Equation of State of Liquid Iron under Extreme Conditions

Abstract

The density of liquid iron has been determined up to 116 GPa and 4350 K via static compression experiments following an innovative analysis of diffuse scattering from liquid. The longitudinal sound velocity was also obtained to 45 GPa and 2700 K based on inelastic x-ray scattering measurements. Combining these results with previous shock-wave data, we determine a thermal equation of state for liquid iron. It indicates that the Earth's outer core exhibits 7.5-7.6% density deficit, 3.7-4.4% velocity excess, and an almost identical adiabatic bulk modulus, with respect to liquid iron. Main text

Phase Relations of Earth’s Core-Forming Materials

“…The longitudinal wave velocity (Vp) and density of iron liquid calculated from the thermodynamic model [9] are compared with those from shock wave experiments [47], DAC experiments [48], and first-principles calculations [49,50], together with seismological models (preliminary reference Earth model (PREM), Dziewonski and Anderson [51]) over the outer core depths between the core-mantle boundary (CMB) and ICB (Figure 3). The different types of measurements, except those by [50], show consistent Vp and density of iron liquid.…”

“…The densities of iron liquid and solid, and the melting point of iron at the ICB can only be simultaneously constrained by internally consistent thermodynamic models (e.g., [9]). [47]); thermodynamic model (Komabayashi [9]); first-principles calculation (Ichikawa et al [50]; Wagle and Steinle-Neumann [49]); static measurements in DAC (Kuwayama et al [48]). Data of the preliminary reference Earth model (PREM, Dziewonski and Anderson [51]) are also shown.…”

“…Their calculations demonstrated that Fe and Fe-10Ni bcc phases showed similar phonon spectra which implied that the effect of Ni is minor, and therefore they expected that high temperature should stabilize the bcc phase due to the entropic effect, same as for pure Fe [56]. [47]); thermodynamic model (Komabayashi [9]); first-principles calculation (Ichikawa et al [50]; Wagle and Steinle-Neumann [49]); static measurements in DAC (Kuwayama et al [48]). Data of the preliminary reference Earth model (PREM, Dziewonski and Anderson [51]) are also shown.…”

“…Figure 3. Density and longitudinal wave velocity (Vp) profiles for pure Fe over the outer core pressure range along each isentrope: shock wave study (Anderson and Ahrens[47]); thermodynamic model (Komabayashi[9]); first-principles calculation (Ichikawa et al[50]; Wagle and Steinle-Neumann[49]); static measurements in DAC (Kuwayama et al[48]). Data of the preliminary reference Earth model (PREM, Dziewonski and Anderson[51]) are also shown.…”

Shock Melting Curve of Iron: A Consensus on the Temperature at the Earth's Inner Core Boundary

“…Earth's core is composed of an iron alloy with a certain amount of light elements, such as Si, S, O, C, and H (Li & Fei, 2014). The melting temperatures of iron and its alloys at high pressures are critical geophysical parameters for understanding the thermal structure, inner core solidification, and energetics of the core (Kuwayama et al, 2020; Nimmo, 2015; Zhang et al, 2019). The liquid outer core and solid inner core are equilibrated at the inner core boundary (ICB, ~330 GPa), where the temperature corresponds to the melting point (Buffett, 2003).…”