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

TL;DR

This study directly measures the structure of Fe, Fe + 4.5 FeO, and Fe$_2$O$_3$ melts under Earth’s outer-core pressures using in situ X-ray diffraction during laser-driven shock at EuXFEL. The melts exhibit predominantly fourfold Fe–O coordination ($CN\approx4.0$–$4.5$) with dense Fe–Fe networks, and the oxidation state modulates oxygen solubility, potentially driving compositional stratification at the top of the outer core. Fe undergoes a bcc-to-hcp transition and melts along the Hugoniot, while FeO remains B1 up to ~170 GPa and Fe$_2$O$_3$ transitions through amorphization before melting; at high pressures Fe$_2$O$_3$ shows convergence toward FeO-like melts, suggesting partial dissociation under core-like conditions. These experimentally constrained structural parameters for Fe–O liquids under extreme pressure–temperature conditions place important limits on oxygen partitioning and transport in the outer core, with implications for core dynamics and magnetic field generation.

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.

Figures (7)

• Figure 1: Left: Experimental setup at the HED endstation of the EuXFEL, showing a 2D raw diffraction image of an Fe$_2$O$_3$ target at ambient conditions prior to shock. The DiPOLE 100-X laser is shown in light green, the VISAR probe in dark green, and the x-ray beam in red. Fe$_2$O$_3$ and Fe + 4.5 FeO layers were deposited on a black Kapton ablator via plasma sputtering, while the Fe foil was glued to the ablator. Right: Dewarped 2D XRD image from the Varex detectors for Fe$_2$O$_3$ melt under shock probed around 0.1 ns before the shock exits the target.
• Figure 2: Fully corrected, azimuthally integrated 1D XRD lineout of Fe under laser-driven shock compression, showing the transformation from bcc to hcp Fe, followed by partial and then complete melting above 280(30) GPa. Vertical dashed lines mark residual ambient bcc peaks, and black stars indicate the main hcp peaks under pressure. Lattice parameters are provided in Section VII and Table 1 of the Supplemental Material. Removal of remaining ambient material produces small dips at the positions of ambient peaks, reflecting sample texture and resulting in differences in relative peak intensity between the shocked and ambient samples. The diffuse scattering region is highlighted in red.
• Figure 3: Fully corrected, azimuthally integrated 1D XRD lineout of Fe + 4.5 FeO under laser-driven shock compression. No phase transition is observed in FeO, which remains in the B1 (rocksalt) structure. Above 170(20) GPa, the disappearance of FeO peaks indicates melting along the Hugoniot. Hcp Fe is seen at 30(20) GPa, but the hcp reflections become indistinct at higher pressures. Only data above 280 GPa (above Fe melting point under shock as seen in Fig. \ref{['Fig:Fe_intensity']}) are taken as fully molten Fe + 4.5 FeO. Vertical dashed lines in black and red mark residual ambient FeO and bcc Fe peaks, respectively; black stars denote the main FeO peaks, and red stars indicate hcp Fe peaks, under pressure. The diffuse scattering region is highlighted in red.
• Figure 4: Fully corrected, azimuthally integrated 1D XRD lineout of Fe$_2$O$_3$ under laser-driven shock compression, showing: (1) the $\alpha$ to $\alpha^\prime$-Fe$_2$O$_3$ phase transition between 33(14) and 82(14) GPa; (2) amorphization from 108(14) GPa; and (3) melting from 177(14) or 217(14) GPa. Two samples of differing crystallinity were used: a well-crystalline sample (black) for most shots, and a poorly crystalline sample (red) for two shots. Vertical dashed lines mark residual ambient Fe$_2$O$_3$ peaks, while black stars denote peaks visible in the amorphous phase. Removal of remaining ambient material produces small dips at the positions of ambient peaks, reflecting sample texture and resulting in variations in relative peak intensity between shocked and ambient samples. The diffuse scattering region is highlighted in red.
• Figure 5: Faber–Ziman structure factor $S_{FZ}(Q)$ and corresponding pair distribution function $g(r)$ for Fe, Fe + 4.5 FeO, and Fe$_2$O$_3$ melts. Data are shifted vertically by 0.5 or 1 for clarity. The density was refined during the calculation of $g(r)$ using an iterative procedure eggert, as detailed in Methods. Denser packing of the melts is primarily explained by a decrease in Fe–Fe interatomic distances in Fe + 4.5 FeO and Fe$_2$O$_3$ melts under shock, relative to the 0 GPa (ambient) melt from Shi et al.shi. For Fe and Fe + 4.5 FeO, the $g(r)$ shifts toward shorter interatomic distances with increasing pressure. In Fe$_2$O$_3$, two distinct changes (highlighted in blue) are observed: first, between 177(14) and 217(14) GPa, where the Q0 peak decreases and the first oscillation (r1) changes shape and width; and second, between 315–333(14) GPa, where the Q2 feature becomes pronounced, r2 increases, and r1 becomes sharper and more intense. The spurious features (sp. feat.) in high-pressure Fe + 4.5 FeO melt data (marked in red) arise from incomplete subtraction of ambient contributions.