Carbon speciation in and partitioning among silicate-saturated C-O-H fluids and (C-O-H)-saturated melts have been determined ~1.7 GPa and 900 °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) 2CH4 + Qn = 2CH3− + H2O + Qn+1 and (2) 2CO32− + H2O + 2Qn+1 = HCO3− + 2Qn, under reducing and oxidizing conditions, and where the superscript, n, in the Qn-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: ΔH(1)fluid = 16 ± 5 kJ/mol, ΔH(1)melt = 50 ± 5 kJ/mol; oxidizing ΔH(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 ΔH = 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.