Helffrich, G; Ballmer, MD; Hirose, K; (2018) Core-Exsolved SiO2 Dispersal in the Earth's Mantle. JGR Solid Earth , 123 (1) pp. 176-188. 10.1002/2017JB014865.

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Abstract

SiO2 may have been expelled from the core directly following core formation in the early stages of Earth's accretion and onward through the present day. On account of SiO2's low density with respect to both the core and 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. Today, if SiO2 is 100–10,000 times more viscous than lower mantle material, the dimensions of SiO2 diapirs formed by the viscous Rayleigh‐Taylor instability at the CMB would cause them to be swept into the mantle as inclusions of 100 m–10 km diameter. Under early Earth conditions of rapid heat loss after core formation, SiO2 diapirs of ∼1 km diameter could have risen independently of mantle flow to their level of neutral buoyancy in the mantle, trapping them there due to a combination of intrinsically high viscosity and neutral buoyancy. We examine the SiO2 yield by assuming Si + O saturation at the conditions found at the base of a magma ocean and find that for a range of conditions, dispersed bodies could reach as high as 8.5 vol % in parts of the lower mantle. At such low concentration, their effect on aggregate seismic wave speeds is within observational seismology uncertainty. However, their presence can account for small‐scale scattering in the lower mantle due to the bodies' large‐velocity contrast. We conclude that the shallow lower mantle (700–1,500 km depth) could harbor SiO2 released in early Earth times.

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