Isotopes of calcium

Calcium (₂₀Ca) has 26 known isotopes, ranging from Ca to Ca. There are five stable isotopes (Ca, Ca, Ca, Ca and Ca), plus one isotope (Ca) with such a long half-life that it is for all practical purposes stable. Two isotopes of calcium, Ca and Ca, are doubly-magic, making them particularly stable nucliides. Calcium also has a cosmogenic isotope, Ca, with half-life 99,400 years. Unlike cosmogenic isotopes produced in the air, Ca is produced by neutron activation of solid Ca in rock and soil. Most of its production is in the upper meter of the soil column, where the cosmogenic neutron flux is still strong enough. Calcium-41 is also produced during stellar nucleosynthesis and decays to K, which can be used to understand formation and mixing between solar system reservoirs . The most stable artificial isotopes are Ca with half-life 162.61 days and Ca with half-life 4.536 days. All other artificial calcium isotopes have half-lives of minutes or less.

Ca comprises about 97% of natural calcium and is mainly created by nucleosynthesis in stars (alpha process). Like Ar, however, some Ca is radiogenic, created by radioactive decay of K. While K–Ar dating has been used extensively in the geological sciences, the prevalence of Ca in nature initially impeded the proliferation of K-Ca dating in early studies, with only a handful of studies in the 20th century. Modern techniques using increasingly precise Thermal-Ionization (TIMS) and Collsion-Cell Multi-Collector Inductively-coupled plasma mass spectrometry (CC-MC-ICP-MS) techniques, however, have been used for successful K–Ca dating similar in method to Rb-Sr dating, as well as determining K losses from the lower continental crust and for source-tracing calcium contributions from various geologic reservoirs.

Stable isotope variations of calcium (most typically Ca/Ca or Ca/Ca, denoted 'δCa' and 'δCa' in delta notation) are also widely used across the natural sciences for a number of applications, ranging from early determination of osteoporosis to quantifying volcanic eruption timescales. Other applications include: quantifying carbon sequestration efficiency in CO₂ injection sites and understanding ocean acidification, exploring both ubiquitous and rare magmatic processes, such as formation of granites and carbonatites, tracing modern and ancient trophic webs including in dinosaurs, assessing weaning practices in ancient humans, and a plethora of other emerging applications.