4.7 Article

Phase Stability and Thermal Equation of State of Iron Carbide Fe3C to 245 GPa

Journal

GEOPHYSICAL RESEARCH LETTERS
Volume 46, Issue 20, Pages 11018-11024

Publisher

AMER GEOPHYSICAL UNION
DOI: 10.1029/2019GL084545

Keywords

Earth core; shockwave compression; iron carbide; core density; core composition

Funding

  1. National Natural Science Foundation of China [U1330111, 41404067]
  2. Fundamental Research Funds for the Central Universities [WUT: 2019IB015]
  3. National Science Foundation [EAR-1619868]

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We conducted shock wave experiments on iron carbide Fe3C up to a Hugoniot pressure of 245 GPa. The correlation between the particle velocity (u(p)) and shock wave velocity (u(s)) can be fitted into a linear relationship, u(s) = 4.627(+/- 0.073) + 1.614(+/- 0.028) u(p). The density-pressure relationship is consistent with a single-phase compression without decomposition. The inference is further supported by the comparison of the observed Hugoniot density with the calculated Hugoniot curves of possible decomposition products. The new Hugoniot data combined with the reported 300-K isothermal compression data yielded a Gruneisen parameter of gamma = 2.23(7.982/rho)(0.29). The thermal equation of state of Fe3C is further used to calculate the density profile of Fe3C along the Earth's adiabatic geotherm. The density of Fe3C was found to be too low (by similar to 5%) to match the observed density in the Earth's inner core, and Fe3C is unlikely a dominant component of the inner core.

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