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

Sun. May 21, 2017 10:45 AM - 12:15 PM A05 (Tokyo Bay Makuhari Hall)

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:Eiji Ohtani(Department of Earth and Planetary Materials Science, Graduate School of Science, Tohoku University)

11:30 AM - 11:45 AM

[SIT22-34] Crystallization of SiO2 from the outer core: A possible means of stratification

*George Helffrich1, Kei Hirose1, Ryosuke Sinmyo1, Guillaume Morard2 (1.Earth-Life Science Institute, Tokyo Institute of Technology, 2.IMPMC UPMC, Paris, France)

Keywords:core, stratification, SiO2 saturation, seismology

The PREM seismic wavespeed model in the outermost core is in near-uniform self-compression. Slight deviations from self-compression constitute evidence for a radial compositional gradient there, and possibly for stable stratification. Based on melting experiments in the Fe-Si-O system in the diamond anvil at outer core pressures and temperatures that show crystallization of SiO2, we developed a thermodynamic model of SiO2 saturation in liquid Fe at high pressure and temperature conditions suitable for modeling magma ocean and outer core processes. Conditions in a magma ocean between 30-50 GPa allow for significant incorporation of Si + O in the metal, which, after the core evolves to its present temperature (3500-4500 K at the CMB), leads to exsolution of SiO2. Using a transition-element hard-sphere model for seismic wavespeeds, we show that the continuous crystallization of SiO2 at the top of the core produces denser, iron-enriched liquid that mixes downward into the core. The net effects of the density and mean atomic weight change in the mixed region leads to reduced wavespeeds in the top of the outer core that require only a small change in concentration of the SiO2 component in the liquid, about 0.15 wt%.