Experiments and thermodynamic modelling on Fe-FeH phase diagram under high pressures: Implications for Earth’s core composition and temperature

2026/07/26 by Shoh Tagawa, Suyu Fu, George Helffrich +2
Earth and Planetary Sciences · Physics and Astronomy · Biochemistry, Genetics and Molecular Biology · #High-pressure geophysics and materials #Astro and Planetary Science #Geomagnetism and Paleomagnetism Studies

paper · doi:10.1016/j.epsl.2026.120243

Abstract

Hydrogen may be one of the major light elements in the metallic cores of terrestrial planets, and the Fe-FeH phase diagram is of great importance to understand core composition and temperature. However, since metallic iron incorporates negligible amounts of hydrogen at ambient pressure, the study of the Fe-FeH system has been limited. Here we obtained the Fe-FeH phase diagram under high pressures to 330 GPa corresponding to Earth’s inner core boundary conditions, based on a combination of high-pressure experiments and thermodynamic modelling. Both melting and subsolidus experiments were carried out up to 130 GPa and 4200 K in a laser-heated diamond-anvil cell combined with in-situ synchrotron X-ray diffraction measurements. We then developed a thermodynamic model based on the present and earlier experiments on the Fe-FeH system including Fe- and FeH-endmembers. It predicts the Fe-FeH phase diagram at 330 GPa, showing the higher Fe-FeH eutectic temperature than the extrapolation of a melting curve reported by earlier experiments, the concentration-dependent hcp-Fe/liquid partition coefficient of hydrogen, and the non-linear effect of hydrogen on the depression of melting temperature. Considering these findings, we estimated the possible ranges of Earth’s core composition and temperature, suggesting that both the outer and inner cores contain hydrogen and silicon as important light elements with a minor amount of oxygen in the former.

Citations

• Phase relations in the Fe–FeSi system at high pressures and temperatures
• High-pressure Raman spectroscopy of diamond anvils to 250GPa: Method for pressure determination in the multimegabar pressure range
• Composition and temperature of the Earth’s core constrained by combining ab initio calculations and seismic data
• High pressure effects on the iron–iron oxide and nickel–nickel oxide oxygen fugacity buffers
• Melting experiments on Fe–Fe 3 S system to 254 GPa
• The elastic properties of hcp-Fe alloys under the conditions of the Earth's inner core
• Chemical compositions of the outer core examined by first principles calculations
• Hydrogen distribution between the Earth's inner and outer core
• Partitioning of silicon and sulfur between solid and liquid iron under core pressures: Constraints on Earth's core composition
• Immiscibility between hydrogen and molten iron in planetary cores

Related

• Fe-FeH Eutectic Melting Curve and the Estimates of Earth's Core Temperature and Composition
• Experimental evidence of a body centered cubic iron at the Earth's core condition
• Model for the Thermodynamics of Iron at High Pressures Near Melting
• Wide-ranged multiphase equation of state for iron and model variations addressing uncertainties in high-pressure melting
• A structural study of hcp and liquid iron under shock compression up to 275 GPa