March 20, 2014
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 (87Rb) and strontium (87Sr, 86Sr).
The utility of the rubidium–strontium isotope system results from the fact that 87Rb (one of two naturally occurring isotopes of rubidium) decays to 87Sr with a half life of 48.8 billion years. In addition, Rb is a highly incompatible element that, during fractional crystallization of the mantle, stays in the magmatic melt rather than becoming part of mantle minerals. The radiogenic daughter, 87Sr, 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 (87Sr/86Sr) through time; and their age can be calculated by measuring the 87Sr/86Sr in a mass spectrometer, knowing the amount of 87Sr present when the rock or mineral formed, and calculating the amount of 87Rb from a measurement of the Rb present and knowledge of the 85Rb/87Rb weight ratio.
If these minerals crystallized from the same silicic melt, each mineral had the same initial 87Sr/86Sr 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 87Sr, measured 87Sr/86Sr values will be different in the minerals, increasing in the same order.