Electrical conductivity of Alm82Py15Grs3 almandine-rich garnet determined by impedance spectroscopy at high temperatures and high pressures

- First experiments on electrical conductivity of almandine-rich garnet are achieved;
- Some characteristic parameters including σ0, △ H, △ U and △ V have been obtained;
- A functional relation for conductivity along with oxygen fugacity was established.

Under conditions of 1.0 ~ 3.0 GPa, 973 ~ 1273 K and frequency range from 10- 2 to 106 Hz, the electrical conductivity of Fe-rich garnet single crystal is measured by virtue of YJ-3000 t multi-anvil high-pressure apparatus and Solartron-1260 Impedance/Gain-phase Analyzer. A series of solid oxygen buffers including Fe3O4 + Fe2O3, Ni + NiO, Fe + Fe3O4, Fe + FeO and Mo + MoO2 are chosen to control oxygen fugacity. Two branches of impedance spectra are obtained in the complex impedance plane, one (102-106 Hz) showing a semicircular arc originated from the origin in the Nyquist and Bode diagram, another small tail in the low frequency of 10- 2-102 Hz. Experimental results show that with the rise of temperature (T), electrical conductivity (σ) tends to increase, and Log σ and 1/T satisfies the Arrhenius relation. Under control of a Ni + NiO oxygen buffer, with the rise of pressure, electrical conductivity decreases, and the pre-exponential factor also reduces, while the activation enthalpy rises, activation energy and activation volume of charge carriers are determined as 1.29 ± 0.12 eV and 2.01 ± 0.57 cm3/mol, respectively. The dominant electrical conduction mechanism of anhydrous almandine-rich garnet is the Fe2 + → Fe3 + hopping of small polaron. At 3.0 GPa, electrical conductivity of almandine-rich garnet increases with increasing oxygen fugacity. The electrical conductivity of this sample can be represented by the relation Log10σ=2.67±0.05+0.054±0.003×log10fO2+−5446±68T, where fO2 is the oxygen fugacity (bar) and T is the absolute temperature (K). At a typical temperature and pressure in the upper mantle, the influence of oxygen fugacity is substantial.