Sound velocities and melting of Fe-Ni-Si system at high pressures under shock loading

Abstract:

The Earth’s liquid outer core is dominantly composed of iron and nickel (~5−10%), with a density lower by ~8% than that of the liquid iron at the core conditions [e.g., 1], requiring the presence of light element(s) [e.g., 2]. Silicon, geochemically abundant, has long been considered as a major potential light element in the Earth’s outer core because of its high solubility in iron, iron-silicate interactions at core-mantle boundary, the Si isotope data, and core formation modeling [3]. To examine effects of Si on physical properties of Fe-Ni system, we directly measured densities, sound velocities, and melting of Fe-9Ni-10Si (in weight percent) system up to ~280 GPa by shock experiments using a two-stage light-gas gun. The sound velocities were determined by the optical analyzer technique [4]. The results, compared with the seismic observations, show that silicon-rich liquid Fe-Ni system can satisfy the observed density deficit and seismological data simultaneously at the physical conditions of the outer core. Analyses of the melting temperatures of Fe-9Ni-10Si system imply the Fe-Ni-Si core will be at lower temperatures by ~600−1000 K than the pure iron core at ~330 GPa of the inner-core boundary.

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