Sound velocity and compressibility of melts along the hedenbergite (CaFeSi2O6)-diopside (CaMgSi2O6) join at high pressure: Implications for stability and seismic signature of Fe-rich melts in the mantle

Abstract

Iron-rich silicate melts play an important role in the magmatic history of the Earth and the Moon. However, their elastic properties at high pressures, especially the sound velocities, are poorly understood. Here we determined the ultrasonic sound velocities of two silicate melts along the hedenbergite (Hd, CaFeSi2O6) – diopside (Di, CaMgSi2O6) join at high pressure and temperature conditions up to 6 GPa and 2329 K, using the high-pressure ultrasonic technique combined with synchrotron radiation in a multi-anvil apparatus. Our results show that Fe can significantly reduce the sound velocity while increasing the density of silicate melts. Comparing the melts of Di, Hd, and a mixture of 50 mol% hedenbergite + 50 mol% diopside (Hd50Di50), we find that although densities of the Hd-Di melts can be well-described by a linear mixing law empirically at high pressures, sound velocities do not vary with composition linearly. Assuming that the low-velocity regions in Earth's upper mantle are mainly due to the presence of partial melt, we applied our results to study the gravitational stability and seismic signature of Fe-rich silicate melts in the mantle, combined with melt geometry and compaction models. For the low-velocity zone (LVZ) in mantle asthenosphere, although the degree of seismic velocity reduction can be explained by the presence of a small amount of partial melt distributed in film/band geometry along grain boundaries, silicate melts formed in this depth range are too light to be gravitationally stable, but may be completely entrained in the convecting mantle due to the small melt fraction and low melt-matrix separation velocity. For the low-velocity layer (LVL) above the mantle transition zone, the presence of Fe-rich melts (with FeO>∼10 wt%) distributed in textural equilibrium with the ambient mantle is a plausible explanation.