[1] Properties of partially molten rocks depend strongly on the grain-scale melt distribution. Experimental samples show a variety of microstructures, such as melt lenses, layers, and multigrain melt pools, which are not readily explained using the theory for melt distribution based on isotropic interface energies. These microstructures affect the melt distribution and the porosity-permeability relation. It is still unclear how the melt distribution changes with increasing melt fraction. In this study, electrical conductivity measurements and microstructural investigation with scanning electron microscopy and electron backscatter diffraction are combined to analyze the melt distribution in synthetic, partially molten, iron-free olivine rocks with 0.01-0.1 melt fraction. The electrical conductivity data are compared with the predictions of geometric models for melt distribution. Both the conductivity data and the microstructural data indicate that there is a gradual change in the melt distribution with melt fraction (X m ) between 0.01 and 0.1. At a melt fraction of 0.01, the melt is situated in a network of triple junction tubes, and almost all grain boundaries are free from melt layers. At 0.1, the melt is situated in a network of grain boundary melt layers, as well as occupying the triple junctions. Between melt fractions 0.01 and 0.1, the number of grain boundary melt layers increases gradually. The electrical conductivity of the partially molten samples is best described by Archie's law (s sample /s melt = CX m n ) with parameters C = 1.47 and n = 1.30.Citation: ten Grotenhuis, S. M., M. R. Drury, C.

[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.

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