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Grain size dependence of the melt geometry in partially molten olivine aggregates

The amount and geometry of the melt in the nominally melt-free areas in the Earth’ s mantle is still controversial. One of the important factors is to understand the melt distribution on the grain scale since it greatly impacts bulk-rock physical properties such as seismic properties and enhances electrical conductivity. It is particularly important to investigate whether the melt resides only in three-grain edge tubules or it exists along two-grain boundaries as well. To estimate the abundance of melt we use the grain boundary wetness, which can be calculated from the total length of solid-liquid and solid-solid boundaries (Takei, JGR, 1998). We have prepared synthetic polycrystalline olivine samples with mean grain sizes ranging from 8 micron to 30 micron with 4 wt% of basaltic melt. High resolution field emission SEM imaging of the coarse-grained sample shows frequent apparently wetted two-grain boundaries. Consistent with this observation the calculated wetness is 0.72, approximately 40% higher value than previously reported (Yoshino et al., JGR, 2005). This value corresponds to a smaller dihedral angle than the 35 deg. reported in the literature. In contrast to the values for the coarse-grained sample, the grain boundary wetness calculated for the fine-grained sample is 0.35 which fits well within the previously reported range. These preliminary results indicate that the grain boundary wetness increases with increasing mean grain size. However, this still leaves the question unresolved whether observed apparently wetted two-grain boundaries are only three grain junctions cut along the axis. To resolve this ambiguity we are performing consecutive serial sectioning (3 micron steps) and high resolution field emission SEM imaging in order to reconstruct the 3D melt geometry

melt; geometry; olivine; SEM; 3D

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