THE CHEMISTRY OF CARBON IN AQUEOUS FLUIDS AT CRUSTAL AND UPPER-MANTLE CONDITIONS: EXPERIMENTAL AND THEORETICAL CONSTRAINTS

Crustal and mantle fluids are made up of a combination of H₂O, dissolved silica and other rock components and gas species. Carbon can be a major constituent as dissolved ionic and molecular species. In H₂O-rich fluids at low pressure (P) and temperature (T), molecular species like CO₂ and CH₄ saturate at low concentration to form a separate phase. However, at relatively modest P & T, these species become miscible with H₂O, leading to a wide range of fluid compositions in the crust and mantle. Carbon species may play an integral role as solvent components and, with H₂O, control the transport and mobility of elements. However, the mechanisms, magnitudes and time variations of carbon transfer from the deep Earth to surface carbon reservoirs remains one of the least understood parts of the global carbon budget.

We review evidence for the presence of carbon in water-rich crustal and mantle fluids, the geochemistry of oxidized carbon in CO₂-H₂O-NaCl fluids, and the geochemistry of reduced carbon species and the role of metastability in C-O-H fluids. It is clear that our understanding of the chemistry of aqueous carbon is advanced for shallow systems; however, the higher pressures relevant to Earth’s deep carbon cycle remain largely unexplored. Progress has been made in the application of equations of state for molecular fluids. Unfortunately this simple framework is inadequate when many other species such as ions, metal carbonate complexes, and metastable organic solutes must be taken into account. The problem is compounded by the limit of P ≤ 5 kbar in the application of aqueous species equations of state and by the paucity of experimental data at P > 20 kbars.

Recent promising advances in this field include: new hydrothermal piston-cylinder and hydrothermal diamond-anvil cell approaches with in situ spectroscopy applied to mineral solubility and fluid speciation; experimental studies of organic solute metastability; the use of simple correlations of mineral solubility and homogeneous equilibria with H₂O density; and the extension of aqueous species equations of state to high P. These advances are opening for the first time the realm of deep fluid flow to robust aqueous geochemical methods. The combined experimental and theoretical avenues thus promise new insights into the terrestrial deep carbon cycle in the coming years.