Conduit entrainment surrounding sinking metal-silicate plumes during core formation

Abstract:

Although the Earth today is segregated into two distinct compositional regions of a silicate mantle and metallic core, the process of differentiation from their original chondritic form is not clear. Continuous meteorite bombardment melted impactors and target bodies causing dissociation of metal from silicates which may have formed magma reservoirs or oceans. The heavy liquid metal phase will sink quickly through a silicate magma ocean and settle at the base as a metal pond. A high density metal pond will be unstable and is expected to sink as a Rayleigh-Taylor instability descending as liquid metal plumes to the core. Previous experiments have shown that lighter silicate material from the magma ocean becomes entrained as a conduit that trails behind sinking metal diapirs. These silicates are then transported to the base of the mantle with implications for mantle and core composition. We perform laboratory fluid experiments which model this process of entrainment and differentiation using glass and gallium spheres in high viscosity liquid sucrose solutions. We use high speed photography and dye to visualize and measure conduit formation and entrainment surrounding a descending sphere. Preliminary experiments indicate that the volume of material that surrounds a sphere is highest at the top of the box when the sphere begins its descent. As the sphere sinks, the entrainment of dyed fluid around the sphere decreases exponentially with falling distance. We suggest this is the result of drag forces acting on walls of the conduit. The volume of entrained material reduces by 80% in the upper 1/3 of the drop trajectory. As much as 8-10% of the original fluid volume is carried to the base of the box and will segregate from the sphere or diffuse after it settles. Higher viscosity fluids are observed to entrain more material on average compared to lower viscosity fluids. These results have important implications for delivery of silicate materials to the base of the mantle and perhaps into the core itself during early formation.