Evidence for Fe-Si-O liquid immiscibility at deep Earth pressures

Area of Science:

• Geophysics
• High-pressure experimental physics
• Computational materials science

Background:

• Seismic data indicate a buoyant, slow-velocity layer at the top of Earth's liquid outer core.
• A stably stratified layer could form via immiscibility in molten iron alloys, but this has not been proven at core pressures.

Purpose of the Study:

• To investigate the immiscibility of molten iron alloys at pressures relevant to Earth's core.
• To determine if silicon and oxygen can coexist as light elements in the core.

Main Methods:

• Laser-heated diamond-anvil cell experiments reaching 140 GPa.
• First-principles molecular dynamics simulations.

Main Results:

• Demonstrated immiscibility between liquid Fe-Si and Fe-Si-O persisting to at least 140 GPa.
• Supported silicon and oxygen as coexisting light elements in the core.
• Indicated that [Formula: see text] does not crystallize from Fe-Si-O at the core-mantle boundary.

Conclusions:

• High-pressure immiscibility in Fe-Si-O systems explains the stratified layer atop the outer core.
• This immiscibility influences deep Earth differentiation, evolution, and the geodynamo.
• Silicon and oxygen are likely coexisting light elements in the core.