Strong Sequestration of Hydrogen Into the Earth's Core During Planetary Differentiation

• Published in: Geophysical research letters Vol. 47; no. 15
• Main Authors: Yuan, Liang, Steinle‐Neumann, Gerd
• Format: Journal Article
• Language: English
• Published: Washington John Wiley & Sons, Inc 16.08.2020
• Subjects: Astrochemistry; core formation; Density functional theory; Earth; Earth core; High pressure; Hydrogen; Hydrogen partitioning; Iron; Iron constituents; Lava; Liquid metals; Low pressure; Magma; Magnesium; Melts; metallic melts; Metals; Molecular dynamics; Planet formation; Planetary cores; Pressure; Quantum mechanics; Segregation; silicate melts; Silicates; Silicon; Simulation; Temperature
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2020GL088303

We explore the partitioning behavior of hydrogen between coexisting metal and silicate melts at conditions of the magma ocean and the current core–mantle boundary with the help of density functional theory molecular dynamics. We perform simulations with the two‐phase and thermodynamic integration methods. We find that hydrogen is weakly siderophile at low pressure (20 GPa and 2,500 K), and becomes much more strongly so with pressure, suggesting that hydrogen is transported to the core in a significant amount during core segregation and is stable there. Based on our results, the core likely contains ~1 wt% H, assuming single‐stage formation and equilibration at 40 GPa. Our two‐phase simulations further suggest that silicon is entrained in the core‐forming metal, while magnesium remains in the silicate phase. This preferred incorporation of silicon hints at an explanation for the elevated Mg/Si ratio of the bulk silicate Earth relative to chondritic compositions. Plain Language Summary Hydrogen is the most abundant element in the universe and may be present in the Earth's core together with its main constituent, iron. Despite its importance, the amount of hydrogen in the core has been poorly understood as it is extremely difficult to measure the hydrogen content in iron metal from experiments at high pressure and temperature. Here, we use advanced quantum mechanical simulations on silicate and metallic melts showing that hydrogen becomes increasingly more incorporated in metal over silicate at high pressure and temperature, conditions under which Earth's core formed. Therefore, hydrogen can be present in the core in high abundance. We further show that silicon is also transported to the core to a smaller extent, while magnesium remains in the mantle material. Key Points We perform density functional theory molecular dynamics simulations to study hydrogen partitioning between liquid metal and silicate Hydrogen is predicted to be weakly siderophile at low pressure (~20 GPa) and becomes significantly more siderophile at higher pressures Core segregating from the magma ocean may have sequestered significant amounts of H and, to a lesser extent, Si, but little or no Mg