AbstractsEarth & Environmental Science

The formation of the Earth and terrestrial planets involved violent meteorite impacts which partially or fully melt the planet???s surface materials, facilitating separation of iron metal from silicates. Liquid-metal ponds may collect at the bottom of magma oceans caused by impact ejecta, but will be unstable. Several important aspects of the differentiation process remain mysteries, including the rapid delivery of metal to the core in under 30 My, an excess abundance of siderophile trace elements in the mantle, and the extreme temperature, or ???superheat,??? of the Earth???s core. Iron delivery to the core by descent of metal-silicate plumes has been proposed, but physical experiments to test theoretical calculations have been difficult to perform. Previous studies have shown that trailing conduits of silicate melt are entrained behind descending metal diapirs. I use a physical fluid model to investigate the formation and evolution of metal-silicate plumes and trailing conduits. I use liquid gallium to represent the metal pond and glucose solutions with viscosities from 10^(-2) ??? 10^5 Pa???s to represent the magma ocean and the silicate mantle. I consider the density and viscosity conditions under which trailing conduits form and remain open behind liquid metal drops. I find that both the descent time of metal diapirs and relaxation time of conduits depend strongly on the density difference between the silicate melt in the conduit and the solid silicate mantle, the mantle viscosity, and the radius of the liquid metal plume. I derive a theoretical expression for the descent speed of a falling drop with a trailing conduit, which is faster than the Stokes flow prediction for a two fluid system when a conduit is present because the conduit reduces drag on the drop. The increased speed of descending metal diapirs in the presence of a trailing conduit will provide more shear heating available for core superheat than estimated by using standard Stokes flow calculations. My results indicate that conduits will remain open and entrain magma-ocean material to the Earth's core-mantle boundary for metal plumes > 300 km in radius and contribute to compositional variations in the mantle.