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Each of the halogens constrains a different aspect of volatile cycling in the solid Earth. F is moderately incompatible in the mantle and has a low mobility at Earth's surface, meaning that it is preferentially retained in the mantle and continental crust. In contrast, Cl, Br, and I are strongly incompatible and highly soluble. Chloride is the dominant anion in seawater and many geofluids and a major component of evaporite minerals. Br and I are essential for life and significantly incorporated into organic matter that accumulates in marine sediments. Surficial fluids circulated into continental and oceanic crust incorporate surface-derived halogens into alteration minerals. As a result, subducting slabs and arc lavas are weakly enriched in F and strongly enriched in Cl, Br, and I. Subduction has maintained mantle Cl and Br concentrations at relatively constant levels since Earth's early differentiation, but mantle I/Cl has decreased over time.
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