Electrical conductivity of fluid-bearing quartzite under lower crustal conditions

摘要

The electrical conductivity of fluid-bearing quartzite was determined as function of temperature and fluid fraction at 1 GPa in order to assess the origin of the high conductivity anomalies observed in the middle to lower crustal levels. Dihedral angles of quartz-fluid-quartz determined from recovered samples were below 60°, suggesting that fluid forms an interconnected network through the quartz aggregate. The electrical conductivity of quartzite increases with increasing temperature, which can be approximately expressed by Arrhenius equation. The apparent activation enthalpy decreases from 0.70 to 0.25 eV with increasing fluid fraction in volume from 0.00043 to 0.32. The electrical conductivity (σ) of the fluid-bearing quartzite increased with fluid fraction (?) proportionally to a power law (σ ∝ ?~(0.56-0.71)) within the temperature range of 900-1000 K. The electrical conductivity of the aqueous fluid-bearing quartzite with the maximum fluid fraction (0.32) was found to be about three orders of magnitude higher than that of dry quartzite at 1000 K. However, its electrical conductivity was definitely lower than the geophysically observed values of high-conductivity anomalies, even if the quartzite contained large fluid fractions (0.32). The present results suggest that fluid-bearing quartzite is unable to account for the high-conductivity anomalies in terms of fluid fraction. A significant amount of other ionic species, such as Na, Cl, and Al in aqueous fluid, in addition to silica phases dissolved in fluid, is required to increase conductivity.