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

Abstract

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) 2CH 4 + Q n = 2CH 3 − + H 2 O + Q n+1 and (2) 2CO 3 2− + H 2 O + 2Q n+1 = HCO 3 − + 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, CH 3 /CH 4 and HCO 3 − /CO 3 2− , increase with temperature. The enthalpy change associated with the species transformation differs for fluids and melts and also for oxidized and reduced carbon [Reducing: Δ H (1) fluid = 16 ± 5 kJ/mol, Δ H (1) melt = 50 ± 5 kJ/mol; oxidizing Δ H (2) fluid = 81 ± 14 kJ/mol]. For the exchange equilibrium of CH 4 and CH 3 species between fluid and melt, the temperature-dependent equilibrium constant, (X CH 4 /X CH 3 ) fluid /(X CH 4 /X CH 3 ) melt , yields Δ H = 34 ± 3 kJ/mol. Increased abundance ratios, CH 4 /CH 3 and HCO 3 − /CO 3 2− , 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.