Introduction to calcium isotope geochemistry: Past lessons and future directions

Calcium (Ca) isotopes have been utilized as geochemical tools since the 1970s, increasing in popularity and scope since the mid- to late 1990s. Research conducted over the past thirty years has revealed a range of applications for Ca isotopes that are highlighted in this introduction and Special Issue, as well as a series of interesting and fundamental problems that have yet to be resolved. For such a chemically-simple system, in comparison to redox-active elements for instance, the level of complexity is both surprising and intellectually attractive. While the advent of multiple collector inductively coupled plasma mass spectrometers made metals such as Fe, Mg, and Zn considerably more accessible analytically, the precision and ease of Ca isotopic analysis did not radically improve in a comparable manner. For a variety of reasons, one of which may be perceived analytical difficulty, the application of Ca isotopes to geological questions remains in the shadows, to some extent, in the Geosciences. In this introduction, we identify (i) areas that we think are most fertile for the application of Ca isotopes as tools and (ii) outstanding questions and future directions that can move the field towards increased utility and broader acceptance by the Geosciences community.