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Study of the isotopic variations of calcium is of interest because Ca is important in geochemical and biochemical processes, and is one of few major cations in rocks and minerals with demonstrated isotopic variability. Calcium is critical to life and a major component of the global geochemical cycles that control climate. Studies to date show that biological processing of calcium produces significant isotopic fractionation (4 to 5‰ variation of the 44Ca/40Ca ratio has been observed). Calcium isotopic fractionation due to inorganic processing at high (e.g., magmatic) temperatures is small. There are few studies of calcium isotope fractionation behavior for low-temperature inorganic processes. The Ca isotopic variations observed in nature, both biological and inorganic, are mostly attributed to kinetic effects, but this inference cannot be confirmed until equilibrium Ca isotope fractionation is more thoroughly investigated. Evaporation of silicate liquids into vacuum at high temperature is expected to produce kinetic Ca isotopic fractionation, as is diffusion of calcium in silicate liquids and aqueous solutions. The evaporation effects have been observed in meteorite samples. Diffusion effects have been observed in the laboratory but not yet in natural samples. The potential value of Ca isotopic studies has barely been tapped. Improvements in measurement precision would increase the attractiveness of Ca isotopes as a geochemical tool, but such improvements have been slow in coming.
Calcium is composed mainly of the isotope 40Ca, which is a highly stable, doubly magic nuclide (both the number of neutrons and the number of protons represent closed nuclear shells). There are a total of six stable isotopes covering a mass range from 40 to 48. Ca has been studied for isotopic variations in three ways. The major isotope, 40Ca is the primary radioactive decay product of radioactive 40K. About 89.5% of the 40K …