Abstract:
An interpretation of the electrical signature of molten rocks within the Earth's interior in terms of nature and temperature conditions of magma requires additional laboratory data on the electrical conductivity of silicate melts. This paper describes an experimental setup and presents measurements of the electrical impedance of dry and hydrous (1-3 wt% H2O) metaluminous obsidians determined in an internally heated pressure vessel, in the range of 50-400 MPa and 350-1325°C. It is shown that the temperature and pressure dependences of electrical conductivity of both hydrous and dry obsidian can be fitted by an Arrhenius law applying in the melt and the glass regions. This suggests that a similar transport mechanism operates in both melt and glass. The determined activation energies are 70 kJ/mol for the dry obsidian and 65 and 61 kJ/mol for the 1 and 3 wt% H2O melts, respectively. The activation volume is 20 cm3/mol. Combination of tracer diffusion and electrical conductivity reveals that sodium is the dominant charge carrier in the hydrous and dry obsidians. The temperature and pressure effects on conductivity are therefore interpreted in terms of activation energy and activation volume for Na mobility in dry and hydrous rhyolites. An increase in conductivity associated with addition of water was observed and is shown to reflect the effect of water incorporation in melts on the mobility of sodium. As a prospective, it is anticipated that both the mobility and content of sodium could control the electrical conductivity of most terrestrial silicate melts. Magma under differentiation becomes more conductive due to sodium and water enrichment associated with fractional crystallization; therefore, its electrical signature must reveal its nature and maturity.
