Structural evolution of iron oxides melts at Earth's outer-core pressures

TL;DR

This study investigates the atomic structure of iron oxide melts at extreme pressures relevant to Earth's outer core, revealing coordination environments and density variations that influence core composition and dynamics.

Contribution

It provides in situ x-ray diffraction data on Fe and Fe-oxides melts at core pressures, showing how oxidation state affects structure and oxygen solubility in the outer core.

Findings

Fe-O coordination numbers are around 4, indicating four-fold coordination environments.

Shorter Fe-Fe distances lead to denser packing in high-pressure melts.

Oxygen content variations may cause stratification in Earth's outer core.

Abstract

Oxygen and other light elements comprise up to 5 wt% of the Earth's outer-core, and may significantly influence its physical properties and the operation of the geodynamo. Here we report in situ x-ray diffraction measurements of Fe, Fe + 4.5 FeO (atomic proportion), and Fe2O3 melts at 177-438 GPa, achieved using laser-driven shock compression at an x-ray free-electron laser. The melts exhibit Fe-O coordination numbers between 4.0(0.4) and 4.5(0.4), indicating predominantly four-fold coordination environments. These coordination states are significantly smaller than those of Fe-bearing lower-mantle phases such as bridgmanite and ferropericlase. Shorter Fe-Fe interatomic distances in compressed iron oxide melts drive the denser packing relative to ambient melts, while the structural differences between Fe + 4.5 FeO and Fe2O3 melts under shock indicate that the oxidation state modulates oxygen solubility in liquid Fe. At around 177 GPa (380 km below the core-mantle boundary), Fe2O3 melts exhibit higher Fe-O coordination, suggesting that local variations in oxygen content could contribute to the stratification in the uppermost outer-core inferred from seismological and geomagnetic observations.