TITLE: Thermal Equation of State of Iron: Constraint on the Density Deficit of Earth’s
Core
AUTHORS (FIRST NAME, LAST NAME): Yingwei Fei1, Caitlin A Murphy1, Yuki
Shibazaki1, Haijun Huang2
INSTITUTIONS (ALL): 1. Geophysical Laboratory, Carnegie Institution of Washington,
Washington, DC, United States.
2. School of Sciences, Wuhan University of Technology, Wuhan, Hubei, China.
ABSTRACT BODY: The seismically inferred densities of Earth’s solid inner core and the
liquid outer core are smaller than the measured densities of solid hcp-iron and liquid iron,
respectively. The inner core density deficit is significantly smaller than the outer core
density deficit, implying different amounts and/or identities of light-elements incorporated
in the inner and outer cores. Accurate measurements of the thermal equation-of-state of iron
over a wide pressure and temperature range are required to precisely quantify the core
density deficits, which are essential for developing a quantitative composition model for the
core. The challenge has been evaluating the experimental uncertainties related to the choice
of pressure scales and the sample environment, such as hydrostaticity at multi-megabar
pressures and extreme temperatures. We have conducted high-pressure experiments on iron
in MgO, NaCl, and Ne pressure media and obtained in-situ X-ray diffraction data up to 200
GPa at room temperature. Using inter-calibrated pressure scales including the MgO, NaCl,
Ne, and Pt scales, we have produced a consistent compression curve of hcp-Fe at room
temperature. We have also performed laser-heated diamond-anvil cell experiments on both
Fe and Pt in a Ne pressure medium. The experiment was designed to quantitatively compare
the thermal expansion of Fe and Pt in the same sample environment using Ne as the
pressure medium. The thermal expansion data of hcp-Fe at high pressure were derived
based on the thermal equation of state of Pt. Using the 300-K isothermal compression curve
of iron derived from our static experiments as a constraint, we have developed a thermal
equation of state of hcp-Fe that is consistent with the static P-V-T data of iron and also
reproduces the shock wave Hugoniot data for pure iron. The thermodynamic model, based
on both static and dynamic data, is further used to calculate the density and bulk sound
velocity of liquid iron. Our results define the solid inner core and liquid outer core density
deficits, which can serve as the basis for any core composition models.