rubidium-strontium dating

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rubidium—strontium dating A radiometric dating method based on the radioactive decay of 87Rb to 87Sr. Rubidium has two isotopes (85Rb 72.15%, 87Rb 27.85%), but only 87Rb is radioactive. 87Rb disintegrates in a single step to 87Sr by the emission of a low-energy beta particle (see BETA DECAY). Unfortunately this low-energy disintegration makes it very difficult to assess the half-life (see DECAY CONSTANT) and two values (5.0 × 1010 years or 4.88 × 1010 years) have been in common use. When a mineral crystallizes, it will usually incorporate both rubidium and strontium ions and the ratio of Rb to Sr will vary depending on the mineral involved. This initial strontium in the mineral is known as ‘common strontium’ (see INITIAL STRONTIUM RATIO) and is normally in the proportion of 88Sr 82.56%, 87Sr 7.02%, 86Sr 9.86%, and 84Sr 0.56%. Using these proportions it is possible to identify the amount of radiogenic 87Sr present. Originally the above proportions were assumed, but today it is more usual to plot 87Sr:86Sr against 87Rb:86Sr to produce a straight-line isochron from which the age of the mineral can be determined. When using the 87Rb:86Sr method it is customary to use whole-rock samples in the analysis, because although 87Sr may leak from one mineral to adjacent minerals over time it usually remains in the system. Micas and potassium feldspars are the most suitable minerals for 87Rb:87Sr age determinations and the results can commonly be compared with 40K:40Ar age determinations from the same sample (see POTASSIUM—ARGON DATING). The method has particularly been applied to ancient metamorphic rocks.

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rubidium–strontium dating A method of dating geological specimens based on the decay of the radioisotope rubidium–87 into the stable isotope strontium–87. Natural rubidium contains 27.85% of rubidium–87, which has a half-life of 4.7 × 1011 years. The ratio 87Rb/87Sr in a specimen gives an estimate of its age (up to several thousand million years).