Carbonate minerals may be recycled into the mantle at subduction zones. However, the evolution of carbonate minerals in equilibrium with aqueous fluids as well as the nature of the chemical species of dissolved carbon in the deep crust and mantle at high PT conditions are still unknown. In this study, we report an integrated experimental and theoretical study of the equilibration of CaCO3 minerals with pure water at subduction zone conditions over the pressure and temperature ranges 5–80 kbar and 300–400 °C. The fluid speciation was studied using in situ Raman spectroscopy. The relative amounts of dissolved carbonate and bicarbonate were estimated from the corrected areas of the Raman bands of the carbonate and bicarbonate ions and used to constrain a theoretical thermodynamic model of the fluid speciation and solubility of aragonite. At 300–400 °C, our results indicate that the proportion of dissolved C present as CO2 strongly decreases in fluids in equilibrium with aragonite at P > 10 kbar. CO2 is replaced by HCO3− and CaHCO3+ which predominate until P > 40 kbar, where CO32− and CaCO30 become the dominant C-species. At higher temperatures, the theoretical model indicates that CO2 again becomes a major species in fluids in equilibrium with aragonite depending on the pressure.