Experimental evidence for hydrogen incorporation into Earth\u2019s core

Shoh Tagawa,Yasuo Ohishi,Naoya Sakamoto,Kei Hirose,Shunpei Yokoo,Hisayoshi Yurimoto,John Hernlund

doi:10.1038/s41467-021-22035-0

Abstract

Hydrogen is one of the possible alloying elements in the Earth’s core, but its siderophile (iron-loving) nature is debated. Here we experimentally examined the partitioning of hydrogen between molten iron and silicate melt at 30–60 gigapascals and 3100–4600 kelvin. We find that hydrogen has a metal/silicate partition coefficient DH ≥ 29 and is therefore strongly siderophile at conditions of core formation. Unless water was delivered only in the final stage of accretion, core formation scenarios suggest that 0.3–0.6 wt% H was incorporated into the core, leaving a relatively small residual H2O concentration in silicates. This amount of H explains 30–60% of the density deficit and sound velocity excess of the outer core relative to pure iron. Our results also suggest that hydrogen may be an important constituent in the metallic cores of any terrestrial planet or moon having a mass in excess of ~10% of the Earth.

Highlights

Hydrogen is one of the possible alloying elements in the Earth’s core, but its siderophile nature is debated
While hydrogen has been found to be strongly siderophile under high pressure[8,9,10], these results were challenged by more recent experiments[11,12]
We carried out metal–silicate partitioning experiments at 30–60 GPa and 3100–4600 K in a diamond-anvil cell (DAC)

Summary

Introduction

Hydrogen is one of the possible alloying elements in the Earth’s core, but its siderophile (ironloving) nature is debated. Existing Earth’s core formation models constrained by partitioning of moderately siderophile elements suggest that metals equilibrated with molten silicates in a deep “magma ocean” under high-pressure and -temperature (P–T) conditions up to about 60 GPa and 4000 K1–6

Results

Conclusion