Hydrogen isotope fractionation during core formation of terrestrial planets

Water strongly modulates the physical and chemical properties of planetary materials, influencing mantle convection, magmatism, and plate tectonics, and thus Earth’s habitability (1). Despite its importance, the timing and mechanism of Earth’s water acquisition remain unresolved. Competing models invoke late delivery by volatile-rich bodies (2), inheritance from water-bearing enstatite chondrites (3), or incorporation of hydrogen from a nebular H₂-rich environment during early accretion (4). Hydrogen isotopes provide a powerful tracer for distinguishing among these scenarios, yet their interpretation is complicated by large-scale hydrogen partitioning during core formation.

Geochemical and geophysical evidence indicates that more than 75% of Earth’s hydrogen was sequestered into the metallic core during differentiation (5, 6), making the core the planet’s largest hydrogen reservoir. Consequently, the bulk Earth’s hydrogen isotope composition depends critically on hydrogen isotope fractionation between silicate and metallic melts at core-forming pressures and temperatures. However, this key fractionation factor remains poorly constrained.

Here, we address this long-standing problem by combining first-principles calculations with machine-learning–accelerated path-integral molecular dynamics to quantify equilibrium hydrogen isotope fractionation between silicate and metallic melts under core-forming conditions. Our simulations explicitly capture nuclear quantum effects and extend to pressures and temperatures relevant to Earth’s early magma ocean and core formation. We incorporate these fractionation factors into models of hydrogen isotope evolution during planetary differentiation and accretion, allowing us to reconstruct the bulk Earth’s D/H ratio. These results provide new constraints on the sources of Earth’s water and clarify the role of metal–silicate equilibration in shaping the planet’s volatile inventory during its earliest history.

References

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