Abstract. Understanding the behavior of halogens (Cl, Br, and I) in subduction zones is critical to constrain the recycling of trace elements and metals, and to quantify the halogen fluxes to the atmosphere via volcanic degassing. Here, the partitioning of bromine between coexisting aqueous fluids and hydrous granitic melts and its speciation in slab-derived fluids have been investigated in situ up to 840 °C and 2.2 GPa by X-ray fluorescence (SXRF) and absorption (XANES and EXAFS) spectroscopy in hydrothermal diamond-anvil cells. The partition coefficients Df/mBr range from 15.3 ± 1.0 to 2.0 ± 0.1, indicating the preferential uptake of Br by aqueous fluids at all investigated conditions. EXAFS analysis further evidences a gradual evolution of Br speciation from hydrated Br ions [Br(H2O)6]− in slab dehydration fluids to more complex structures invoving both Na ions and water molecules, [BrNax(H2O)y], in hydrous silicate melts and supercritical fluids released at greather depth (> 200 km). In dense fluids containing 60 wt % dissolved alkali-silicates and in hydrous Na2Si2O5 melts (10 wt % H2O), Br is found in a salt-like structure involving 6 nearest Na ions and several next-nearest O neighbors that are either from water molecules or the tetrahedral silicate network. Bromine (and likely chlorine and iodine) complexation with alkalis is thus an efficient mechanism for the mobilization and transport of halogens by hydrous silicate melts and supercritical fluids, which can carry high amounts of Br, up to the 1000 ppm level. Overall, our results suggest that both shallow dehydration fluids and deeper silicate-bearing fluids efficiently remove halogens from the slab in the sub-arc region, thus controling an efficient recycling of halogens in subduction zones.
Bromine speciation and partitioning in slab-derived fluids and melts: Implications for halogens recycling in subduction zones
