Periodic Table--Calcium

Calcium has six stable isotopes, two of which occur in nature: stable ⁴⁰Ca and radioactive ⁴¹Ca with a half-life = 106 years. 97% of the element is in the form of ⁴⁰Ca. ⁴⁰Ca is one of the daughter products of ⁴⁰K decay, along with ⁴⁰Ar. While K-Ar dating has been used extensively in the geological sciences, the prevalence of ⁴⁰Ca in nature has impeded its use in dating. Techniques using mass spectrometry and a double spike isotope dilution have been used for K-Ca age dating. Russell et al. (1978) compared ⁴⁰Ca to ⁴⁴Ca; Marshall and DePaolo (1982) used ⁴⁰Ca and another stable isotope, ⁴²Ca, with a good deal of success.

The isotopic composition of calcium in rocks and minerals varies because of the formation of ⁴⁰Ca by beta decay of ⁴⁰K and because of the fractionation of Ca isotopes by natural processes. The possibility for fractionation is enhanced by the large differences in mass of Ca isotopes. This fractionation is important not only because it interferes with the K-Ca dating method, but because of hopes that the variation in Ca isotopes might convey process information.

Unlike cosmogenic isotopes that are produced in the atmosphere, ⁴¹Ca is produced by neutron activation of ⁴⁰Ca. Most of its production is in the upper meter or so of the soil column where the cosmogenic neutron flux is still sufficiently strong. Variations in the isotopic ratios of calcium are due to particle-induced changes in the nucleus of the atom. The geochemical behavior of Ca at low temperatures is mainly controlled by calcite formation and dissolution. This would suggest that ⁴¹Ca may be useful in studies that are focused on the carbon cycle. Calcium may eventually be one of the most useful elements to hydrologic studies, participating in a wide variety of hydrochemical processes. Currently, no published studies have applied ⁴¹Ca to hydrology.

⁴¹Ca has received much attention in stellar studies because ⁴¹Ca decays to ⁴¹K, a critical indicator of solar-system anomalies. ⁴¹K excesses found in solar-system material suggests that stellar core material may have been added to the solar nebula Allende as late as 2 to 20 Myr before condensation (Hutcheon et al., 1984).

Source of text: This review was assembled by Dan Snyder, Carol Kendall and Eric Caldwell, primarily from Faure (1986), Dicken (1995) and Nimz (1998).