Raman spectroscopic determination of carbon speciation and quartz solubility in H2O + Na2CO3 and H2O + NaHCO3 fluids to 600 °C and 1.53 GPa

The concentrations of the species CO32-(aq), NaCO3-(aq), and HCO3-(aq) and the solubility of quartz in 4.65 m NaHCO3 and 1.6 m Na2CO3 solutions have been determined at temperatures (T) to 600 degrees C and pressures (P) up to 1.53 GPa using Raman spectroscopy and a Bassett-type hydrothermal diamond-anvil cell. Most Raman spectra of the 1.6 m Na2CO3 fluid and the spectra of the 4.65 m NaHCO3 solution at T >= 400 degrees C showed evidence of Na+-CO32- contact ion pairs. In the 1.6 m Na2CO3 solution, the molal fractions alpha(HCO3)- and alpha(NaCO3)- of the species HCO3-(aq) and NaCO3-(aq) increased with temperature. At vapor pressure during first heating, HCO3-(aq) was not detectable at 23 degrees C and NaCO3-(aq) not at 23-200 degrees C, and alpha(HCO3)- was 0.106 at 200 degrees C. After equilibration with quartz at high temperature, these fractions were higher and showed little dependence on pressure (e.g., alpha(HCO3)- was similar to 0.13 at 23 degrees C, similar to 0.29 at 200 degrees C, similar to 0.39 at 400 degrees C, and similar to 0.50 at 600 degrees C, and alpha(NaCO3)- similar to 0.11 at 23 degrees C, similar to 0.23 at 200 degrees C, similar to 0.34 at 400 degrees C, and similar to 0.41 at 600 degrees C). Consequently, alpha CO32-, the fraction of CO32-(aq), decreased strongly with temperature to similar to 0.09 at 600 degrees C. Carbon dioxide was not detected in preceding tests. In contrast to the Na2CO3 solution, significant CO2 formed in runs with H2O + NaHCO3 via autoprotolysis 2 HCO3-(aq) = CO32-(aq) + CO2(aq) + H2O. In the 4.65 m NaHCO3 solution, the HCO3-(aq) fraction decreased with temperature and increased nonlinearly with pressure along all studied isotherms from 200 to 600 degrees C (e. g. at 600 degrees C from 0.66 at 0.63 GPa to 0.75 at 1.1 GPa to 0.80 at 1.5 GPa), whereas alpha(NaCO3)-, alpha(CO3)2-, and alpha(CO2) showed the opposite behavior. These results indicate that HCO3-(aq) is a significant oxidized carbon species in aqueous fluids in the lower crust and upper mantle. The only detectable Raman band from dissolved silica was from symmetric Si-OH stretching of monomers and consisted of two components assigned to the species SiO(OH)(3)(-)(aq) and Si(OH)(4)(aq). The calibrated normalized integrated intensity of this band at similar to 770 cm(-1) was used to determine the SiO2(aq) molality in the fluid. These data showed that the quartz solubility in 1.6 m Na2CO3 increased with P and T, and was always much higher than the solubility of quartz in water at the same P-T condition. The quartz solubility in 4.65 m NaHCO3 increased with temperature. At lower pressures, it was generally higher than in water, which indicates a basic pH. Along the 500 and 600 degrees C isotherms, the quartz solubility decreased with pressure to values below that in water. (C) 2014 Elsevier Ltd. All rights reserved.