Time-dependent changes of the electrical conductivity of basaltic melts with redox state

Abstract : The electrical conductivity of basaltic melts has been measured in real-time after fO2 step-changes in order to investigate redox kinetics. Experimental investigations were performed at 1 atm in a vertical furnace between 1200°C and 1400°C using air, pure CO2 or CO/CO2 gas mixtures to buffer oxygen fugacity in the range 10-8 to 0.2 bars. Ferric/ferrous ratios were determined by wet chemical titrations. A small but detectable effect of fO2 on the electrical conductivity is observed. The more reduced the melt, the higher the conductivity. A modified Arrhenian equation accounts for both T and fO2 effects on the electrical conductivity. We show that time-dependent changes in electrical conductivity following fO2 step-changes monitor the rate of Fe2+/Fe3+ changes. The conductivity change with time corresponds to a diffusion-limited process in the case of reduction in CO-CO2 gas mixtures and oxidation in air. However, a reaction at the gas-melt interface probably rate limits oxidation of the melt under pure CO2. Reduction and oxidation rates are similar and both increase with temperature. Those rates range from 10-9 to 10-8m2/s for the temperature interval 1200-1400°C and show activation energy of about 200kJ/mol. The redox mechanism that best explains our results involves a cooperative motion of cations and oxygen, allowing such fast oxidation-reduction rates.