Global halogen flux of subducting oceanic crust.

• Altered seafloor basalts preferentially incorporate F relative to altered gabbros, which contain more Cl and Br. • The efficiency of halogen devolatilization from subducting ocean crust is Cl ≈ Br > I ≫ F. • At high P-T, lower crustal gabbros can be converted to halogen-enriched eclogites. • Subducted ocean crust represents the largest flux of F, Cl, Br, and I to the mantle. • Approximately 50% of the initial Cl and 95% of initial Br and I is lost before the arc. In order to constrain the geochemical cycling of halogens (F, Cl, Br, I) during subduction of altered ocean crust (AOC), this study compares bulk halogen concentrations from seven seafloor AOC drill cores (n = 21) and exhumed eclogites (n = 22) from three paleo-subduction settings (Raspas Complex, Ecuador; Zambezi Belt, Zambia; Cabo Ortegal, Spain). As ocean crust undergoes hydrothermal alteration, basalts and gabbros preferentially incorporate different halogens. Fluorine is predominantly hosted in basalts (averaging 155 μg/g); Cl and Br are enriched in gabbros (averaging 241 μg/g and 225 ng/g, respectively). During prograde metamorphism of AOC, F behaves compatibly and is decoupled from the heavy halogens (Cl, Br, I). Eclogite samples retain F in concentrations (30–160 μg/g) similar to seafloor values (20–190 μg/g). Chlorine and Br are strongly coupled and effectively mobilized during prograde metamorphism, with high-pressure (HP) samples containing between 5–15 μg/g Cl and 17–69 ng/g Br. Elevated F/Cl, Br/Cl, and I/Cl in eclogites relative to MORB and AOC ratios support the decoupling of halogens during metamorphism as Cl is most efficiently expelled from the slab; the mobility of halogens in subducting AOC is as follows: Cl ≈ Br > I ≫ F. In conjunction with published values of halogen abundances in seafloor serpentinites, marine sediments, and HP metaserpentinites and metasediments, the results of this study are used to estimate global halogen fluxes. Mass balance estimates indicate that up to 2% of the initial F, 50% of the initial Cl, 93% of the initial Br, and 97% of initial I entering subduction zones is lost between the trench and eclogite facies. Subducted AOC represents a substantial halogen reservoir to arc magma depths and may represent the most significant carrier of halogens to the deep mantle. [ABSTRACT FROM AUTHOR]