Isotopes of calcium

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Calcium (₂₀Ca) has 26 known isotopes, ranging from ³⁵Ca to ⁶⁰Ca. There are five stable isotopes (⁴⁰Ca, ⁴²Ca, ⁴³Ca, ⁴⁴Ca and ⁴⁶Ca), plus one isotope (⁴⁸Ca) with such a long half-life that it is for all practical purposes stable. The most abundant isotope, ⁴⁰Ca, as well as the rare ⁴⁶Ca, are theoretically unstable on energetic grounds, but their decay has not been observed. Calcium also has a cosmogenic isotope, ⁴¹Ca, with half-life 99,400 years. Unlike cosmogenic isotopes that are produced in the air, ⁴¹Ca is produced by neutron activation of ⁴⁰Ca. Most of its production is in the upper metre of the soil column, where the cosmogenic neutron flux is still strong enough. ⁴¹Ca has received much attention in stellar studies because it decays to ⁴¹K, a critical indicator of solar system anomalies. The most stable artificial isotopes are ⁴⁵Ca with half-life 163 days and ⁴⁷Ca with half-life 4.5 days. All other calcium isotopes have half-lives of minutes or less.^[4]

Stable ⁴⁰Ca comprises about 97% of natural calcium and is mainly created by nucleosynthesis in large stars. Similarly to ⁴⁰Ar, however, some atoms of ⁴⁰Ca are radiogenic, created through the radioactive decay of ⁴⁰K. While K–Ar dating has been used extensively in the geological sciences, the prevalence of ⁴⁰Ca 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 Collision-Cell Multi-Collector Inductively-coupled plasma mass spectrometry (CC-MC-ICP-MS) techniques, however, have been used for successful K–Ca age dating,^[5][6] as well as determining K losses from the lower continental crust^[7] and for source-tracing calcium contributions from various geologic reservoirs^[8][9] similar to Rb-Sr.

Stable isotope variations of calcium (most typically ⁴⁴Ca/⁴⁰Ca or ⁴⁴Ca/⁴²Ca, denoted as 'δ⁴⁴Ca' and 'δ^44/42Ca' in delta notation) are also widely used across the natural sciences for a number of applications, ranging from early determination of osteoporosis^[10] to quantifying volcanic eruption timescales.^[11] Other applications include: quantifying carbon sequestration efficiency in CO₂ injection sites^[12] and understanding ocean acidification,^[13] exploring both ubiquitous and rare magmatic processes, such as formation of granites^[14] and carbonatites,^[15] tracing modern and ancient trophic webs including in dinosaurs,^[16][17][18] assessing weaning practices in ancient humans,^[19] and a plethora of other emerging applications.