Bromine solubility in silicate melts and its implications for volcanic HBr degassing

We have determined the solubility of bromine in silicate melts using Ag-AgBr(±AgI) buffers to control f Br 2 , together with C-CO-CO 2 and Re–ReO 2 buffers to control f O 2 over a broad range of melt compositions at 0.5–2.5 GPa and 1200–1500 °C. In the dilute region, melt Br concentrations vary with fB r 2 0.5 and f O 2 − 0.25 , consistent with dissolution through the exchange of two Br − ions for one O 2− ion, analogous to the mechanism proposed for chlorine. Henry's law behaviour extends to 1.1 wt% Br, well above concentrations in natural melts. We use these relationships to define bromide capacity and, by combining our results with published hydrous fluid-melt partitioning data, derive a model applicable from 790 to 1500 °C and 0.1–2.5 GPa. The resulting equation is: log C Br = 3.94 + − 972 X SiO 2 + 3360 X FeO tot + 3382 X CaO − 659 X H O 0.5 + 201 X Na O 0.5 − 11 X K O 0.5 − 1277 P − 6322 / T where T is in K, P in GPa, and X i denotes single cation oxide mole fraction. Bromide capacity decreases with increasing pressure and temperature, decreases with increasing SiO 2 and K 2 O contents, and increases with CaO and FeO, so that bromine solubility decreases in the order basalt > andesite > dacite > rhyolite. Bromide capacity is lower than chloride capacity for all compositions investigated, showing that bromine is consistently less soluble than chlorine in silicate melts. Despite this lower melt solubility, application of the bromide capacity model to H 2 O-rich low-pressure fluids indicate that Br and Cl degas similarly as HBr and HCl, respectively, so the HBr/HCl ratio of volcanic gases should closely reflect the Br/Cl ratio of the melt with little fractionation. These results provide a thermodynamic framework for modelling bromine behaviour in magmatic and volcanic systems.