Equation of State for Fe‐9.0 wt% O up to 246 GPa: Implications for Oxygen in the Earth's Outer Core

Abstract

Oxygen has been proposed as a potential major light element because it is the most abundant element on Earth. The partition experiments of siderophile elements between silicate melts and liquid metals also indicate that oxygen is a plausible candidate for a light element dissolved in the Earth's outer core (Fischer, 2015; Frost et al., 2010; Ozawa et al., 2009; Takafuji et al., 2005). The impact of oxygen on the density and sound velocity of Fe-rich melts is critical for interpreting the composition, dynamics, and evolution of the Earth's core. However, the amount of oxygen in the core is still a topic of debate. To examine the possibility of oxygen being the main light element in the Earth's outer core, the equation of sate (EoS) and phase relations of FeO were measured by static compression and shock wave loading. Fischer et al. (2011a, 2011b) performed X-ray diffraction measurements using a laser-heated diamond anvil cell to achieve the EoS of FeO, and determined that ∼7.7 wt% oxygen is required in the outer core to match the seismologically determined density. Our previous shockwave data in the Fe-SO system showed negligible oxygen in the outer core since adding oxygen into liquid iron would not simultaneously reproduce the observed density and sound velocity profiles of the outer core (Huang et al., 2011). The liquid phase relation, especially, the Fe-FeO eutectic alloys, provides another clue to constrain the outer core composition. The latest melting data on Fe-O alloys up to 204 GPa in a diamond-anvil cell supported ∼13 wt% oxygen in the liquid outer core (Oka et al., 2019). A thermodynamic model developed by Komabayashi (2014) predicted ∼12 wt% oxygen in the eutectic liquid composition at 330 GPa, which contradicts the experimental results reported by Morard et al. (2017). Therefore, the constraint on the concentration of oxygen in the Earth's

A Review on Elemental and Isotopic Geochemistry

“…%. Because oxygen is the most representative element on earth, it has been postulated as a possible main light element [50].…”