The Effect of CO2 Reduction in Low-Water Melts at Low Hydrogen Fugacity: Experiment at 500 MPa and Thermodynamic Model

Formation of graphite was observed in experiments on synthesis of dry carbon-bearing albite glasses in platinum capsules in an Internally Heated Pressure Vessel at 500 MPa and Т = 1200–1250°С. A thermodynamic model is proposed that explains the achievement of low oxygen fugacity near QFM-2 in the melt at low fugacity of hydrogen formed due to the decomposition of trace amounts of water in a compression medium (Ar gas). The unexpectedly low fugacity of oxygen is explained by the shift of equilibrium between the gases dissolved in the melt CO₂ + H₂ = H₂O + CO to the right due to the low activity of molecular water at a low total content of H₂O ~ 0.1–0.5 wt %. The high local СО concentrations in the melt lead to the platinum dissolution in form of carbonyl, corrosion of capsule walls, and redeposition of the metal at the contact with melt. With increase of water concentration in the melt (>1 wt %), the effect of reduction disappears.