[S-IT18] Planetary cores: Structure, formation, and evolution

[SIT18-P05] Stratification of the Earth's core by SiO₂ crystallization

*George Helffrich¹, Guillaume Morard³, Kei Hirose^1,2, Satoshi Kaneshima⁴ (1.Earth-Life Science Institute, Tokyo Institute of Technology, 2.Department of Earth and Planetary Science, University of Tokyo, 3.IMPMC, Universite Pierre et Marie Curie - Paris 6, 4.Department of Earth and Planetary Sciences, University of Kyushu)

Keywords:core, stratification, secular variation, seismic wavespeeds

The topmost ~300 km of the Earth's core features slightly lower wavespeeds than the one dimensional radial earth model PREM. One way this could arise is if this lower speed layer represents a crystallization boundary inside the core, similar to the cloud base in the atmosphere where water-saturated air condenses out water. In the case of the Earth's core, saturation in Si+O could crystallize SiO₂ that rises upward to the CMB and leaves the core due to SiO₂ being even less dense than the base of the Earth's present mantle. This mechanism addresses a criticism of stable core stratification arising from study of the secular variation of the Earth's magnetic field, which seems to require radial fluid motions that strict stratification forbids. Motion of core liquid through a saturation front, which changes its composition, temperature and wavespeeds, facilitates patterns of secular variation attributable to radial motion. We explore this scenario by finding SiO₂ saturation parameters consistent with experimental partitioning of Si and O in metal, and derive the temperature and core compositions compatible with the observed wavespeed profile in the outer core.