Scale limits of free-silica seismic scatterers in the lower mantle

Lower mantle seismic scatterers are often interpreted in terms of subducted basaltic crust, which should contain free-silica phases that are seismically fast. However, free-silica phases are unstable in a peridotite lower mantle, as they react with ferropericlase (magnesiowustite) to form silicate perovskite. Nonetheless, conservative estimates and recent measurements of diffusion coefficients in silicate perovskite suggest that free-silica phases may persist as polycrystalline armored relics in lower mantle peridotite at length scales equivalent to those observed for seismic scatterers. Simple numerical simulations also suggest that polycrystalline silica may persist within subducted metabasalts.