JpGU-AGU Joint Meeting 2017

Presentation information

[EE] Oral

S (Solid Earth Sciences) » S-IT Science of the Earth's Interior & Tectonophysics

[S-IT22] [EE] Interaction and Coevolution of the Core and Mantle in the Earth and Planets

Mon. May 22, 2017 3:30 PM - 5:00 PM Convention Hall B (International Conference Hall 2F)

convener:Taku Tsuchiya(Geodynamics Research Center, Ehime University), Hidenori Terasaki(Graduate School of Science, Osaka University), Madhusoodhan Satish-Kumar(Department of Geology, Faculty of Science, Niigata University), Tetsuo Irifune(Geodynamics Research Center, Ehime University), John Hernlund(Earth-Life Science Institute, Tokyo Institute of Technology), Eiji Ohtani(Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University), Chairperson:Taku Tsuchiya(Geodynamics Research Center, Ehime University)

3:45 PM - 4:00 PM

[SIT22-38] Composition of the core: Geochemical and mineral physics constraints

*Eiji Ohtani1, Takanori Sakairi1, Tatsuya Sakamaki1, Seiji Kamada2, Ryosuke Tanaka1, Hiroshi Fukui3, Alfred Q.R. Baron4 (1.Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University, 2.Frontier Research Institute of Interdisciplinary Sciences, Tohoku University, 3.Graduate School of Material Science, University of Hyogo, 4. RIKEN SPring-8 Center)

Keywords:Inner Core, Outer core , composition, light elements, sound velocity

The Earth’s core is believed to contain certain amount of light elements based on seismological observations and mineral physics data. The major potential candidates of the light elements of the core are considered to be S, Si and O. Recent studies on the Fe-Si-O system revealed that Si and O have mutual avoidable nature in metallic liquid (1,2), and precipitation of silicates such as SiO2 or FeSiO3 occurred during cooling of the liquid core (e.g., (3)). Therefore, the composition of the inner core coexisting with metallic liquid outer core should be ether Fe-O-S or Fe-Si-S alloy, i.e., coexistence of Si and O are prohibited to occur in the core crystallizing metallic solid inner core.
Our sound velocity measurements of FeO revealed that O is not likely to be the major light element of the inner core (4). Thus the most plausible candidates of the light elements in the core are likely to be S and Si, without O.
Based on our measurements of the sound velocity of iron (5), iron-silicon alloy (6), and Fe3S (7), and the solid-liquid partitioning in the Fe-Si-S system at high pressure and temperature, we constrained the composition of the inner and outer cores. The present experiments on the solid-liquid partitioning of S and Si revealed that the major element of the inner core is silicon whereas that in the outer core is sulfur.
The present results on sound velocity measurements and solid-liquid partitioning of iron alloy indicate that an iron alloy with about 5 wt.% of Si and 0.1 wt. % of S can explain the physical properties of the PREM inner core at the ICB condition, whereas the outer core contains both S and Si (about 7 wt.% S and 3 wt% Si) without O.

(1) Takafuji et al. (2005). Geophys. Res. Lett. 32, L06313. (2) Sakai et al. (2006). Geophys. Res. Lett., 33, L15317. (3) Hirose et al. (2015). AGU 2015 fall meeting, abstract, DI41B-03, San Francisco, December, 14-18, 2015. (4) Tanaka et al. This meeting. (5) Sakamaki et al. (2016), Science Advances, 2: e1500802. (6) Sakairi et al. (2017) in review, (7) Kamada et al. (2014). Am. Mineral. 99, 98-101, 342.