Halogen Abundances and Isotopes

Halogen (F, Cl, Br and I) abundance and isotope geochemistry may be used to gain insights into the complex cycling of volatiles on Earth. Despite the typically low abundances of the halogens in Earth and planetary materials, they play an important role in numerous geochemical processes. Each of the halogens has unique chemical properties, making them collectively suitable for informing on elemental and isotopic exchange in diverse terrestrial and extraterrestrial environments, from the surfaces of planets to their interiors. These properties, including incompatibility, fluid solubility, and specific and distinct abundance ratios in geological reservoirs, make them powerful geochemical tracers. Examples include terrestrial volatile origin, recycling of volatiles at subduction zones, and atmosphere evolution from volcanic and anthropogenic emissions. In tracking these processes, halogens contribute to our understanding of large-scale planetary processing. The halogens are useful not only as elemental tracers, but also as isotopic tracers with isotopic fractionation occurring during fluid-rock and fluid-mineral reactions, such as alteration and redox reactions. Chlorine isotopes (and the burgeoning field of Br isotope geochemistry) can therefore be used to distinguish such processes on the surfaces (e.g., evaporation, serpentinization) and interiors (e.g., metasomatism) of Earth and other bodies in the Solar System (e.g., Mars, the Moon).