Acoustic properties of partially molten media studied on a simple binary system with a controllable dihedral angle

Borneol‐diphenylamine, a binary eutectic system of the organic compounds, provides an appropriate analogue of melting in the Earth's mantle. Eutectic temperature is just above room temperature (43°C), and at this temperature the dihedral angle is about 40°. As the temperature increases, the dihedral angle gradually decreases at a rate of about 1.5° per 1°C, and becomes nearly zero at 70°C. Melt fraction change is small at this temperature range; this system is therefore appropriate in investigating a systematic effect of dihedral angle. Using this system, reduction of the shear and longitudinal wave velocities caused by grain boundary melt, having nearly equilibrium textures, was measured accurately as functions of both melt fraction and dihedral angle. The results demonstrate the significant effect of equilibrium melt geometry on shear wave velocities, while also showing that the effects of melting and dihedral angles are much smaller on the longitudinal waves. The quantitative effects of the melt fraction and dihedral angles on the acoustic wave velocities can be predicted theoretically using the elasticities of granular media derived as functions of grain‐boundary contiguity [ Takei , 1998]. The present experimental results described in this paper agree well with the theoretical predictions and demonstrate the validity of the theory. Clarifying the analogy and difference between the present organic system and the Earth's materials, the shear and longitudinal wave velocities of the partially molten rocks in the Earth were predicted as functions of melt fraction, dihedral angle, and the compressibility ratio between solid and melt.