Frequency dependent electrical properties of polycrystalline olivine compacts

Abstract

The complex electrical properties of poly crystalline San Carlos olivine compacts were measured over the range of frequency l0−4–104Hz from 800° to 1400°C under controlled oxygen fugacity. The impedance data display a strong frequency dependence that is evidenced most clearly when the results are displayed in the complex impedance plane. A parameterized model of the frequency dependent electrical response using equivalent electrical circuits is presented. Two distinct conduction mechanisms of the sample are observed: grain interior and grain boundary conduction. Each occurs over a different range of frequency. The resistance of each mechanism adds in series resulting in a lower total DC conductivity for polycrystalline olivine than for either mechanism separately. The total DC conductivity is dominated by the grain interior conductivity above 1200°C, whereas the grain boundary conductivity has the strongest influence below 1000° C. Impedance spectra of natural dunite samples exhibit a similar type of frequency dependence. The grain interior conductivity displays a change in slope at 1344°C and has activation energies of 1.45 eV (800°–1344°C) and 4.87 eV (>1344°C). The grain boundary conductivity has an activation energy of 2.47 eV. In these cases, the ƒO2 for each experimental run was controlled at that of the wustite‐magnetite oxygen buffer. Experiments on samples with different grain sizes reveal no dependence of DC conductivity on grain size for either mechanism, although the relaxation time and real relative permittivity of the grain boundary mechanism are dependent on grain size. Because of the electrical response observed at low frequencies, care must be taken in the inversion of electromagnetic field observations using laboratory measurements made in the kilohertz range since they may not be the same as DC measurements. Impedance measurements must be performed over a range of relatively low frequencies to assess the role of grain boundaries on the overall electrical response of polycrystalline materials.

Velocity–conductivity relationships for mantle mineral assemblages in Archean cratonic lithosphere based on a review of laboratory data and Hashin–Shtrikman extremal bounds

“…This observation remains controversial, however, as other workers have measured conductivities on single crystals and samples with a range of grain sizes and have not seen any systematic variation in conductivity with grain size (Roberts and Tyburczy, 1991;Xu et al, 2000b;Wang et al, 2006 Boyd, 1987). In addition, Li et al (2005) analysed grain interior and grain boundary conduction in pyroxenes, and found that grain-boundary conduction is important at high frequencies, above 100 Hz.…”

Effect of chemical composition on the electrical conductivity of gneiss at high temperatures and pressures

“…The values of imaginary parts almost remain unchanged within a frequency range of 10 6 -10 5 Hz, and the values gradually increased at 10 5 -10 3 Hz and decreased at 10 3 -10 1 Hz; these values then slowly increased in the 10 1 to 10 −1 Hz lower-frequency region. Roberts and Tyburczy (1991) and Saltas et al (2013) have suggested that the ideal semicircle represents the bulk electrical properties of a sample, and the additional tail is characteristic of diffusion processes at the sample-electrode interface. Hence, the bulk sample resistance can be obtained by fitting the ideal semicircle in the high-frequency domain.…”

Constraints on Mantle Electrical Conductivity from Field and Laboratory Measurements.

“…ROBERTS and TYBURCZY (1991) show that grain boundaries act in series with grain interior conduction and at the low frequencies of geomagnetic induction decrease conductivity by about a factor of 2 (one-third an order of magnitude), although the effect of pressure on the grain boundary conduction mechanism is completely unknown. However, the conductivity of a mantle lherzolite measured recently ,by DuBA and CONSTABLE (1993) varied between S02 and 0.15 orders of magnitude more conductive.…”