Sound Velocity and Density of Hcp-Fe Under Earth’s Core Conditions

Abstract:

Earth’s inner core consists of mainly hexagonal close-packed iron (hcp-Fe). Therefore, the physical properties of hcp-Fe can provide significant knowledge about the core. Despite seismological observations providing density-sound velocity data of Earth’s core, there are few experimental reports about the sound velocity of hcp-Fe at ultrahigh pressure and temperature. The sound velocity of pure hcp-Fe at high temperature provides an important constraint on the composition of the Earth's core: any model of the impurity composition of the core must satisfy both the density deficit from seismology and give an appropriate change in velocity. Here, we report the compressional sound velocity ( V_P ) of hcp-Fe up to 163 GPa and 3000 K based on a combination of the laser-heated diamond anvil cell and inelastic X-ray scattering measurements. The pressure and temperature dependence suggest both the density and V_P of hcp-Fe are higher than PREM. In other words, Earth’s inner core shows a velocity deficit in addition to the density deficit. Thus, we conclude that light elements or a combination of light elements and nickel in the inner core decreases not only density but also V_P of hcp-Fe simultaneously under the inner core conditions. We will discuss the Earth’s core composition based on our results.