Beaudoin, G. M., Barnes, J. D., Orlandini, O. F., Chatterjee, R., Stockli, D. F., and John, T.
In order to examine the progressive chemical evolution of halogens (F, Cl, Br, I) in altered ocean crust (AOC) during prograde subduction, this study compares bulk and in situ halogen concentrations in mafic samples from three petrogenetically related exhumed terrains in the Western Alps (the Chenaillet ophiolite, the Queyras ophiolites of the Schistes Lustrés, and the Monviso ophiolite). Samples from the Chenaillet ophiolite represent oceanic crust unaffected by metamorphic halogen loss and define a protolith halogen content (122 μg/g F, 29 μg/g Cl, 82 ng/g Br, and 98 ng/g I). Samples from the Queyras ophiolites experienced blueschist facies conditions, undergoing recrystallization and halogen loss (74 μg/g F, 19 μg/g Cl, 70 ng/g Br, and 63 ng/g I). Eclogite facies samples from the Monviso meta‐ophiolite exhibit markedly reduced Cl (8 μg/g Cl) and Br (42 ng/g Br) contents relative to samples from Chenaillet and Queyras. Using electron probe microanalysis (EPMA), F and Cl host minerals (e.g., amphibole, chlorite, epidote) are identified and characterized in selected samples, showing a broad distribution of F and Cl, lending support to the view that halogen devolatilization in the subducting slab occurs continuously and is not dependent on the breakdown of a particular phase. In situ Cl concentrations decrease significantly between sub‐greenschist and blueschist assemblages. Fluorine is retained within subducting AOC and is decoupled from the heavy halogens (Cl, Br, I), which undergo continuous devolatilization during prograde metamorphism. Plain Language Summary: Halogens are volatile elements found predominantly in the Earth's oceans and sediments. During hydrothermal processes, they bond with other elements to form volatile‐bearing minerals. Tectonic processes can bring halogen‐enriched rocks into subduction zones, regions where one tectonic plate submerges beneath another. Subduction governs the transport of material from Earth's surface to its interior, influencing the chemical evolution of the crust, atmosphere, hydrosphere, and mantle. As the subducting plate carries rocks to greater depths beneath the surface, volatile‐bearing minerals destabilize, releasing volatile components as fluids. Halogens alter the properties of subduction‐derived fluids and impact the behavior of other biologically, climatically, and economically important elements (e.g., H2O, CO2, Au). Due to the influential role of halogens in subduction zone environments, the timing and extent of halogen loss must be constrained. This study investigates metamorphic rock samples from the Western Alps which underwent halogen‐enrichment followed by subduction and devolatilization. Results show that halogens behave differently from each other. Fluorine is not removed; it is retained until deep in the subduction zone and is returned to the Earth's mantle. The efficient removal of the other halogens occurs early in subduction (<60 km depth) and is not linked to the destabilization of a specific mineral. Key Points: Exhumed ophiolites in the Western Alps record prograde subduction zone metamorphism and halogen devolatilization of altered ocean crustResults show halogen decoupling. Fluorine is retained, whereas heavy halogens are mobilized from mafic rocks in the subducting slabDevolatilization of altered ocean crust occurs early in subduction; a significant portion of Cl, Br, and I is lost prior to eclogite facies [ABSTRACT FROM AUTHOR]