** Progress in Earth and Planetary Science is the official journal of the Japan Geoscience Union, published in collaboration with its 50 society members.

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    Solid earth sciences

    201503201503

    Sound velocity of hcp-Fe at high pressure: experimental constraints, extrapolations and comparison with seismic models

    Antonangeli D, Ohtani E

    Sound velocity measurements, High pressure, hcp-Fe, Extrapolation schemes, Earth’s inner core, Comparison with seismic models

    Figure 1: Various previous experimental data on sound velocity and density of hcp-Fe are corrected based on the same pressure scale and the equation of state of hcp-Fe. Most of previous data are consistent with each other.

    Figure 2: Compressional sound velocity as a function of density under static compression at 300 K is compared with the shock wave data along Hugoniot. The temperature effect on the Birch's law is clearly observed in this figure.

    Determining the sound velocity of iron under extreme thermodynamic conditions is essential for a proper interpretation of seismic observations of the Earth’s core but is experimentally challenging. Here, we review techniques and methodologies used to measure sound velocities in metals at megabar pressures, with specific focus on the compressional sound velocity of hexagonal close-packed iron. A critical comparison of literature results, coherently analyzed using consistent metrology (pressure scale, equation of state), allows us to propose reference relations for the pressure and density dependence of the compressional velocity of hexagonal close-packed iron at ambient temperature. This provides a key base line upon which to add complexity, including high-temperature effects, pre-melting effects, effects of nickel and/or light element incorporation, necessary for an accurate comparison with seismic models, and ultimately to constrain Earth’s inner core composition.