87Rb87Sr

The isobar pair 87Rb87Sr refers to the atomic nuclei with mass number 87, where one is the element Rubidium (Rb) with 37 protons and the other is Strontium (Sr) with 38 protons. Specifically, 87Rb is a radioactive isotope of Rubidium, and 87Sr is a stable isotope of Strontium.

The significance of this isobar pair lies in the radioactive decay of 87Rb. Rubidium-87 (87Rb) undergoes beta decay to form Strontium-87 (87Sr). The half-life of this decay is approximately 4.92 billion years. This long half-life makes the 87Rb-87Sr decay system a valuable tool in geochronology, particularly for dating ancient rocks and minerals.

In geological applications, the ratio of 87Sr to 86Sr (a stable, non-radiogenic isotope of Strontium) in a sample can be used to determine its age. The initial abundance of 87Sr in a rock is primarily determined by the initial 87Rb content and the age of the rock. Over time, the decay of 87Rb increases the amount of 87Sr. By measuring the present-day ratio of 87Sr to 86Sr and knowing the initial 87Sr/86Sr ratio (which can be estimated from minerals with low initial Rb content), geologists can calculate the time elapsed since the rock formed. This technique is widely used in dating meteorites, Earth rocks, and lunar samples.