[S-MP06_28PM2] Impact of volatiles on the processes of formation and evolution of the Earth's interior

[SMP06-09] In-situ characterization of carbon-speciation in silicate-C-O-H fluid and melt with temperature, pressure, and redox con

*Bjorn MYSEN¹ (1.Geophysical Laboratory, CIW, USA)

Keywords:COH volatiles, fluid structure, melt structure, high pressure, high temperature, redox conditions

Speciation and partitioning of C-bearing volatiles species in and between silicate-saturated C-O-H fluids and (C-O-H)-saturated melts have been determined in-situ with the samples to pressures and temperatures of ~2GPa and 900^oC, respectively. Structural characterization was conducted with vibrational spectroscopy of samples contained in externally-heated, hydrothermal diamond anvil cells. The redox conditions were controlled near that of the Fe+H₂O=FeO+H₂ (reducing, RED) and Ni+H₂O=NiO+H₂ (oxidizing, OX) equilibria, respectively. Melts are, therefore saturated in H₂O, H₂, and C-bearing species (redox dependent) and coexisting fluids saturated in silicate components. Solubility of volatile and silicate components depend on both temperature and pressure. The melt/fluid partition coefficients of the C-bearing species vary with redox conditions and temperature with the ∆H_RED^melt/fluid = 44(7) kJ/mol and ∆H_OX^melt/fluid = -70(32) kJ/mol. Pressure is a dependent variable and increases with increasing temperature. It is assumed no pressure effect of the partition coefficients.The solution equilibria under reducing and oxidizing conditions, respectively, were; (1) 2CH₃^-+H₂O+Qⁿ⁺¹=2CH₄+Qⁿ and (2) 2CO₃^2-+H₂O+2Qⁿ⁺¹=HCO₃^-+2Qⁿ, where the superscript, n, in the Q-species denotes number of bridging oxygen in the silicate species (Q-species). In the absence of H₂O equilibrium (1) changes to CH₃^-+Qⁿ=CH₄+Qⁿ⁺¹. For oxidized carbon, there is an analogous expression expressing equilibrium between molecular CO₂ and structurally bound CO₃^2--groups. Under both oxidizing and reducing conditions, the abundance ratios, CH₄/CH₃^- and HCO₃^-/CO₃^2- increase with temperature. The enthalpy change associated with the species transformation does, however, differ for fluids and melts and also for oxidized and reduced carbon (∆H₍₁₎^fluid = -16(5) kJ/mol, ∆H₍₁₎^melt = -49(5) kJ/mol, ∆H₍₂₎^fluid=81(14) kJ/mol). For the exchange equilibrium of CH₄ and CH₃ species, the temperature-dependent equilibrium constant yields ∆H=34(3) kJ/mol.Reactions (1) and (2) involve changes in silicate polymerization where increasing abundance ratios, CH₄/CH₃^- and CO₃^2-/HCO₃^- lead to increased silicate melt polymerization. As a result of the relations between speciation of C-bearing species and melt and fluid structure, stable isotope (C and H) and element partition coefficients between melts and fluids, which depend on and silicate polymerization and silicate speciation, also vary with speciation of C-bearing species in silicate-C-O-H systems. Pressure, temperature, and redox control on the C-speciation also govern those (and other) properties.