Hidden carbon in Earth’s inner core revealed by shear softening in dense Fe 7 C 3

• Published in: Proceedings of the National Academy of Sciences - PNAS 2014-12, Vol.111 (50), p.17755-17758
• Main Authors: Chen, Bin, Li, Zeyu, Zhang, Dongzhou, Liu, Jiachao, Hu, Michael Y., Zhao, Jiyong, Bi, Wenli, Alp, E. Ercan, Xiao, Yuming, Chow, Paul, Li, Jie
• Format: Article
• Language: English
• ISSN: 0027-8424; 1091-6490
• DOI: 10.1073/pnas.1411154111

Seismic studies revealed that shear wave ( S wave) travels through the inner core at an anomalously low speed, thus challenging the notion of its solidity. Here we show that for the candidate inner core component Fe 7 C 3 , shear softening associated with a pressure-induced spin-pairing transition leads to exceptionally low S -wave velocity ( v S ) in its low-spin and nonmagnetic phase. An Fe 7 C 3 -dominant inner core would match seismic observations and imply a major carbon reservoir in Earth’s deepest interior. Earth’s inner core is known to consist of crystalline iron alloyed with a small amount of nickel and lighter elements, but the shear wave ( S wave) travels through the inner core at about half the speed expected for most iron-rich alloys under relevant pressures. The anomalously low S -wave velocity ( v S ) has been attributed to the presence of liquid, hence questioning the solidity of the inner core. Here we report new experimental data up to core pressures on iron carbide Fe 7 C 3 , a candidate component of the inner core, showing that its sound velocities dropped significantly near the end of a pressure-induced spin-pairing transition, which took place gradually between 10 GPa and 53 GPa. Following the transition, the sound velocities increased with density at an exceptionally low rate. Extrapolating the data to the inner core pressure and accounting for the temperature effect, we found that low-spin Fe 7 C 3 can reproduce the observed v S of the inner core, thus eliminating the need to invoke partial melting or a postulated large temperature effect. The model of a carbon-rich inner core may be consistent with existing constraints on the Earth's carbon budget and would imply that as much as two thirds of the planet's carbon is hidden in its center sphere.