[S17-P] Poster

[S17-P-02] Outer core stratification by crystallization of SiO2

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

Wavespeeds lower than PREM's near-uniform self-compression profile in the outermost outer core constitute evidence for a radial compositional gradient there, and possibly for stable stratification. Melting experiments in the Fe-Si-O system in the diamond anvil cell at outer core pressures and temperatures show crystallization of SiO2 from a variety of starting compositions. We developed a thermodynamic model of SiO2 saturation in liquid Fe at high pressure and temperature conditions suitable for modelling magma ocean and outer core processes. Significant incorporation of Si+O in the metal occurs in magma ocean conditions, 30-50 GPa, which, after the core evolves to its present temperature (3500-4500 K at the CMB), leads to exsolution of SiO2. Driving a dynamo by crystallization is quite efficient and allows one of today's strength to be powered throughout Earth history. 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. This leads to reduced wavespeeds in the top of the outer core from the net effects of the density and mean atomic weight change. Only a small change in concentration of the SiO2 component in the bulk liquid is required about 0.15 wt%.