Calcium isotopes in high-temperature terrestrial processes

This chapter explores the growing body of stable and radiogenic Ca isotope measurements in high temperature terrestrial materials and covers the emerging applications for Ca isotope variability in igneous and metamorphic rocks and minerals. Calcium isotope fractionation at high temperature has been found to lead to larger effects than initially theorized, sometimes even exceeding those found in low temperature near-surface environments, yet many of the underlying fractionation mechanisms still remain poorly understood. Igneous whole-rocks span a delta Ca-44 range of similar to 2 parts per thousand (-0.9 to +0.9 parts per thousand, relative to bulk-silicate Earth) and their constituent minerals have a range of similar to 2.5% (-1.2 to +1.3 parts per thousand). Metamorphic whole rocks span a larger delta Ca-44 range of similar to 6 parts per thousand (-2.5 to +3.3 parts per thousand), while metamorphic mineral separates span similar to 8 parts per thousand (-2.2 to +5.8 parts per thousand). Observed Ca isotope variations can stem from a variety of sources including: (i) isolation of isotopically-distinct mineral components, such as during melting or crystallization, (ii) mixing of isotopically-distinct reservoirs into magmatic sources, such as during mantle metasomatism or assimilation of distinct crustal components, and (iii) isotopic rate differences (kinetic effects) during melting, crystallization, transport, and metamorphism, which are often governed by Ca diffusion. The emerging picture is that Ca isotopes are sensitive to a large number of high-temperature processes and can be used to understand the evolution of crust and mantle reservoirs, along with mechanisms leading to the formation of igneous, metamorphic, and hydrothermal rocks and minerals, through geologic time.