HR: 11:15h
AN: T41F-04    [PDF]
TI: Electrical properties of polycrystalline olivine: evidence for grain boundary transport
AU: * Ten Grotenhuis, S M
EM: saskiatg@geo.uu.nl
AF: Utrecht University, Department of Earth Sciences P.O. Box 80021, Utrecht, 3508 TA Netherlands
AU: Drury, M R
EM: martynd@geo.uu.nl
AF: Utrecht University, Department of Earth Sciences P.O. Box 80021, Utrecht, 3508 TA Netherlands
AU: Peach, C J
EM: cpeach@geo.uu.nl
AF: Utrecht University, Department of Earth Sciences P.O. Box 80021, Utrecht, 3508 TA Netherlands
AU: Spiers, C J
EM: cspiers@geo.uu.nl
AF: Utrecht University, Department of Earth Sciences P.O. Box 80021, Utrecht, 3508 TA Netherlands
AB: The physical and chemical properties of grain boundaries are known to play an important role in determining the electrical properties of polycrystalline oxides. Grain boundaries can either enhance conductivity if the transport of charge carriers along the grain boundaries is faster than through the lattice, or grain boundaries can reduce conductivity if the grain boundaries block the transport of charge carriers. The purpose of the experiments presented here is to deduce the mechanisms responsible for electrical conductivity in fine-grained forsterite, the Mg-end member of olivine, in order to get a better understanding of the contribution of grain boundary transport, of the properties of the grain boundaries, and to determine any relation between grain size and conductivity. A relationship between grain size and conductivity at high temperature could potentially be used to interpret zones of anomalous conductivity in the upper mantle. The materials studied consist of fine-grained forsterite (Mg2SiO4) with a minor amount (5%) of enstatite (MgSiO3) added. The electrical conductivity of three melt-free synthetic polycrystalline samples, with grain sizes between 1.1 and 4.7 mm, was measured at temperatures up to 1470$\deg$C. 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 grain boundary diffusion of the charge carriers is controlling the electrical conductivity of the samples. The activation energy for diffusion of the charge carriers lies between 315 and 323 kJ/mol. This resembles previous data on grain boundary diffusion of Mg in forsterite and grain boundary diffusion creep. A geometrical model of less conducting cubic grains and more conducting grain boundaries agrees well with the experimental data. This model is applied to a natural mantle shear zone 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 to 2 orders of magnitude higher conductivity than less deformed regions in the lithosphere. This may mean that fine-grained shear zones can be detected using magnetotelluric methods.
DE: 3904 Defects
DE: 3914 Electrical properties
DE: 3947 Surfaces and interfaces
DE: 5112 Microstructure
DE: 8030 Microstructures
SC: Tectonophysics [T]
MN: 2003 Fall Meeting