Evidence for Fe-Si-O liquid immiscibility at deep Earth pressures

Abstract

SignificanceEarth's outer core is composed of a liquid iron alloy with up to 10% of unknown light elements, likely silicon, oxygen, sulfur, carbon, or hydrogen. The release of these light elements upon freezing of the solid iron inner core plays an important role in sustaining Earth's magnetic field, but the exact chemical makeup of the core is widely debated. In this paper, we perform high-pressure, high-temperature melting experiments and first-principles simulations on iron alloys containing silicon and oxygen and find that two distinct liquids form at high pressures. The presence of immiscible liquids may explain a seismically observed stratified layer atop the outer core and suggests that an Fe-Si-O composition can explain multiple observations of the outer core.

Ab Initio Molecular Dynamics Investigation of Molten Fe–Si–O in Earth's Core

“…This is in contrast with a recent experimental and first-principle calculation study (Arveson et al, 2019) where SiO 2 exsolution was not observed but where the presence of Fe-Si and Fe-Si-O immiscible liquids was proposed. This is in contrast with a recent experimental and first-principle calculation study (Arveson et al, 2019) where SiO 2 exsolution was not observed but where the presence of Fe-Si and Fe-Si-O immiscible liquids was proposed.…”

“…Partial RDFs and coordination numbers agree well with previously reported liquid structures and show no sign of SiO 2 clustering. Shorter (10 ps) simulations give the impression of immiscibility, and those durations are comparable to the ones (6 to 14 ps) used in Arveson et al (2019). Ternaries formed by ideal volume mixing of Fe-Si and Fe-O binaries have identical density and bulk sound velocity as those obtained in the calculated ternary, implying that equations of state of binary liquids (Fe-Si and Fe-O) can be used to obtain reliable equations of state of Fe-Si-O ternary alloys at core conditions.…”

“…Here no immiscibility between Fe-Si and Fe-Si-O liquids was observed in the liquid state as described above, but instead liquid Fe-Si-O is well mixed at temperatures down to 3800 K and 136 GPa. These are significantly longer than the simulations in Arveson et al (2019), whose trajectories showing immiscibility were run for 6.6 to 14.8 ps. Si and O were initially clustered (as described previously), and the trajectories are plotted after 10, 20, and 29 ps in Figure 4, showing full mixing in the longest simulation runs.…”

“…Si and O were initially clustered (as described previously), and the trajectories are plotted after 10, 20, and 29 ps in Figure 4, showing full mixing in the longest simulation runs. These are significantly longer than the simulations in Arveson et al (2019), whose trajectories showing immiscibility were run for 6.6 to 14.8 ps. Such short simulation times do not allow to fully mix the system, as confirmed by our trajectories after 10 and 20 ps.…”

Mass Transport and Structural Properties of Binary Liquid Iron Alloys at High Pressure

“…No evidence of atomic clustering (i.e., exsolution) is observed visually or in the RDF in any of the cells, suggesting that the 4 at% of alloying element is below their solubility limit in Fe at the V and T studied. In recent work, Arveson et al () have shown, however, that computations tend to require significantly larger concentrations of alloying elements than experiments to show exsolution.…”