Complex electrical conductivity measurements of a KTB amphibolite sample at elevated temperatures

In the present work, complex electrical conductivity measurements in the frequency range of 10 mHz–1 MHz and at elevated temperatures (423 K–1373 K) were carried out in amphibolite sample, originating from the KTB drilling. Impedance Cole–Cole plots were used to discriminate the contributions of grain interior, grain boundaries and electrode polarization effects to the measured conductivity spectra. At frequencies above 10 Hz where the electrode effects are negligible, ac-conductivity exhibits frequency dispersion according to a two-term power law behaviour, in most cases of the isothermal spectra. A conductivity relaxation step is observed during heating of the sample up to 773 K, which is attributed to strongly bound water, either in the form of hydroxyls in the crystal lattice of grains, or in the form of molecules trapped between the grain boundaries. The hysteresis plot of dc-conductivity shows an anomalous behaviour at around 1000 K during the heating procedure, due to the dehydroxylation of the sample. The electric modulus M* representation provides additional information over the low temperature range and the conductivity relaxation times were estimated for a limited temperature range. Thermal activation energies of Arrhenius type vary from 0.58 eV up to 1.50 eV, and they are ascribed to proton conduction at low temperature and hopping conduction of small polarons at higher temperatures.

► ac-Conductivity is investigated from 10 mHz up to 1 MHz and from RT up to 1373 K.
► dc-Conductivity shows anomalous behaviour around 1000 K during heating.
► Thermal activation energies of conduction mechanisms vary from 0.58 eV to 1.50 eV.
► Protons and hopping of small polarons are the dominant conduction mechanisms.