ALP Seminar: Investigating iron and iron oxides at Earth’s lower mantle and outer-core conditions via laser-driven compression

Iron and iron oxides are fundamental constituents of the Earth’s lower mantle and core. (Fe,Mg)O is a major mineral of the Earth’s lower mantle, and other iron oxides could be present down to the core-mantle boundary via subduction of Banded Iron Formation (large iron oxide formations deposited in the oceans during the Great Oxidation of the Earth) or reaction between the silicate mantle and the iron-rich core at the core-mantle boundary (136 GPa). Iron oxides are transition metal oxides, exhibiting structural and electronic transitions (from high to low spin and from insulator to metal) upon increasing pressure and temperature, which could be at the origin of seismological heterogeneities observed at depth. Moreover, the iron-rich outer core of the Earth contains up to 5% O. Study of iron oxide melts is thus of importance for the understanding of the Fe-O bonding environment whose evolution with pressure would help to uncover potential layering and transport properties of the outer-core of significance for the Earth’s Geodynamo and by extension for magnetic field in terrestrial exoplanets, a key parameter for habitability.  

In this seminar, following a summary of the tools used to investigate the Earth’s interior as well as a summary of the current knowledge related to iron oxides in the deep Earth, I will present experimental results obtained on solid and liquid iron and iron oxide melts. Our main technique is laser-driven shock compression in combination with X-ray diffraction and X-ray emission spectroscopy at an X-ray Free Electron Laser (XFEL). Differences between static and dynamic compression results for FeO and Fe₂O₃ will be discussed, as well as their relevance in the study of planetary interiors. Liquid structures and densities will be presented, based on liquid diffraction analysis, still in its infancy for non-monoatomic metals under dynamic compression. Differences observed between Fe²⁺ and Fe³⁺ oxide melts and changes with pressure will be discussed.