87Sr

87Sr is a stable isotope of the element strontium (Sr) with atomic number 38 and mass number 87. It consists of 38 protons and 49 neutrons. In natural strontium, 87Sr accounts for about 7 percent of the isotopic composition, with the remaining isotopes being 84Sr, 86Sr, and 88Sr.

87Sr is produced in geological time as the radiogenic daughter product of 87Rb, which beta decays to 87Sr with a long half-life of about 4.88 × 10^10 years. As 87Rb decays, the 87Sr/86Sr ratio in rocks and minerals increases, allowing 87Sr to function as a geological clock in closed systems.

One primary application of 87Sr is rubidium–strontium dating. By measuring 87Sr/86Sr and 87Rb/86Sr in minerals, scientists construct an isochron from which the age of the rock can be inferred and the initial 87Sr/86Sr ratio estimated. This method is particularly valuable for dating ancient rocks and meteorites and for studying crustal differentiation and mantle processes through Sr isotopic geochemistry.

Analytical measurements are typically performed with mass spectrometry (such as TIMS or MC-ICP-MS). The 87Sr/86Sr ratio also serves as a tracer for geological reservoirs and is used to distinguish sources in planetary materials and other geochemical investigations. Interpretations rely on assumptions about closed-system behavior and the initial isotopic composition.