The extreme acoustic anisotropy and fast sound velocities of cubic high-pressure ice polymorphs at Mbar pressure

Abstract

We presented the experimentally determined single-crystal elasticity model of ice up to 103(3) GPa, based on the sound velocity measurements of high-P ice polymorphs within multiple diamond anvil cells using Brillouin spectroscopy. We have not observed any discontinuities of the P-wave (Vp) or S-wave (Vs) velocities over the entire P range. The elastic moduli of high-P ice show a close to linear P dependence. In comparison with the high-P silicate minerals in terrestrial planetary bodies, the Vp and Vs values of ice exceed those of both bridgmanite and ferropericlase at P > 80–90 GPa, counter-intuitively indicating that the high-P ice, if existing in the deep terrestrial planets' interior, is not a slow phase. Instead, the high-P ice shows extremely strong elastic anisotropy, reaching 27% and 74% at 100 GPa for Vp and Vs, respectively. The presence of high-P ice in terrestrial planets' interior, even on a small scale may lead to the observable anisotropic signatures, such as the 25% Vs anisotropy in the deep earthquake-generating zone in subducting slabs. We anticipate our measurements to serve as an important base for explaining and modeling the geophysical observations for various types of planetary bodies.