The effect of CO₂ on the speciation of RbBr in solution at temperatures to 650°C and pressures to 0.65 GPa

Katy Evans¹, John Mavrogenes², Robert Gordon<sup>3

1</sup> Research School of Earth Sciences, Australian National University, Canberra, ACT 0200, Australia.  Now at School of Applied Geology, Curtin University, GPO Box U1987, WA6845
² Research School of Earth Sciences, Australian National University, Canberra, ACT 0200,
³ PNC-CAT, APS, Argonne National Laboratory, 9700 South Cass Avenue - Building 46, Argonne, IL 60439, USA

Carbon dioxide- and salt-bearing solutions are common in geological environments.  The presence of CO\2 affects mineral solubilities, fluid miscibility, and viscosity/wetting properties, and is expected to affect salt speciation. EXAFS measurements of RbBr-H\2O-CO\2 fluids contained in corundum-hosted synthetic fluid inclusions (SFLINCs) have been used to investigate the effect of CO₂on salt speciation at temperatures to 650°C and pressures to around 0.65 GPa. 

Surprisingly, results for Rb in CO₂-free and CO₂-bearing solutions are effectively identical.  This is attributed to exclusion of CO₂ from the vicinity of the solvated cation, and is consistent with significant variation in fluid properties in the vicinity of solvated cations. This result has implications for thermodynamic modelling of CO\2-bearing fluids as it suggests that bulk fluid properties may be inappropriate for calculation of solute parameters.  Results for Br in CO₂-free solutions are different to those from CO₂-bearing solutions.  The contrast between the results for cations and anions is consistent with the characteristics of ion-water bonding; the geometry of anion-water complexes permit greater continuity between the bulk solvent and solvated waters than is possible for cation-water complexes.

Otherwise, the results are consistent with those of previous work.  Increases in pressure and temperature cause an apparent decrease in the number of waters of solvation for Rb of up to 67%, and a contraction in bond lengths of around 4%.  Results for Br also show apparent solute dehydration.  However, forward modelling indicates that solute dehydration might be difficult to distinguish from ion-pairing.   Advances in conceptual models based on the results presented here are pertinent to CO₂-bearing solutions in a range of geological environments that include granulite, ore-forming and magmatic environments.