Carbon-depleted outer core revealed by sound velocity measurements of liquid iron–carbon alloy

Yoichi Nakajima,Saori Imada,Haruka Ozawa,Kei Hirose,Alfred Q. R. Baron,Shigehiko Tateno,Satoshi Tsutsui,Yasuhiro Kuwayama,Tetsuya Komabayashi

doi:10.1038/ncomms9942

Abstract

The relative abundance of light elements in the Earth's core has long been controversial. Recently, the presence of carbon in the core has been emphasized, because the density and sound velocities of the inner core may be consistent with solid Fe7C3. Here we report the longitudinal wave velocity of liquid Fe84C16 up to 70 GPa based on inelastic X-ray scattering measurements. We find the velocity to be substantially slower than that of solid iron and Fe3C and to be faster than that of liquid iron. The thermodynamic equation of state for liquid Fe84C16 is also obtained from the velocity data combined with previous density measurements at 1 bar. The longitudinal velocity of the outer core, about 4% faster than that of liquid iron, is consistent with the presence of 4–5 at.% carbon. However, that amount of carbon is too small to account for the outer core density deficit, suggesting that carbon cannot be a predominant light element in the core.

Highlights

The relative abundance of light elements in the Earth’s core has long been controversial
While properties of solid iron alloys have been extensively examined by laboratory studies to core pressures (4136 GPa)[1,2,3], little is known for liquid alloys because of experimental difficulties
The core is predominantly molten, and the longitudinal wave (P-wave) velocity of liquid iron alloy is the key to constraining its composition

Summary

Introduction

The relative abundance of light elements in the Earth’s core has long been controversial. The presence of carbon in the core has been emphasized, because the density and sound velocities of the inner core may be consistent with solid Fe7C3. Recent experimental and theoretical studies[12,13] have suggested that solid Fe7C3 may explain the properties of the inner core, in particular its high Poisson’s ratio[14,15], supporting the presence of carbon in the core. We determine the P-wave velocity (VP) (equivalent to bulk sound velocity, VF, in a liquid) of liquid Fe84C16 at high P–T based on inelastic X-ray scattering (IXS) measurements. 16) both velocity and density (r) profiles of liquid Fe84C16 along adiabatic compression are obtained They are compared with seismological observations, indicating that both VP and r in the Earth’s outer core are not explained simultaneously by liquid Fe–C

Methods

Results

Conclusion