Grain boundary wetness of partially molten dunite

The grain scale melt distribution plays a key role for physical properties of partially molten regions in Earth's upper mantle, but our current understanding of the distribution of basaltic melt at the grain scale is still incomplete. A recent experimental study shows that wetted two-grain boundaries are a common feature of partially molten dunite at small melt fractions (Garapic et al., G3, 2013). In early ideal models which assume isotropic surface energy, the grain scale melt distribution is uniquely determined by knowing the melt fraction and the dihedral angle between two crystalline grains and the melt (von Bargen and Waff, JGR, 1986). Olivine is anisotropic in surface energy, hence the grain scale melt distribution at given melt fraction cannot be characterized by the dihedral angle alone. The grain boundary wetness, which is defined as the ratio of solid-liquid boundary area over the total interfacial area (Takei, JGR, 1998), is a more objective measure of the grain scale melt distribution. The aim of this study is to quantify the relationship between grain size, melt fraction, temperature and grain boundary wetness of partially molten dunite under dry conditions. We annealed olivine-basalt aggregates with melt fractions from 0.03% to 6% at a range of temperatures and 1 GPa in a piston cylinder for 1 to 336 hours, with resulting mean grain sizes of 10 to 60 μm. The samples were sectioned, polished and imaged at high resolution by using a field emission SEM. Each image had a size of 2048 x 1536 pixels with a resolution of 0.014 to 0.029 μm/pixel, depending on magnification. For each sample, depending on grain sizes, we made mosaics of 3 x 3 or 6 x 6 overlapping images. Measurements of melt fraction, grain boundary wetness and grain size were carried out on these high resolution mosaics by using ImageJ software. Analyses of mosaics show that grain boundary wetness increases with increasing melt fraction at constant grain size to values well above those predicted based on dihedral angles. This is consistent with the occurrence of wetted two-grain boundaries, and observations for hydrous systems (Yoshino et al., EPSL, 2007). A key new result is that at constant melt fraction, grain boundary wetness increases with increasing grain size. This emphasizes that the dihedral angle is not adequate to characterize the melt distribution. Extrapolation to upper mantle grain sizes suggests that grain boundary wetness may be higher at lower melt fractions relative to the experimental grain sizes.