87Rb86Sr

87Rb86Sr refers to the rubidium–strontium dating system in geochronology, which uses the beta decay of rubidium-87 to strontium-87 and the stable isotope 86Sr as a reference. In this framework, 86Sr serves as a denominator in isotope ratios measured in minerals, enabling the construction of isochrons that track time since crystallization.

The key decay relationship is 87Rb decaying to 87Sr with a half-life of about 4.88 × 10^10 years. Because 86Sr is stable and non-radiogenic in this system, the evolving ratio 87Sr/86Sr in a sample reflects the accumulation of radiogenic 87Sr over time. The rubidium-to-strontium system is exploited by plotting 87Sr/86Sr against 87Rb/86Sr for multiple minerals or samples from the same rock. The isochron slope equals e^(λt) − 1, where λ is the decay constant, and the intercept yields the initial 87Sr/86Sr ratio. From the slope, the age t can be calculated as t = (1/λ) ln(1 + slope).

Applications of the 87Rb86Sr approach include dating old igneous and metamorphic rocks, as well as meteorites, benefiting from the long half-life of 87Rb and the widespread presence of Rb and Sr in minerals like feldspars and micas. Advantages include its ability to provide an age independently of absolute Sr initial compositions when a mineral pair or set of minerals with varying Rb/Sr ratios is analyzed. Limitations arise from open-system behavior (loss or gain of Rb or Sr), inheritance of different initial Sr compositions, and analytical uncertainties, all of which can complicate isochron interpretation.

See also: rubidium–strontium dating, isotope geochemistry.