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Persistence of Metal Sediments at the Core-Mantle Boundary and Implications for the Seismic Ultra-Low Velocity Zone
The discovery of Ultra Low Velocity Zone's (ULVZ), at the core-mantle boundary (CMB), reveal a sharper velocity contrast observed across any boundary layer in the Earth's interior except the outer core. Modern lavas in Baffin Island and several ocean island basalts (OIB), exhibiting the highest 3He/4He ratios ever observed and primordial lead isotope ages are linked to plumes rising from the ULVZ. The composition and method responsible for the creation of these ULVZ's remain unexplained. This shear velocity reduction of 30 % is too large to be caused by molten silicate magma and too small to be caused by molten iron at the CMB. Here I propose that the ULVZ at the Earth's CMB is formed by residual heavy metal sediments formed during meteorite impacts and delivered to the core by iron descent during planetary accretion. I present laboratory experiments that show metal flakes are created rapidly by viscous shearing and oxidation of liquid gallium with glucose solutions. Metal flakes resting at the interface between two convecting fluid layers of glucose and liquid metal are shown to survive indefinitely in the presence of vigorous convective circulation scaled to solid state convection dynamics in the Earth. I propose that iron metal sediments may form during turbulent agitation in a magma ocean during meteorite impacts and are carried to the CMB within descending metal-silicate diapirs. Experimental results presented here indicate these iron metal sediments will survive indefinitely at the CMB and may be observed today beneath convecting mantle plumes. Density differences between metal flakes and liquid metal suggested in our experiments and extrapolated to the Earth suggest that a layer of iron metal sediments would produce seismic velocity reductions smaller than molten iron, but larger than silicate magma, consistent with velocities observed in the ULVZ. These primordial metal sediments have been shown to be intermixed with primordial silicate magmas transported to the core that may feed mantle convective plumes to produce primordial geochemical signatures observed in ocean island basalts (OIB's), Greenland, and other locations above large mantle plumes today.