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) 2CH₄ + Qⁿ = 2CH₃⁻ + H₂O + Qⁿ⁺¹ and (2) 2CO₃^2- + 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⁻/CO₃^2-, increase with temperature. The enthalpy change associated with the species transformation differs for fluids and melts and also for oxidized and reduced carbon [Reducing: 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, (X_CH₄/X_CH₃)fluid/(X_CH₄/ X_CH₃)melt, yields ΔH = 34 ± 3 kJ/mol. \

Increased abundance ratios, C_H₄/C_H₃ and HCO₃⁻ /CO₃^2-, 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.