Saori Imada

Liquid iron‐sulfur alloys at outer core conditions by first‐principles calculations

We examined the density, bulk sound (compressional) velocity, and Gruneisen parameter of liquid pure Fe, Fe100H28 (0.50 wt % H), Fe88H40 (0.81 wt % H), and Fe76H52 (1.22 wt % H) at Earths outer core pressure and temperature (P-T) conditions (~100 to 350 GPa, 4000 to 7000 K) based on first-principles molecular dynamics calculations. The results demonstrate that the thermodynamic Gruneisen parameter of liquid iron alloy decreases with increasing pressure, temperature, and hydrogen concentration, indicating a relatively small temperature gradient in the outer core when hydrogen is present. Along such temperature profile, both the density and compressional velocity of liquid iron containing ~1 wt % hydrogen match seismological observations. It suggests that hydrogen could be a primary light element in the core, although the shear velocity of the inner core is not reconciled with solid Fe-H alloy and thus requires another impurity element.

Carbon-depleted outer core revealed by sound velocity measurements of liquid iron–carbon alloy

The relative abundance of light elements in the Earths core has long been controversial. Recently, the presence of carbon in the core has been emphasized, because the density and sound velocities of the inner core may be consistent with solid Fe7C3. Here we report the longitudinal wave velocity of liquid Fe84C16 up to 70 GPa based on inelastic X-ray scattering measurements. We find the velocity to be substantially slower than that of solid iron and Fe3C and to be faster than that of liquid iron. The thermodynamic equation of state for liquid Fe84C16 is also obtained from the velocity data combined with previous density measurements at 1 bar. The longitudinal velocity of the outer core, about 4% faster than that of liquid iron, is consistent with the presence of 4–5 at.% carbon. However, that amount of carbon is too small to account for the outer core density deficit, suggesting that carbon cannot be a predominant light element in the core.

Measurements of lattice thermal conductivity of MgO to core‐mantle boundary pressures

The pressure response of lattice thermal conduction in MgO periclase has been a matter of interest for many decades to estimate the thermal conductivity profile of the lower mantle. Using the pulsed light heating thermoreflectance technique, we measured the lattice thermal diffusivity of MgO at pressures up to 137 GPa at 300 K to determine its lattice thermal conductivity under deep lower mantle conditions. Considering the temperature effect estimated by previous high-temperature measurements, we calculated the lattice part of the thermal conductivity of MgO to be 17.9 ± 1.1 W/m/K at 135 GPa and 3600 K. Additionally, we observed that the lattice conductivity of MgO has little dependence on its grain size under core-mantle boundary conditions.

Temperature dependence of the velocity-density relation for liquid metals under high pressure: Implications for the Earth’s outer core

Abstract The relationship between the sound velocity, density, and temperature of liquid metals is important when one tries to interpret the seismic velocity profile and infer the chemical compositions of the Earth’s outer core. We, therefore, have experimentally measured the longitudinal acoustic (LA) velocity of liquid indium under high P-T conditions. Also, we examined a Hugoniot data of liquid iron by comparing with an existing equation of state (EoS). The LA velocities of liquid and solid indium at pressures up to 6.7 GPa and temperatures mostly at 710 K were measured using inelastic X‑ray scattering (IXS) to probe samples in an externally heated diamond-anvil cell. A thermal EoS for liquid indium derived from existing literature was used to calculate the density for the IXS measurements and to provide an independent check on the sound velocities. The IXS data are consistent with the hydrodynamic LA velocity derived from the liquid EoS, implying that the positive dispersion is minimal in liquid indium. The velocity-density relation for liquid indium derived from the EoS has temperature dependence, implying that Birch’s law does not hold for the liquid phase. Similarly we calculated the temperaturevelocity- density relation of liquid iron over the Earth’s core range from a recently reported EoS. The resulting velocity-density relation is also temperature dependent, indicating that liquid iron thus does not follow Birch’s law. The violation of Birch’s law implies that the Hugoniot data cannot be directly compared with seismological observations because of the different temperature ranges. Formulation of the temperature-velocity-density of liquid iron-alloys supported by experimental measurements provides better understanding of the thermodynamic state of the Earth’s core.