High‐resolution imaging of the melt distribution in partially molten upper mantle rocks: evidence for wetted two‐grain boundaries

We determine the 3‐D melt geometry of partially molten samples of olivine containing 1.6 and 3.6 vol.% of basaltic melt that were held in a piston cylinder apparatus at upper mantle conditions for 430 h. Our approach involves serial sectioning and high‐resolution field emission SEM imaging. Resolution is such that melt pockets approaching ~30 nm in size were resolved while covering an area of ~300 by 230  µ m. The principal result of this study is to show that thin layers (typically 100 nm or less in thickness) between adjacent grains observed in 2‐D images persist with depth and are therefore wetted two‐grain boundaries. Melt geometries most closely resembling triple junction tubules of the isotropic equilibrium model occur at all three‐grain edges but are small compared to larger pockets. The wetted grain boundaries at a dihedral angle >0° for this system are inferred to be due to slow expulsion of melt from dynamically reorganizing grain boundaries during steady state grain growth. The attenuation peak observed in forced torsional oscillation experiments on similar samples is likely related to the wetted grain boundaries. Grain growth, driven by surface energy reduction, occurs also at the larger grain sizes expected for the mantle. This suggests the presence of wetted grain boundaries and significant velocity reduction and attenuation in partially molten upper mantle, as observed for example in back‐arc basins.