The effect of CO₂ on the speciation of bromine in low-temperature geological solutions: An XANES study

CO2-rich solutions are common in geological environments. An XANES (X-ray absorption near-edge structure) study of Br in CO2-bearing synthetic fluid inclusions has revealed that Br exhibits a strong pre-edge feature at temperatures from 298 to 423 K. Br in CO2-free solutions does not show such a feature. The feature becomes smaller and disappears as temperature increases, but reappears when temperature is reduced. The size of the feature increases with increasing X(CO2) in the fluid inclusion, where X(CO2) is the mole fraction of CO2 in the solution [n CO2 /(n CO2 + n H2 O + n RbBr); n indicates the number of moles]. The pre-edge feature is similar to that shown by covalently bonded Br, but observed and calculated concentrations of plausible Br-bearing covalent compounds (Br2, CH3Br and HBr) are vanishingly small. An alternative possibility is that CO2 affects the hydration of Br sufficiently that the charge density changes to favour the 1s-p level transitions that are thought to cause the pre-edge peak. The distance between the first two post-edge maxima in the XANES also decreases with increasing X(CO2). This is attributed to a CO2-related decrease in the polarity of the solvent. The proposed causes of the observed features are not integrated into existing geochemical models; thus CO2-bearing solutions could be predicted poorly by such models, with significant consequences for models of geological processes such as ore-formation and metamorphism.