The behavior of halogens (F, Cl, Br, I) in altered oceanic crust during prograde subduction zone metamorphism and devolatilization

Halogens (F, Cl, Br, I) are volatile elements enriched in Earth’s surface reservoirs. Following seafloor alteration and tectonic convergence, halogens are carried into subduction zones. Prograde metamorphism of altered ocean crust (AOC) results in hydrous mineral breakdown and the release of volatiles. Halogen devolatilization is not well constrained, particularly for Br and I. To explore the halogens fluxes and behaviors during subduction of AOC, this study investigates bulk rock concentrations from seafloor AOC drill cores (n=21) and paleo-subduction settings that expose exhumed metamorphic rocks (n=44) that record the chemical evolution of AOC during prograde metamorphism. Chapter 2 juxtaposes eclogitic and AOC samples to gauge broadscale changes to the halogen budget between the trench and the depths of ~80 km. Chapter 3 focuses on a suite of petrogenetically-related meta-ophiolites from the Western Alps that record progressive metamorphism and shed light on halogen loss across metamorphic grade, especially at low P-T conditions. To study how halogens partition during phase breakdown and fluid transport, Chapter 4 examines a ~1 m transect of eclogitic samples that preserve evidence of high pressure vein formation and fluid-rock interactions. Findings from this work reveal that halogens are not evenly distributed in AOC. Basalts are F-rich and gabbros are Cl- and Br-rich. During subduction, F is decoupled from the heavy halogens (Cl, Br, I) displaying compatibility and immobility. Chlorine and Br are closely coupled; they preferentially partition into fluid phases and are efficiently removed from the mafic slab. Iodine behaviors are more cryptic, with many high-P samples preserving protolith-level abundances. Halogen mobility in devolatilizing AOC is as follows: Cl ≈ Br > I ≫ F. New flux estimates find that ≤40% of initial F and I and ~70% of initial Cl and Br is removed from subducting AOC prior to depths of arc magma genesis, with much of this loss occurring early in the forearc, before the transition to blueschist assemblages. Fluorine is broadly distributed among phases. The heavy halogen inventory is not controlled by a dominant phase (i.e., amphibole). Rather, these elements are distributed at low concentrations among many phases or are predominantly hosted in non-lattice sites.