The effect of CO 2 on the speciation of bromine in low‐temperature geological solutions: an XANES study

CO 2 ‐rich solutions are common in geological environments. An XANES (X‐ray absorption near‐edge structure) study of Br in CO 2 ‐bearing synthetic fluid inclusions has revealed that Br exhibits a strong pre‐edge feature at temperatures from 298 to 423 K. Br in CO 2 ‐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 (CO 2 ) in the fluid inclusion, where X (CO 2 ) is the mole fraction of CO 2 in the solution [ n CO2 /( n CO2 + n H2O + 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 (Br 2 , CH 3 Br and HBr) are vanishingly small. An alternative possibility is that CO 2 affects the hydration of Br sufficiently that the charge density changes to favour the 1 s – 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 (CO 2 ). This is attributed to a CO 2 ‐related decrease in the polarity of the solvent. The proposed causes of the observed features are not integrated into existing geochemical models; thus CO 2 ‐bearing solutions could be predicted poorly by such models, with significant consequences for models of geological processes such as ore‐formation and metamorphism.