[S-IT27] [EE] Carbon in Planetary Interiors

[SIT27-P05] Existence of Carbon in the Earth’s inner core

Suguru Takahashi¹, *Eiji Ohtani¹, Seiji Kamada², Tatsuya Sakamaki¹, Naohisa Hirao³ (1.Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, 2.Frontier Research Institute of Interdisciplinary Sciences, Tohoku University, 3.Japan Synchrotoron Research Institute)

Keywords:Carbon, Fe3C, Melting relation, equation of state, Sound velocity

Carbon is an important candidate for light element in the Earth’s core. There have been contradictory results on the stability of Fe–C compounds, such as Fe₃C and Fe₇C₃ under the core conditions. We observed Fe₃C melts incongruently to form Fe₇C₃ and liquid at 4200 K and 200 GPa, Additionally, it was shown that the C content of the liquid coexisting with Fe₇C₃ decreases and the Fe–Fe₃C eutectic composition shifts toward the Fe-rich direction with increasing pressure. The present result revealed that Fe₃C and Fe₇C₃ are plausible constituents of the inner core, with a possible layering of Fe₇C₃ in the central region and Fe₃C in the outer region of the inner core formed during cooling of the core. We also conducted the equation of state and sound velocity measurements at high pressure and temperature by using XRD and IXS measurements at Spring-8. The results indicate that the sound velocity of the mixture of Fe-Fe3C is higher than the PREM inner core under the Earth’s core conditions. Thus, it is difficult to explain that the Earth’s inner core is composed of only a Fe–Fe₃C mixture. Since the inner core temperature is very close to the eutectic temperature of the Fe-Fe₃C system, pre-melting effect of Fe3C could be important to account for the sound velocity of the PREM inner core.