Thermal Equation of State of Fe3C to 327 GPa and Carbon in the Core

Suguru Takahashi,Daijo Ikuta,Seiji Kamada,Yasuo Ohishi,Naohisa Hirao,Tatsuya Sakamaki,Eiji Ohtani

doi:10.3390/min9120744

Suguru Takahashi, Daijo Ikuta + Show 5 more

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https://doi.org/10.3390/min9120744

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Abstract

The density and sound velocity structure of the Earth’s interior is modeled on seismological observations and is known as the preliminary reference Earth model (PREM). The density of the core is lower than that of pure Fe, which suggests that the Earth’s core contains light elements. Carbon is one plausible light element that may exist in the core. We determined the equation of state (EOS) of Fe3C based on in situ high-pressure and high-temperature X-ray diffraction experiments using a diamond anvil cell. We obtained the P–V data of Fe3C up to 327 GPa at 300 K and 70–180 GPa up to around 2300 K. The EOS of nonmagnetic (NM) Fe3C was expressed by two models using two different pressure scales and the third-order Birch–Murnaghan EOS at 300 K with the Mie–Grüneisen–Debye EOS under high-temperature conditions. The EOS can be expressed with parameters of V0 = 148.8(±1.0) Å3, K0 = 311.1(±17.1) GPa, K0′ = 3.40(±0.1), γ0 = 1.06(±0.42), and q = 1.92(±1.73), with a fixed value of θ0 = 314 K using the KBr pressure scale (Model 1), and V0 = 147.3(±1.0) Å3, K0 = 323.0(±16.6) GPa, K0′ = 3.43(±0.09), γ0 = 1.37(±0.33), and q = 0.98(±1.01), with a fixed value of θ0 = 314 K using the MgO pressure scale (Model 2). The density of Fe3C under inner core conditions (assuming P = 329 GPa and T = 5000 K) calculated from the EOS is compatible with the PREM inner core.

Highlights

The density of the Earth’s interior has been determined using the preliminary reference Earth model (PREM) [1]
Previous studies show that the magnetic transition of Fe3 C
X-ray diffraction (XRD) patterns of Fe3 C, KBr, and MgO were observed at all present experimental conditions

Summary

Introduction

The density of the Earth’s interior has been determined using the preliminary reference Earth model (PREM) [1]. The density of iron at specific pressure and temperature conditions has been determined in high-pressure and high-temperature experiments and from theoretical calculations (e.g., [2,3]). These studies implied that the density of pure hcp-Fe is higher than that obtained from seismological models such as PREM [1]. The density deficit is estimated to be ~10% for the outer core [4,5,6,7] and 2–5% for the inner core [8,9]. Light elements may be contained within the Earth’s inner core. Liu et al [13] reported that Fe3 C is stable up to 150 GPa and that it melts incongruently into Fe7 C3 and liquid at high temperatures in this pressure range and decomposes to Fe7 C3 and

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