Experimental study on electrical conductivity of biotite- and plagioclase-bearing gneiss at high temperature and high pressure

We have investigated the electrical properties of rock sample from Yinshan orogenic zone by in situ complex impedance spectroscopy using a multianvil apparatus. The electrical conductivity was measured at pressure of 1 GPa and temperature from 250C to 1100C at frequencies from 0.1 Hz to 1 MHz. The data of resistance were obtained by using an equivalent circuit fitting technique. From these measurements we determined the electrical conductivity of the Archean metamorphic biotite- and plagioclase-bearing gneiss. The experimental results indicate that (1) each spectrum consists of an almost semicircular high-frequency arc Iand a depressed high-frequency arc II. The impedance spectrum I is dominated by grain-interior response; the impedance spectrum II is dominated by the sample-electrode interface impedance. (2) Electrical conductivity displays Arrhenian behavior respectively over investigated temperature range from 250C to 700C and from 750C to 1100C. However, it rapidly increases from 700C to 750C. This jump is due to dehydration-melting of biotite within sample based on the comparative analysis of SEM photoes and electron probe data for pre-run and post-run samples. (3) Over higher temperature range (750-1100C), sodium is the dominating charge carrier in partially molten samples. (4) The activation energy of biotite- and plagioclase-bearing gneiss is 0.53 eV over range of temperature from 250C to 700C. However, the activation energy of partially molten sample is 1.41 eV over range of temperature from 750C to 1100C. This change of activation energy has relations with sodium content and mobility in melt. It is possibly due to the change of structure between mineral and melt within samples.