Solubility and speciation of carbonate in aqueous fluids at HP-HT

We report an integrated experimental and theoretical study of the equilibration of CaCO$_{3}$ minerals with aqueous solutions (pure H$_{2}$O or NaCl solutions) at HP-HT (0.5 to 8 GPa, 250 to 500$^{\circ}$ C). The fluid speciation was studied using in situ Raman spectroscopy in an externally heated membrane-type diamond anvil cell. The aqueous fluid and a calcite crystal were loaded in a rhenium gasket. P was determined from the calibrated shift of the carbonate $\nu $1 symmetric stretching mode of aragonite and the T measured with a K-type thermocouple. Spectra were recorded using a LabRam HR800 Raman spectrometer (Horiba Jobin-Yvon) coupled to a Spectra Physics Ar$^{+}$ laser. At equilibrium with an aragonite crystal, the Raman data show that bicarbonate is the most abundant species in low-pressure fluids below 4 GPa whereas carbonate becomes progressively dominant at higher pressure. After correction from their Raman cross-sections, the relative amounts of dissolved carbonate and bicarbonate were estimated from the areas of the $\nu $1 and $\nu $5 symmetric stretching Raman modes of the carbonate and bicarbonate ions, respectively. The presence of sodium chloride extends the pressure range of predominance for bicarbonate species in the fluid. The Raman data were also used to constrain a theoretical thermodynamic model of the fluid speciation in equilibrium with CaCO$_{3}$. Revised thermodynamic properties of aqueous CO$_{2}$ and HCO$_{3}^{-}$, data for the aqueous CaHCO$_{3}^{+}$ complex from 4 to 90$^{\circ}$ C, and estimated dielectric constants of water, enabled an equation of state characterization of the standard Gibbs free energy of CaHCO$_{3}^{+}$ at the conditions of the Raman study. At 300-400$^{\circ}$ C, the Raman speciation results were used to constrain equilibrium constants involving the carbonate ion. The results indicate that CO$_{2}$ is a minor species in fluids in equilibrium with aragonite at 300-500$^{\circ}$ C and P \textgreater 1 GPa. Instead, the CaHCO$_{3}^{+}$ species becomes important at high pressures until carbonate ion becomes the dominant C-species.