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

• Published in: Nature communications 2015-11, Vol.6 (1), p.8942-8942, Article 8942
• Main Authors: Nakajima, Yoichi, Imada, Saori, Hirose, Kei, Komabayashi, Tetsuya, Ozawa, Haruka, Tateno, Shigehiko, Tsutsui, Satoshi, Kuwayama, Yasuhiro, Baron, Alfred Q. R.
• Format: Article
• Language: English
• Subjects: 704/2151/213; 704/2151/2809; 704/445/123; Humanities and Social Sciences; multidisciplinary; Science; Science (multidisciplinary)
• ISSN: 2041-1723; 2041-1723
• DOI: 10.1038/ncomms9942

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 Fe 7 C 3 . Here we report the longitudinal wave velocity of liquid Fe 84 C 16 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 Fe 3 C and to be faster than that of liquid iron. The thermodynamic equation of state for liquid Fe 84 C 16 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. The composition of the Earth's core, particularly the light elements present, is not well constrained. Here, the authors report sound velocities of liquid iron-carbon alloy as measured at very high pressures using inelastic X-ray scattering and suggest that carbon cannot be predominant in the outer core.