Anisotropic Texture of Iron Metal Flakes in the Ultra Low Velocity Zone (ULVZ) at the Earth’s Core-Mantle Boundary

Abstract:

The discovery of the ULVZ reveals a striking 30% S velocity contrast across the core-mantle boundary (CMB). However, the physical composition of this thin 5-30 km layer is elusive. Molten metal or silicates alone produce excessive velocity reductions. Recent studies of differential SKS-SKKS phases indicate seismic anisotropy in the ULVZ at the root of the African plume. Many ULVZ regions are observed beneath large low velocity provinces (LLSVP) suggesting a link. Geochemical studies of modern lava's indicate ³He/⁴He and lead-isotope ratios which may be consistent with a primordial (4.55 Ga) origin for LLSVP’s. Here, we present results from laboratory fluid convection experiments using liquid gallium and glucose solutions which indicate that solid iron metal sediments may form in the turbulent environment of meteorite impacts and are entrained by sinking molten iron diapirs to the CMB during the Earth’s core differentiation process. Geodynamic fluid experiments model turbulent mixing, iron sediment settling in a silicate magma ocean, delivery to the CMB, and metal sediment behavior during mantle and core convection. Our observations indicate that solid metal flakes form during agitation of liquid metal with viscous fluids. Increasing agitation rate, agitation time, and ambient fluid viscosities increases the volume of metal flakes produced. Metal sediments consist of planar flakes and are observed to settle with a preferred orientation forming a textured sediment layer at the top of a liquid metal layer and do not re-dissolve. Convection is not observed to significantly disturb these heavy metal sediments. We are currently investigating variations in metal flake orientation in the presence of convection currents and upward migration of silicates entrained during core formation which may be the source of LLSVP’s. We expect a textured layer of metal sediments formed during core differentiation to produce seismic anisotropy in the ULVZ that may be preserved today.