Abstract
AbstractWe 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.
Highlights
The Earth's early accretion events, in particular magma ocean formation, had a great impact on the chemical and thermal evolution of the Earth
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
At all P–T conditions considered, we observe the diffusion of hydrogen atoms from the silicate to the metallic phase, leading to an accumulation of hydrogen in iron (SI, Figure S4 and Movies S2–S4) that is documented by the time‐development of the partial radial distribution function (RDF) g(r) between both hydrogen species (HS and HM) and Fe and O, i.e., gOHS ðrÞ, gOHM ðrÞ, gFeHS ðrÞ and gFeHM ðrÞ (SI, Figure S5)
Summary
The Earth's early accretion events, in particular magma ocean formation, had a great impact on the chemical and thermal evolution of the Earth. Segregation of metal from silicate to form a core during the magma ocean stage has removed elements besides iron and nickel from the mantle (e.g., Righter, 2003). The affinity to iron (siderophile behavior) at high P suggests that hydrogen is a strong candidate for the light element in the core This is reflected by decades of work on the Fe–H system at high P, including melting point depression and phase diagram (Okuchi, 1998; Sakamaki et al, 2009), hydrogen partitioning experiments between silicate and iron (Clesi et al, 2018; Malavergne et al, 2019; Okuchi, 1997), and by extension studies of water–iron reactions (Fukai, 1984; Ohtani et al, 2005; Yagi & Hishinuma, 1995; Yuan et al, 2018). We present results from molecular dynamics (MD) simulations using density functional theory (DFT) based methods on the interaction between hydrogen‐bearing silicate and metallic liquids, determining hydrogen partitioning at 20–130 GPa and 2,500–4,000 K, which covers the whole range of P–T conditions of metal–silicate equilibration/segregation during core formation (Li & Agee, 1996; Siebert et al, 2012)
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