The Chemistry of Carbon in Aqueous Fluids at Crustal and Upper-Mantle Conditions: Experimental and Theoretical Constraints

Carbon can be a major constituent of crustal and mantle fluids, occurring both as dissolved ionic species (e.g., carbonate ions or organic acids) and molecular species (e.g., CO2, CO, CH4, and more complex organic compounds). The chemistry of dissolved carbon changes dramatically with pressure (P) and temperature (T). In aqueous fluids at low P and T, molecular carbon gas species such as CO2 and CH4 saturate at low concentration to form a separate phase. With modest increases in P and T, these molecular species become fully miscible with H2O, enabling deep crustal and mantle fluids to become highly concentrated in carbon. At such high concentrations, carbon species play an integral role as solvent components and, with H2O, control the mobility of rock-forming elements in a wide range of geologic settings. The migration of carbon-bearing crustal and mantle fluids contributes to Earth’s carbon cycle; however, the mechanisms, magnitudes, and time variations of carbon transfer from depth to the surface remain least understood parts of the global carbon budget (Berner 1991, 1994; Berner and Kothavala 2001).