Electrical conductivity of fluid-bearing quartzite at high pressure and high temperature

It has been reported that the electrical conductivities determined by magnetotelluric methods are much higher than those determined by dry laboratory sample of crustal rocks (e.g., Shankland and Ander, 1983). The possible reasons of the high-conductivity anomaly regions are interconnected graphite films, partial melt, and aqueous fluid (e.g., Glover and Vine, 1994). Because the solubility of silicate component in the aqueous fluid should increase with increasing pressure, the electrical conductivity of fluid-bearing rocks can be higher at pressure of the lower crust even if there is no brine. To clarify the effect of the soluble ionic species in aqueous fluid on the bulk rock conductivity, we measured the electrical conductivity of fluid-bearing quartzite as functions of temperature and fluid content at high pressure. Based on the result, we discuss the fluid content in the crust. High-pressure experiments were conducted using a DIA-type high-pressure apparatus. Pressure medium is pyrophyllite and heater is graphite. The starting materials were mixture of quartz powder plus silicic acid, or quartz aggregate synthesized using Piston-cylinder apparatus plus silicic acid. Electrical conductivity was determined by impedance spectroscopy method. To prevent a loss of water during the electrical conductivity measurements, we used a diamond single crystal capsule. The pressure is 1 GPa, and temperature range is 700-1100K. The texture of the recovered samples was observed using field-emission scanning electron microscope, and the fluid content was measured from the obtained SEM images. The electrical conductivity of fluid-bearing quartzite (fluid fraction=0.29) is about 4 orders of magnitude higher than that of dry quartzite at 1000K. Our result suggests that the observational electrical conductivity at Tohoku, Japan (Ogawa et al. 2001) and New Zealand (Wannamaker et al. 2009) in the middle crust is unable to account for quartz plus H2O. Therefore, plausible explanations of high-conductivity anomaly are presence of saline fluid and/or the other ionic species.