Compressional behavior of hydrous orthoenstatite: Insight into the nature of LVZ under continental plate

A suite of hydrous orthoenstatite crystals were synthesized at 5–7 GPa and 1100–1300 °C, corresponding to the mid upper mantle conditions in continental regions. The synthetic crystals presented a clear, inclusion-free, and euhedral form with a size range from 100 to a few hundred microns. The Al- and water content of crystals were less than 2 wt.% and ranging from ~500 ppm to 1000 ppm, respectively, characterized by Raman and IR spectroscopy, electron microscopy, and SIMS. The analysis shows that the capability of water incorporation for this suite of hydrous orthoenstatite is correlated to the Al-content in the crystal structure. To understand how the detailed crystal structure reflects the influence of water and Al, single crystal X ray diffraction analysis was performed for this suite of hydrous orthoenstatite. By comparing the results obtained at ambient conditions, we find that for low-level of Al-content, <2 wt.%, the Al³⁺ cation tends to occupy one of tetrahedral sites (TB) only. Analysis of the X ray diffraction results under high pressure indicates that the elasticity of orthoenstatites is insensitive to the presence of low-level water and Al. We use this finding to evaluate the velocity profile at the mid upper mantle of continental regions to compare with seismic observation. The comparison indicates that the cause of the low velocity zone in continental regions originates from the geotherm profile rather than the effect of water on the elasticity of mantle phases.