RubidiumStrontium

Rubidium-strontium dating, or Rb-Sr dating, is a radiometric dating method that relies on the radioactive decay of rubidium-87 (Rb-87) to strontium-87 (Sr-87) to determine the age of rocks and minerals containing rubidium. It is especially useful for dating old igneous and metamorphic rocks and for studying crustal evolution.

Natural rubidium consists of two isotopes, 85Rb and 87Rb; only 87Rb is radioactive. 87Rb decays to 87Sr with a half-life of about 4.88 × 10^10 years. The dating scheme compares the ratio 87Sr/86Sr, a stable reference, with 87Rb/86Sr. Because 87Sr is produced by decay, the 87Rb/86Sr ratio increases over time, allowing calculation of the elapsed time since the system closed.

The most common approach is the isochron method. By analyzing multiple minerals or rock samples from the same geological event and plotting 87Sr/86Sr versus 87Rb/86Sr, the data should fall on a straight line. The slope of this line equals e^(λt) − 1, where λ is the decay constant for 87Rb. The intercept provides the initial 87Sr/86Sr ratio, and the age t is derived from the slope.

Applications and limitations: Rb-Sr dating is widely used to determine ages of ancient rocks and to investigate magma genesis and crustal processes. It requires a closed system for rubidium and strontium since formation; weathering, metamorphism, or alteration can compromise results. Analytical methods include thermal ionization mass spectrometry (TIMS) and multi-collector inductively coupled plasma mass spectrometry (MC-ICP-MS).