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

Abstract

We perform first-principles calculations to investigate liquid iron-sulfur alloys (Fe, Fe 56 S 8 , Fe 52 S 12 , and Fe 48 S 16 ) under high-pressure and high-temperature (150-300 GPa and 4000-6000 K) conditions corresponding to the Earth's outer core. Considering only the density profile, the best match with the preliminary reference Earth model is by liquid Fe-14 wt % S (Fe 50 S 14 ), assuming sulfur is the only light element. However, its bulk sound velocity is too high, in particular in the deep outer core, suggesting that another light component such as oxygen is required. An experimental check using inelastic X-ray scattering shows good agreement with the calculations. In addition, a present study demonstrates that the Birch's law does not hold for liquid iron-sulfur alloy, consistent with a previous report on pure liquid iron.

Sound velocity of liquid Fe‐Ni‐S at high pressure

“…A temperature effect on the V P of liquid Fe‐C was not found in the recent laser‐heated DAC study performed by Nakajima et al []. Theoretical calculations at core pressures have also suggested that the sound velocities of liquid Fe and Fe‐S alloys are not sensitive to temperature [ Vočadlo et al , ; Ichikawa et al , ; Umemoto et al , ].…”

“…The shaded areas around curves show the uncertainties of the extrapolations (see text for detail). The open and closed squares indicate the sound velocities of liquid Fe 75 S 25 [ Umemoto et al , ] and Fe 75 S 25 [ Badro et al , ].…”

“…Ultrasonic wave measurements in a large‐volume press also reported the sound velocity of liquid Fe‐S alloys, although the pressure range examined was limited to below 8 GPa [ Jing et al , ; Nishida et al , ]. Calculations [ Badro et al , ; Umemoto et al , ] suggest that liquid Fe‐S is not consistent with both the density and velocity of the Earth's outer core.…”

The influence of sulfur on the electrical resistivity of hcp iron: Implications for the core conductivity of Mars and Earth

“…Density measurements on liquid Fe-Ni-S alloys demonstrated that the Earth's outer core composition with 19.2 at. However, its bulk sound velocity is much higher than the seismic observation in particular in deep outer core, suggesting other light elements on top of S [Umemoto et al, 2014]. Recent first-principle calculation supports this estimate of S content in the outer core.…”

Nitrogen Content in the Earth's Outer Core

“…We have also estimated the amount of other light elements including hydrogen, carbon, oxygen, silicon, and sulfur required to explain the ρ and V P observations of the Earth's outer core (Badro et al, 2014;Umemoto et al, 2014;Umemoto & Hirose, 2015; Figure 3). We determine the light element content in the outer core with the formalism: Δρ ð Þ FeÀCMB ¼ dρ dX ÂX ρ (mol %) where X ρ is the mol % of light element required to match the observed density and ΔV P ð Þ FeÀCMB ¼ dVP dX À Á ÂX VP (mol %), where X VP is the mol % of light element required to match the observed P wave velocity (V P ).…”