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The Elasticity of Iron Alloys at Extreme Conditions

Abstract : For nearly 40 years, there has been a robust seismological model (PREM - Preliminary Reference Earth Model) which reports the elastic properties (compressional velocity - Vp, shear velocity - Vs, and density - ρ) of the Earth’s core, and yet there is still no clear consensus on the chemical composition which can match such properties at relevant pressures and temperatures. While iron has elastic properties which are close to those of the solid inner core, iron alone is too heavy to match the density of the inner core. In light of this, and based on a series of cosmochemical and geochemical arguments, Si has been proposed for many years in varying quantities as a material which could explain the difference in material properties between Fe and PREM. Using a combination of Synchrotron X-ray Diffraction and Picosecond Acoustics, ρ and Vp of Fe-Si and Fe-Ni-Si alloys have been measured to pressures and temperatures exceeding 1 Mbar and 2000 K. Obtained data allows an accurate determination of Velocity-Density relations, both P-V and P-V-T equations of state, and axial ratios of a variety of geophysically important Fe-Si and Fe-Ni-Si alloys. These results are used to discuss the elastic properties of Fe-Si alloys at high pressure - high temperature conditions to place tight constraints on Si abundance in the Earth’s inner core.
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Eric Edmund. The Elasticity of Iron Alloys at Extreme Conditions. Geophysics [physics.geo-ph]. Sorbonne Université, 2018. English. ⟨NNT : 2018SORUS531⟩. ⟨tel-02926161⟩

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