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 fO 2 step-changes in order to investigate redox kinetics. Experimental investigations were performed at 1 atm in a vertical furnace between 1200 and 1400 °C using air, pure CO 2 or CO/CO 2 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 fO 2 on the electrical conductivity is observed. The more reduced the melt, the higher the conductivity. A modified Arrhenian equation accounts for both T and fO 2 effects on the electrical conductivity. We show that time-dependent changes in electrical conductivity following fO 2 step-changes monitor the rate of Fe 2+/Fe 3+ changes. The conductivity change with time corresponds to a diffusion-limited process in the case of reduction in CO–CO 2 gas mixtures and oxidation in air. However, a reaction at the gas–melt interface probably rate limits oxidation of the melt under pure CO 2. Reduction and oxidation rates are similar and both increase with temperature. Those rates range from 10 −9 to 10 −8 m 2/s for the temperature interval 1200–1400 °C and show activation energy of about 200 kJ/mol. The redox mechanism that best explains our results involves a cooperative motion of cations and oxygen, allowing such fast oxidation–reduction rates.