Electrical properties of fine‐grained olivine: Evidence for grain boundary transport

Abstract

[1] To investigate the mechanisms responsible for electrical conduction and deduce the relation between grain size and conductivity, the electrical conductivity of synthetic polycrystalline forsterite, with grain sizes between 1.1 ± 0.4 and 4.7 ± 2.4 mm, was measured at temperatures up to 1470°C and at 0.1 MPa pressure. The complex impedance plots display one clear arc, indicating a single dominant conduction mechanism. Bulk conductivity is inversely proportional to the grain size of the different samples. This relation suggests that the electrical conductivity of the samples is controlled by grain boundary diffusion of the charge carriers. The apparent activation energy (Q) for diffusion of the charge carriers between 1180°and 1470°C lies between 315 ± 39 and 323 ± 15 kJ/mol. This resembles previous data on grain boundary diffusion of Mg in forsterite. A geometrical model of less-conducting cubic grains and more-conducting grain boundaries agrees well with the experimental data. This model is applied to predict the conductivity contrast between fine-grained shear zones and less-deformed regions in the lithosphere. Upper mantle shear zones are predicted to have 1.5-2 orders of magnitude higher conductivity than less-deformed regions in the lithosphere. This means grain boundary transport probably has an important role in the existence of anomalously high conductivity zones in the upper mantle and that fine-grained shear zones might be detected using magnetotelluric methods.

Electrical conductivity of synthetic iron-bearing olivine

“…The magnitude of the effect (six-order increase in cell capacitance) and the depression of all arcs together suggest that the effect does not lie in the sample itself but at the sample/electrode interface. It was previously confirmed that the porosity does not appear to influence the impedance arcs (ten Grotenhuis et al 2004). Although minor porosity (as shown in Fig.…”

“…There does not seem to be a coherent differentiation between the electrical conductivity of Fo 100 and Fo 90 (this study) for high temperatures. That is, Fo 90 a with the same grain size as 1a (ten Grotenhuis et al 2004) almost overlap. At lower temperatures, Fo 100 shows 1-3 orders of magnitude lower conductivity than Fo 90 .…”

“…Also it appears that the difference between the Red Sea olivine (single crystal) (Schock et al 1989) and San Carlos olivine (single crystal) (Du Frane et al 2005) is almost one order of magnitude, thus highlighting the natural compositional variation and possibly more oxidized state of San Carlos material. The data from ten Grotenhuis et al (2004) show, in Fig. 4a, a decreasing conductivity with increasing grain size.…”

“…Unlike experiments on synthetic forsterite (Fo 100 ), (Hinze et al 1981;ten Grotenhuis et al 2004), such experiments must be carried out in a reducing environment to keep Fo 90 within its stability field. However, they may be carried out under atmospheric pressure because the electrical conductivity in olivine is not significantly affected by pressure (Xu et al 2000) and only up to 0.6 log units of conductivity from 1 to 4 GPa for a lherzolite (Dai et al 2008a).…”

Electrical impedance measurement of hydrous microcrystalline quartz

“…Systematic measurement errors with the combined apparatus are less than 2%. The conductivity cell, constructed of refractory alumina (Ten Grotenhuis et al, 2004), was equipped with 0.5 mm thick Pt electrodes and guard ring, of similar areal dimensions to the sputtered sample electrodes, to ensure good connection via shielded platinum wires. This was situated inside a 60 mm internal diameter tube furnace, with temperature controlled and monitored using Pt/Pt -10%Rh thermocouples.…”