Laboratory Fluid Experiments on Metal-Silicate Plumes and Core Formation

Differentiation and formation of the Earth's iron core from proto-planetary material is suggested to have occured rapidly (< 30 My) under very energetic and violent accretionary conditions. Such high impact bombardments and evidence from mantle siderophile abundances suggest the presence of a deep magma ocean ({≥ 400} km) in the early Earth that facilitated metal-silicate equilibration. High temperatures of formation are also implied by the ancient dynamo, indicating a superheated core in the past. Conventional core formation models, however, have difficulty satisfying the simultaneous requirement of rapid formation with the residence time needed for metal-silicate equilibration. A favored model for core formation envisions impact-induced metal drops that equilibrate with silicates in the magma ocean, concentrate at the magma ocean base, and then descend to the core as large diapirs or instabilities. We report results from fluid experiments of liquid gallium in stratified sucrose solutions. To model core formation processes, we consider three cases of liquid metal instabilities through the stratified fluids: 1) descending single, small diameter metal drops; 2) metal ponding, instabililty, and descent of large diameter metal diapirs; and 3) ponding of tiny metal droplets in an emulsion, instability, and descent of emulsion diapirs. The experimental parameters represent viscosity ratios ({μsucrose/μga}) up to {106}, Reynolds numbers from {10-6} to {10-4}, and Bond numbers from 2.5 {10-1} - {101}. The most interesting observation in all experiments is the formation of trailing conduits behind sinking metal drops which fill with the upper layer, low viscosity material. These conduits provide extended residence time for chemical exchange and equilibrium between metal and silicate material during descent, after the metal pond goes unstable. We suggest the presence and long life of trailing, fluid-filled conduits may be a mechanism for reconciliation of models which require rapid core differentiation and formation and simultaneous metal-silicate equilibrium. Our experiments also indicate that metal-silicate mantle plumes may evolve into buoyant thermal plumes, connecting core formation to ancient hotspot activity on terrestrial planets.