[S-IT20] Structure and Dynamics of Earth and Planetary Mantles

[SIT20-11] Core-mantle interaction evidence from SiO₂ dispersal in Earth's lower mantle

*George Helffrich¹, Maxim Ballmer^1,2, Kei Hirose^1,3 (1.Earth-Life Science Institute, Tokyo Institute of Technology, 2.Institute of Geophysics, ETH Zurich, 3.Department of Earth and Planetary Science, University of Tokyo)

Keywords:SiO2, core, mantle

As the Earth accreted and progressively grew its core, Si and O probably dissolved into the metal and were kept there for some time. As the core subsequently cooled, SiO₂ would have been expelled due to oversaturation, a process that could continue today. On account of SiO₂'s low density with respect to the lowermost mantle, we examine the process of SiO₂ accumulation at the core-mantle boundary (CMB) and its incorporation into the mantle by buoyant rise. Diapirs formed by the viscous Rayleigh-Taylor instability in the SiO₂ collected at the CMB would cause them to be swept into the mantle as inclusions of 100 m - 10 km diameter today, using estimates of SiO₂ viscosity in the lower mantle. Under early Earth conditions of rapid heat loss after core formation, smaller, ~1 km diameter diapirs could have risen independently of mantle flow to a level of neutral buoyancy in the lower mantle, trapping them there. SiO₂ presence could account for small-scale scattering in the lower mantle due to the bodies' large velocity contrast with peridotite.