Ab Initio Thermoelasticity of Liquid Iron-Nickel-Light Element Alloys

Abstract

The earth’s core is thought to be composed of Fe-Ni alloy including substantially large amounts of light elements. Although oxygen, silicon, carbon, nitrogen, sulfur, and hydrogen have been proposed as candidates for the light elements, little is known about the amount and the species so far, primarily because of the difficulties in measurements of liquid properties under the outer core pressure and temperature condition. Here, we carry out massive ab initio computations of liquid Fe-Ni-light element alloys with various compositions under the whole outer core P, T condition in order to quantitatively evaluate their thermoelasticity. Calculated results indicate that Si and S have larger effects on the density of liquid iron than O and H, but the seismological reference values of the outer core can be reproduced simultaneously by any light elements except for C. In order to place further constraints on the outer core chemistry, other information, in particular melting phase relations of iron light elements alloys at the inner core-outer core boundary, are necessary. The optimized best-fit compositions demonstrate that the major element composition of the bulk earth is expected to be CI chondritic for the Si-rich core with the pyrolytic mantle or for the Si-poor core and the (Mg,Fe)SiO3-dominant mantle. But the H-rich core likely causes a distinct Fe depletion for the bulk Earth composition.

Thermodynamics of the system Fe–Si–O under high pressure and temperature and its implications for Earth’s core

“…The effect of oxygen on Ks is more pronounced than on ρ, resulting in a reduction of Vp (this study; Komabayashi 2014) a positive effect. Another recent first-principle calculations by Ichikawa and Tsuchiya (2020) predicted similar trends to those in Badro et al (2014) (Fig. 10).…”

“…Similarly the effects of silicon on the bulk modulus are in contrast between the experiment-based (this study) and theory-based models (Badro et al 2014;Ichikawa and Tsuchiya 2020) although all the papers show positive effects on the Vp (Fig. 10a).…”

“…The result depends on the magnitude of reduction of the bulk modulus of iron liquid due to oxygen. All Badro et al (2014), Komabayashi (2014), and Ichikawa and Tsuchiya (2020) show that the addition of oxygen reduces both the bulk modulus and density (Fig. 10b).…”

“…10 Effects of a silicon and b oxygen on the P-wave velocity (Vp), adiabatic bulk modulus (Ks) and density (ρ) of liquid iron at 330 GPa. Data from the first principles studies are compared (Badro et al 2014;Ichikawa and Tsuchiya 2020). Note that the parameters are normalized to those for pure iron.…”

Structures of Liquid Iron–Light‐Element Mixtures under High Pressure

“…[ 25 ] In addition, Ichikawa and Tsuchiya evaluated the thermoelasticity of liquid FeNiLE alloys using ab initio MD simulations. [ 26 ] It was found that Si and S have a greater effect on the density of liquid iron than O and H.…”