Core composition

The Earth's core is comprised mostly of an Fe–Ni alloy, but seismological evidence shows that the core must also contain ~10% of one or more element(s) less dense than iron. Understanding the identity and abundance of light elements in the core is important to our knowledge of the core formation process and chemical history of the planet's interior; dynamics of the outer core and magnetic field generation; inner core structure and properties; and thermal structure of the Earth. Geochemical and cosmochemical data suggest that Si, O, S, C, and H are the most likely core light elements.

Using synchrotron X-ray diffraction in a laser-heated diamond anvil cell, I have mapped out the pressure–temperature phase diagrams of FeO, FeSi, and Fe–Si alloys containing 9 and 16 wt% silicon, up to pressures of the Earth's outer core (>135 GPa) and several thousand K. These phase diagrams reveal which phases are stable under the conditions of the Earth's core. I then determined the equations of state of each of these phases, describing how their densities vary with pressure and temperature. The densities of these materials can be compared to the density of iron and that of the core to determine how much silicon or oxygen the core would need to contain to match its seismologically-observed density. Using these methods, I have established that the Earth's outer core can contain a maximum of 8 wt% oxygen or 11 wt% silicon.

Earth's interior
The Earth's interior
FeO phase diagram
The phase diagram of FeO

Related publications:
Fischer, 2016
Thompson et al., 2016
Fischer and Campbell, 2015
Fischer et al., 2014
Fischer et al., 2013
Fischer et al., 2012
Fischer et al., 2011b
Fischer et al., 2011a
Fischer and Campbell, 2010
Lin et al., 2009

Selected press coverage:
Optics and Photonics Focus
Science News