RubidiumStrontiumdatering

Rubidium-Strontium dating, often abbreviated Rb-Sr dating, is a radiometric dating method used to determine the ages of rocks and minerals. It relies on the radioactive decay of rubidium-87 (87Rb) to strontium-87 (87Sr) with a half-life of about 4.88 × 10^10 years, making it suitable for dating very old materials.

The most common approach is the isochron method. In this approach, several samples with different rubidium-to-strontium ratios are analyzed for their 87Rb/86Sr and 87Sr/86Sr ratios. When plotted, these data form a straight line (an isochron). The slope of the line equals e^(λt) − 1, where t is the time since crystallization and λ is the decay constant of 87Rb. The intercept provides the initial 87Sr/86Sr ratio of the samples. This method allows dating without knowing the initial Sr isotopic composition.

Rb-Sr dating requires a closed system since formation, a well-characterized decay constant, and sufficient variation in Rb/Sr among samples. Measurements are typically made with mass spectrometry, such as TIMS or MC-ICP-MS, using the stable isotope 86Sr as a reference.

Applications include dating ancient terrestrial rocks, differentiating crustal formation events, and dating meteorites and lunar rocks. Benefits of the method include the ability to produce isochron ages that do not rely on a single sample’s initial composition. Limitations involve possible disturbance by metamorphism, weathering, or mineral alteration, which can reset or mix isotopic ratios. Rubidium-Strontium dating remains a foundational tool in geochronology for constraining the timing of geological processes.