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 uids, occurring both as dissolved ionic species (e.g., carbonate ions or organic acids) and molecular species (e.g., CO[subscript 2], CO, CH[subscript 4], and more complex organic compounds). The chemistry of dissolved carbon changes dramatically with pressure (P) and temperature (T). In aqueous uids at low P and T, molecular carbon gas species such as CO[subscript 2] and CH[subscript 4] saturate at low concentration to form a separate phase. With modest increases in P and T, these molecular species become fully miscible with H[subscript 2]O, enabling deep crustal and mantle uids to become highly concentrated in carbon. At such high concentrations, carbon species play an integral role as solvent components and, with H[subscript 2]O, control the mobility of rock-forming elements in a wide range of geologic settings. The migration of carbon-bearing crustal and mantle uids 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).