Settling of Metal Droplets Through a Magma Ocean and Metal Plume Conduits During Core Formation PDF Download

Author: Danielle Brand
Publisher:
ISBN:
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Languages : en
Pages : 59

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Book Description
The gravitational settling dynamics of molten iron droplets through silicate material is an important process during the early impact history of the Earth where settling may have occurred within magma oceans or within metal plume conduits descending rapidly to form the metallic cores in terrestrial bodies. Implications for equilibration of liquid metal in the silicate mantle are key in aiding our understanding of the thermo-chemical evolution of Earth. To simultaneously meet geochemical constraints for rapid core formation as well as siderophile trace element distribution throughout the upper mantle, an emulsion of iron droplets or an emulsion metal plume may be important. Previous experiments have shown that descending metal silicate plumes entrain magma ocean material in trailing conduits that travel to the core-mantle boundary. It has also been shown that the style of metal emulsions will descend through these conduits in two stages, as a coalesced group within the plume head and later settling through the conduit column. However, the nature of iron settling through a magma ocean and within conduits is only understood in a rudimentary way. Here, I consider physical fluid models which study the settling of liquid iron droplets through silicate melts using liquid gallium emulsions and glucose solutions. I test the effect of several physical properties including the metal volumetric ratio, density difference, fluid viscosity, metal droplet diameter, and liquid versus solid metal spheres. Three stages are observed during gravitational settling. Regime 1 reveals rapid sinking of liquid metal droplets and entrainment of low-density (light element) fluids into a metal pond and into the core itself, regime 2 is characterized by upward migration of entrained fluid and regime 3 couples slow compaction of metal droplets at the base with final segregation of residual glucose solution. Results show that high volumetric ratios and low viscosity ratios of metal to magmas will have faster sinking velocities and metal pond or core formation times. I find that increased metal volumetric ratio and liquid (versus solid) metal spheres demonstrates more entrainment of magma into a metal pond or into the core. Higher levels of entrainment predicts the presence of light elements in the core during its formation and suggests “bottom-up” migration of light elements and metal-silicate segregation at high pressures during post core forming events. Upward migration of light elements will leave behind higher bulk density metals that initiate overturn in the outer core and can assist in powering the geodynamo. Finally, the settling process of emulsion metal droplets through magma oceans, plume conduits, and entrainment into the core provide a large surface areas and longer residence time for metal-silicate equilibration to address the excess siderophile observation while still descending rapidly enough to form the Earth's core in 30 My. I observe a metal sediment layer that forms above the metallic core after settling. If this sediment layer is stable, it may be entrained in upwelling mantle plumes over the Earth’s history and contribute towards mixing of siderophile elements with mantle minerals.