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. Understanding the behavior of halogens (Cl, Br, and I) insubduction zones is critical to constrain the geochemical cycle of thesevolatiles and associated trace metals, as well as to quantify the halogen fluxesto the atmosphere via volcanic degassing. Here, the partitioning of brominebetween coexisting aqueous fluids and hydrous granitic melts and itsspeciation in slab-derived fluids have been investigated in situ up to840  ∘ C and 2.2 GPa by synchrotron x-ray fluorescence (SXRF) and x-rayabsorption spectroscopy (XAS) in diamond anvil cells. The partitioncoefficients D Br f / m range from ∼2 to ∼15 , with an average value of 6.7±3.6 ( 1σ )over the whole pressure–temperature ( P – T ) range, indicating a moderate Brenrichment in aqueous fluids, in agreement with previous work. Extended x-ray-absorption fine-structure (EXAFS)analysis further evidences a gradual evolution of Br speciation fromhydrated Br ions [Br( H2O ) 6 ] − in slab dehydration fluids tomore complex structures involving both Na ions and water molecules,[ BrNax(H2O)y ], in hydrous silicate melts and supercriticalfluids released at greater depth (> 200 km). In denser fluids( ρ  > 1.5 g cm −3 ) containing 60 wt % dissolvedalkali–silicates and in hydrous Na2Si2O5 melts (10 wt % H2O ), Br is found to be in a “salt-like” structure involving the sixnearest Na ions and several next-nearest O neighbors that are either fromwater molecules and/or the silicate network. Bromine (and likely chlorineand iodine) complexing with alkalis is thus an efficient mechanism for themobilization and transport of halogens by hydrous silicate melts andsilica-rich supercritical fluids. Our results suggest that both shallowdehydration fluids and deeper silicate-bearing fluids efficiently removehalogens from the slab in the sub-arc region, thus favoring an efficienttransfer of halogens across subduction zones.

Bromine speciation and partitioning in slab-derived aqueous fluids and silicate melts and implications for halogen transfer in subduction zones