Ultrasonic wave from high pressure and temperature experiments

The Earth’s inner core shows distinct features, including low shear velocities and a high Poisson’s ratio, often linked to hcp-Fe (hexagonal close-packed iron) through high-pressure experiments and calculations. Premelting is proposed as a key mechanism behind the observed shear softening in hcp-Fe. This study uses magnesium, an hcp-structured analogue, to explore the effects of premelting on ultrasonic velocities. Premelting begins at ~0.97 times the melting temperature(Tm), leading to sharp nonlinear drops in shear (VS) and compressional (VP) velocities by 14.5% and 4%, respectively, with a ~13.3% increase in Poisson’s ratio. These findings, together with seismic and mineralogical data, suggest that premelting could account for the reduced VP and VS and increased Poisson’s ratio observed in the Earth’s inner core.