Evidence for an oxygen-depleted liquid outer core of the Earth

Abstract

On the basis of geophysical observations, cosmochemical constraints, and high-pressure experimental data, the Earth's liquid outer core consists of mainly liquid iron alloyed with about ten per cent (by weight) of light elements. Although the concentrations of the light elements are small, they nevertheless affect the Earth's core: its rate of cooling, the growth of the inner core, the dynamics of core convection, and the evolution of the geodynamo. Several light elements-including sulphur, oxygen, silicon, carbon and hydrogen-have been suggested, but the precise identity of the light elements in the Earth's core is still unclear. Oxygen has been proposed as a major light element in the core on the basis of cosmochemical arguments and chemical reactions during accretion. Its presence in the core has direct implications for Earth accretion conditions of oxidation state, pressure and temperature. Here we report new shockwave data in the Fe-S-O system that are directly applicable to the outer core. The data include both density and sound velocity measurements, which we compare with the observed density and velocity profiles of the liquid outer core. The results show that we can rule out oxygen as a major light element in the liquid outer core because adding oxygen into liquid iron would not reproduce simultaneously the observed density and sound velocity profiles of the outer core. An oxygen-depleted core would imply a more reduced environment during early Earth accretion.

Sound velocities of Fe and Fe-Si alloy in the Earth’s core

“…Comparison between V P − ρ of hcp-Fe and Earth's core indicates significant V P − ρ differences that call for the addition of approximately 8-10 wt % light elements in the outer core and 4 wt % in the inner core (e.g., [12][13][14][15]23) (Fig. 3B) (Fig.…”

“…Deciphering these observations requires solid knowledge about the composition of the Earth's inner core and, therefore, the elasticity of candidate Fe alloys (7)(8)(9)(10)(11)(12)(13)(14)(15)(16). Since F. Birch pointed out in the 1950s that Earth's core is too dense if composed of Fe or Fe-Ni alloy alone (13), a number of candidate major light elements, including oxygen (O), silicon (Si), sulfur (S), carbon (C), and hydrogen (H), have been suggested via cosmochemical, geochemical, and geophysical evidence (17).…”

A seismologically consistent compositional model of Earth’s core

“…Ideal mixing has been the standard working hypothesis in this kind of study (6,19,22) and will need to be verified by future work. However, our study reinforces this hypothesis by showing that (i) the binary systems are perfectly ideal (as can be seen by the perfectly linear fits of density versus concentration) and (ii) our calculations were compared with existing shockwave data (19,(22)(23)(24) on molten Fe, Fe-O, and Fe-S alloys and found them to be in excellent agreement (SI Appendix, section 3).…”

“…Ideal mixing has been the standard working hypothesis in this kind of study (6,19,22) and will need to be verified by future work. However, our study reinforces this hypothesis by showing that (i) the binary systems are perfectly ideal (as can be seen by the perfectly linear fits of density versus concentration) and (ii) our calculations were compared with existing shockwave data (19,(22)(23)(24) on molten Fe, Fe-O, and Fe-S alloys and found them to be in excellent agreement (SI Appendix, section 3). It should also be noted that high-pressure experiments have shown that miscibility gaps vanish at high pressures (25)(26)(27)(28), hence also indicating that high-density liquids tend to have a simpler thermodynamic behavior than their low-pressure counterpart.…”

“…The literature offers a wide range (3,2,7,8,10,11,18,19) of plausible estimates for the light-element composition of the core (SI Appendix, section 1). To constrain these further, we need to assess whether the compositional model for the core matches the seismically observed density and sound velocity of the core.…”