who later went on to discover nuclear fission in December 1938.

The utility of the rubidium- strontium isotope system results from the fact that Rb (one of two naturally occurring isotopes of rubidium) decays to Sr with a half life of 48.8 billion years. In addition, Rb is a highly incompatible element that, during partial melting of the mantle, prefers to join the magmatic melt rather than remain in mantle minerals. As a result, Rb is enriched in crustal rocks. The radiogenic daughter, Sr, is produced in this decay process and was produced in rounds of stellar nucleosynthesis predating the creation of the Solar System.

Different minerals in a given geologic setting can acquire distinctly different ratios of radiogenic strontium-87 to naturally occurring strontium-86 (Sr/Sr) through time; and their age can be calculated by measuring the Sr/Sr in a mass spectrometer, knowing the amount of Sr present when the rock or mineral formed, and calculating the amount of Rb from a measurement of the Rb present and knowledge of the Rb/Rb weight ratio.

If these minerals crystallized from the same silicic melt, each mineral had the same initial Sr/Sr as the parent melt. However, because Rb substitutes for K in minerals and these minerals have different K/ Ca ratios, the minerals will have had different Rb/Sr ratios.

During fractional crystallization, Sr tends to become concentrated in plagioclase, leaving Rb in the liquid phase. Hence, the Rb/Sr ratio in residual magma may increase over time, resulting in rocks with increasing Rb/Sr ratios with increasing differentiation. Highest ratios (10 or higher) occur in pegmatites.

Typically, Rb/Sr increases in the order plagioclase, hornblende, K-feldspar, biotite, muscovite. Therefore, given sufficient time for significant production (ingrowth) of radiogenic Sr, measured Sr/Sr values will be different in the minerals, increasing in the same order.