Thematic Articles

Halogens are mobile in geological fluids, making them excellent tracers of volatile activity. Halogen-bearing minerals in diverse planetary materials, coupled with chlorine isotope compositions of bulk samples and minerals, can be used to infer the presence of fluids on planetary surfaces, crusts, and interiors. Halogen element and isotopic evidence helps define the role that halogens play in diverse planetary environments (e.g., asteroids, the Moon, and Mars), which offers insights into fluid activity in the early Solar System and in the role such fluids have played in volatile transport, alteration processes, and habitability throughout geological history.

The halogen group elements (F, Cl, Br, and I) and the stable isotopes of
Cl and Br collectively are powerful tracers of terrestrial volatile cycling.
Individually, their distinct geochemical affinities inform on a variety of fluid-mediated and magmatic processes. They form a wide-range of halogenbearing minerals whose composition reflects the source fluids from which they evaporated or crystallized. Fluorine’s geochemical cycle is generally decoupled from that of the heavier Cl, Br, and I, which are concentrated into Earth’s surface reservoirs. Throughout history, the salt-forming halogens have been integral to human health and are key constituents of many industries. These common elements have an important role in tracing geochemical processes across many geologic environments – from the surface to the deep planetary interior.