Carbon speciation in silicate-C-O-H melt and fluid as a function of redox conditions: An experimental study, in situ to 1.7 GPa and 900 °C

Carbon speciation in and partitioning among silicate-saturated C-O-H fluids and (C-0-H)-saturated melts have been determined similar to 1.7 GPa and 900 degrees C under reducing and oxidizing conditions. The measurements were conducted in situ while the samples were at the conditions of interest. The solution equilibria were (1) 204(4) + Q(n) = 2CH(3)(-) + H2O + Q(n+1) and (2) 2CO(3)(2-) + H2O + 2Q(n+1) = HCO3- + 2Q(n), under reducing and oxidizing conditions, and where the superscript, n, in the Q(n)species denotes number of bridging oxygen in the silicate species (Q-species). The abundance ratios, CH3/CH4 and HCO3-/CO32-, increase with temperature. The enthalpy change associated with the species transformation differs for fluids and melts and also for oxidized and reduced carbon [Reducing: Delta H-(1)(fluid) = 16 +/- 5 kJ/mol, Delta H-(1)(melt) = 50 +/- 5 kJ/mol; oxidizing Delta H-(2)(fluid) = 81 +/- 14 kJ/mol]. For the exchange equilibrium of CH4 and CH3 species between fluid and melt, the temperature-dependent equilibrium constant (XCH4/XCH3)(fluid)/(XCH4/XCH3)(melt), yields Delta H = 34 +/- 3 kJ/mol. Increased abundance ratios, CH4/CH3 and HCO3-/CO32-, lead to increased polymerization of silicate+(C-O-H) melt. Because of such relations, melt transport properties (e.g., viscosity) and element partition coefficients between magmatic liquids, C-O-H fluids, and crystalline phases can vary by more than 100% with speciation changes of C-bearing volatiles upper mantle. These structure effects are more pronounced the higher the pressure and the more mafic the magma.