A Study of Liquid Iron Settling and Descent During Planetary Core Formation

Comparative Tectonics and Geodynamics (2015) 5015.pdf

A Study of Liquid Iron Settling and Descent During Planetary Core Formation. D. Brand1 and D. S. We- eraratne2, 1California State University Northridge (18111 Nordhoff St, Northridge, CA 91330, United States)

Abstract: In the early impact history of the Earth iron metal ejecta likely segregated from silicate phases during meteorite break up. Settling of molten iron droplets may have occurred within magma oceans or within metal plume conduits descending rapidly to form the metallic cores in terrestrial or rocky planets. Implications for equilibration of liquid metal in the silicate mantle are key in aiding our un- derstanding of the chemical evolution of planetary mantles, including the siderophile trace element abundance in mantle interiors, the nature of metal-silicate segregation, as well as timing for core formation. Previous experiments have shown that descending metal silicate plumes entrain magma ocean material in trailing conduits that travel to the core-mantle boundary. However, the geodynamics of iron settling through a magma ocean and within conduits is only under- stood in a rudimentary way. Here, we consider physical fluid models which study the settling of liquid iron droplets through silicate melts using liquid gallium emulsions and glucose solutions. We test the effect of several physical properties including the metal volumetric ratio, density dif- ference, fluid viscosity, metal droplet diameter, and liquid versus solid metal spheres. Three stages are observed during gravitational settling. Regime 1 reveals rapid sinking of liq- uid metal droplets and entrainment of low-density fluids into a metal pond, regime 2 is characterized by the first stage of upward migration of entrained fluid and regime 3 couples slow compaction of metal droplets at the base with final seg- regation and upward migrating 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. We find that in- creased metal volumetric ratio and liquid metal spheres demonstrate more entrainment of magma into a metal pond. The settling process indicates that each metal droplet is coat- ed by glucose solution and collectively provide a large sur- face area and longer residence time for metal-silicate equili- bration during descent. An emulsion metal pond is shown to go unstable as a Rayleigh-Taylor instability that sinks rapidly to the core, but our results shows that the coating of iron droplets by silicate magma may persist to lower mantle depths and provide an answer to the excess siderophile prob- lem while still descending rapidly to form the Earth's core in 30 My.