Limitations on silicon in the outer core: Ultrasonic measurements at high temperatures and high dK/dP values of Fe‐Ni‐Si liquids at high pressures

Abstract

The sound velocities of four iron‐nickel‐silicon liquids (Fe‐5 wt %Ni‐6 wt %Si, Fe‐5 wt %Ni‐10 wt %Si, Fe‐5 wt %Ni‐14 wt %Si, and Fe‐5 wt %Ni‐20 wt %Si) are measured between 1460 and 1925 K at ambient pressures using ultrasonic interferometry. The results constrain both the dependence on Si content of the bulk modulus of these liquids and the temperature dependence of their elasticity. These elastic data are utilized to assess both relatively low pressure (to 12 GPa) compressional data on Fe‐Si liquids and to extrapolate to higher‐pressure and higher‐temperature conditions. If a single equation of state for Fe‐Ni‐Si liquids of a given composition applies from low pressure to near core conditions, then our results imply that the isothermal pressure derivative of the bulk modulus of these liquids is high: likely 8 and above at high temperatures. This high value of the pressure derivative of the bulk modulus at low pressures in Fe‐Si liquids causes marked stiffening at higher pressures, leading to notable incompressibility and apparent low values of the pressure derivative of the bulk modulus at core conditions. These results reinforce the conclusion that silicon is not a major alloying component of Earth's core.

Pressure and Composition Effects on Sound Velocity and Density of Core‐Forming Liquids: Implication to Core Compositions of Terrestrial Planets

“…On the other hand, the V P of liquid Fe‐Ni‐Si decreases slightly with increasing temperature with d V P / d T of −0.42 to −0.57 ms −1 ·K −1 (see Table ). The d V P / d T found in this study is in agreement with that measured at ambient pressure (−0.36 to −0.52 ms −1 ·K −1 ; Williams et al, ).…”

“…Different symbol shapes represent different temperatures as shown in the legend. Data at ambient pressure are taken from Williams et al (2015).…”

“…In previous studies on planetary core modeling, ideal mixing behavior has been assumed to obtain the core thermoelastic properties as a function of LE content. However, the nonideality of the Fe‐S and the Fe‐Ni‐Si systems are suggested from phase relations and density measured at ambient and moderate pressures (Buono & Walker, ; Kawai et al, ; Nagamori, ; Nishida et al, ; Williams et al, ), and ab initio calculations at the Earth's core conditions (Alfè et al, ). Thus, we examine the effect of different mixing models (ideal and nonideal mixing) on density and V P for Fe‐Ni‐S and Fe‐Ni‐Si liquids in order to assess which model can best summarize the present data.…”

“…Journal of Geophysical Research: Planets Nagamori, 1969;Nishida et al, 2008;Williams et al, 2015), and ab initio calculations at the Earth's core conditions (Alfè et al, 2003). Thus, we examine the effect of different mixing models (ideal and nonideal mixing) on density and V P for Fe-Ni-S and Fe-Ni-Si liquids in order to assess which model can best summarize the present data.…”

Thermoelastic properties of liquid Fe‐C revealed by sound velocity and density measurements at high pressure

“…The relationship between ρ and V P is characterized by the adiabatic bulk modulus (K S ), i.e., V P = (K S /ρ) 1/2 . The effect of light elements on the bulk modulus has been reported by Terasaki et al [2010], Jing et al [2014], and Williams et al [2015]. Carbon and sulfur significantly decrease the bulk modulus of iron, while silicon has only a minor effect on the bulk modulus.…”