Halogens

The four stable halogens (F, Cl, Br, and I) are low-abundance elements that are widely distributed in nature. Two of the halogens, Cl and Br, each have two stable isotopes showing a range in natural isotope variation of up to a few parts per thousand. A variety of analytical techniques have been developed to determine the abundance and isotopic ratios of the halogens: these include in situ techniques for high spatial resolution studies and bulk determinations, and they have been applied to a range of materials, including whole rocks, minerals, glasses, and fluid inclusions. Here, we summarise some of the established methods for determining halogen abundances and isotopes and highlight key advances.

Halogens are volatile elements present in trace amounts in the Earth’s crust, mantle, and core. They show volatile behavior and tend to be incompatible except for fluorine, which makes them key tracers of fluid-mediated and/or melt-mediated chemical transport processes. Even small quantities of halogens can profoundly affect many physicochemical processes such as melt viscosity, the temperature stability of mineral phases, the behavior of trace elements in aqueous fluids, or the composition of the atmosphere through magma degassing. Experiments allow us to simulate deep-Earth conditions. A comparison of experimental results with natural rocks helps us to unravel the role and behavior of halogens in the Earth’s interior.

Understanding the atmospheric geochemical cycle of both natural and anthropogenic halogens is important because of the detrimental effect halogens have on the environment, notably on tropospheric and stratospheric ozone. Oceans are the primary natural source for atmospheric Cl, F, Br, and I, but anthropogenic emissions are still important, especially for Cl. While emissions of human-made halocarbons (e.g., chlorofluorocarbons or CFCs) are expected to continue to decrease allowing progressive stratospheric ozone recovery, volcanic activity (e.g., clusters of mid-scale explosive eruptions or large-scale explosive eruptions) might disturb this recovery over the next decades. This review provides a synthesis of natural halogen fluxes from oceanic, terrestrial, and volcanic sources, and discusses the role of natural halogen species on atmosphere chemistry and their environmental impact.

Halogens are important elements that participate in a variety of biogeochemical processes and influence the solubility of metals in subduction-zone fluids. Halogens are powerful tracers of subducted volatiles in the Earth’s mantle because they have high abundances in seawater, sediments, and altered oceanic lithosphere but low concentrations in the mantle. Additionally, Br/Cl and I/Cl ratios, as well as Cl-isotope ratios, have characteristic ranges in different surface reservoirs that are not easily fractionated in the mantle. Current data suggest that subduction of serpentinised lithosphere is a major source of halogens in the Earth’s mantle.

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.