Abstract

Since Francis Birch (1952) first proposed the idea that Earth’s core should contain light elements, scientists have been trying to identify their nature and abundance. This is one of the biggest and long standing arguments about the deep Earth.

In the seminar I will talk about my recent high-pressure experiments on pure iron in a newly developed internally resistive heated diamond anvil cell (DAC) and thermodynamic modelling of the system Fe-FeO. I will discuss the implications of these results for the distribution of oxygen in the core of our planet.

For the pure iron experiments, we developed a new heating technique in the DAC , namely internal resistive heating system. This technique produces much higher temperature than external heating systems, and much more stable heating than the conventional laser heating technique. We constrained the pressure (P)-temperature (T) location of a phase transition boundary between the hexagonal close-packed (HCP) structure and face-centred cubic (FCC) structure. The thermodynamics of melting relations in the system Fe-FeO was investigated to the outer core-inner core boundary condition from a self-consistent thermodynamic database, which was evaluated from the latest static high-P-T experiments including our own HCP -FCC boundary. From the Gibbs free energy for the Fe-FeO liquids, I calculated the density, sound velocity, and isentropic temperature gradient of a hypothetical oxygen-bearing outer core.

Under the outer core conditions, the addition of oxygen reduces the compressional wave velocity of iron liquid, moving it away from seismologically constrained values. An overall O-rich bulk outer core model is thus excluded. Seismological observations however, suggest the presence of a low-velocity layer with a thickness of 60-70 km at the top of the outer core. The origin of such a low-velocity layer can be explained by an enrichment of oxygen, which might be a consequence of chemical interactions between the core and mantle.