The Behavior of Halogens During Subduction-Zone Processes

Halogens (Cl, F, I, Br) are enriched in surface reservoirs compared to the mantle. The subduction of these reservoirs in the form of sedimentary pore fluids, sediments, altered oceanic crust, and serpentinized mantle lithosphere returns halogens to the mantle and to regions of arc magma genesis. Pore fluids are particularly enriched in I, yet shallow pore fluid loss in subduction zones due to compaction, as indicated by I-129/I ratios, makes pore fluids a negligible halogen source at depths > similar to 5 km. Sediments can host large quantities of halogens, particularly I. However, serpentinites +/- altered oceanic crust subduct the largest amount of halogens to depths of magma genesis. Due to their hydrophilic nature, halogens are lost to aqueous slab-derived fluids during prograde metamorphic reactions. The addition of halogens, particularly Cl, increases the ability of subduction-zone fluids to transport metals and trace elements. The amount of Cl in solution is a function of the P-T conditions of the subduction zone, such that higher temperatures at a given depth and lower pressures at a given temperature favor ion pair formation (NaClaq, KClaq). Therefore, ion pairing will be more important in subduction zones with warmer geotherms, such as Cascadia, compared to those with cooler geotherms, such as Alaska. High halogen concentrations in melt inclusions and volcanic gas emissions from the arc front support the efficiency of fluid loss and transport from the slab to the region of magma genesis. Despite this high efficiency, mass balance calculations and halogen concentrations in back-arc basalts and ocean island basalts show that more halogens are subducted than returned to the Earth's surface through volcanic arc fronts, implying transport of halogens into the upper mantle. Chlorine is the halogen most efficiently recycled to the surface, and F the least. Shallow loss of I and Br, through fore-arc fluids that are not accounted for in the mass balance calculation, likely explain the imbalance in these cycles.