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Influence Of The Redox State On The Electrical Conductivity Of Basaltic Melts

Abstract : AB: The electrical conductivity is an efficient probe of mass transfer processes within silicate melts and magmas. Previous studies have established that the electrical conductivity is sensitive to parameters such as temperature, melt composition and pressure. In contrast to what is known for Fe-bearing minerals, little attention has been given to the influence of redox state on the electrical conductivity of melts. Experiments were performed on tephritic and basaltic compositions respectively from Mt. Vesuvius and Pu'u ‘O'o. Measurements were carried out on cylindrical glass samples (OD: 6 mm, ID: 1 mm, L: 8 mm) drilled from glass obtained by fusing each rock sample at 1400°C in air. A two-electrode configuration was adopted, with the electrical impedance being radially measured. A Pt wire was used as the internal electrode whereas a Pt tube served as the external electrode. Experiments were conducted at 1 atm in a vertical furnace between 1200°C and 1300°C, both in air and in a CO-CO2 atmosphere at a fO2 corresponding to NNO+1. Both reduction and oxidation experiments were performed. In reduction experiments (pure CO2 then CO-CO2 gas mixture), electrical conductivities progressively increase with time. The reverse is observed in oxidation experiments (CO-CO2 gas mixture then pure CO2). These variations of electrical conductivities are correlated with modifications of the Fe2+/Fe3+ ratio in the melt, and are consistent with the respective structural roles of Fe2+ and Fe3+. In both types of experiments, a minimum of about 400 mn is necessary before a plateau is reached, interpreted to reflect the kinetics of attainment of the equilibrium Fe2+/Fe3+ ratio in the melt. Differences between plateau and initial values are typically of a few ohms, much higher than the sensitivity of our measurements (better than 0.1 ohm). When increasing temperature, the time required for reaching plateau values decreases. At NNO+1, the electrical activation energy (Ea) was determined for both compositions: Ea=137 kJ/mol (tephrite) and 73 kJ/mol (basalt). Further experiments are necessary to quantify the influence of redox state on electrical conductivity, especially at fO2 below NNO+1.
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Submitted on : Tuesday, January 22, 2008 - 2:48:16 PM
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  • HAL Id : insu-00212120, version 1

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Anne Pommier, Fabrice Gaillard, Michel Pichavant. Influence Of The Redox State On The Electrical Conductivity Of Basaltic Melts. AGU Meeting San Francisco, Dec 2007, San Francisco, United States. 1 p. ⟨insu-00212120⟩

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