Differentiation and delivery of an enriched deep mantle reservoir during iron descent to the core.

Abstract:

Planetary interior differentiation from a bulk silicate chondrite composition is shown by geochemical studies to occur early in planetary evolution producing separated enriched and depleted mantle reservoirs with important implications for the mantle and crustal compositions that we observe today. The absence of an enriched component at the Earth's surface, however, and has lead to implications of a reservoir at the base of the mantle, but the mechanism of differentiation or downward transport of this enriched material is unknown. Here we present results from laboratory fluid dynamic experiments using liquid metal to show that metal-silicate segregation from a metal pond which forms in a magma ocean following meteorite impacts will entrain magma ocean silicate material to the base of the mantle during metal descent to the core. We model liquid iron and silicate magma using emulsified liquid metal gallium in high viscosity glucose solutions which provide the buoyancy ratios and Stokes flow regimes expected for planetary interiors. Preliminary results indicate that emulsion metal droplets sink together as a Rayleigh-Taylor instability and forms a trailing conduit of buoyant solution. Metal droplets form a pile at the base of the box where the low density solution collects, grows, and initially rises back to the surface as a thermo-chemical plume. The remaining buoyant material, which surrounds each droplet, slowly migrates upwards and rises out of the metal pile. These physical experiments scaled to planetary interiors provide important tests of purely theoretical or numerical approximations and indicate that metal-silicate segregation is consistent with rapid core formation times and contributes simultaneously to complex mantle differentiation at all depths. Our observation of entrainment of a silicate-metal conduit provides a model for differentiation and sequestration of an enriched reservoir from a magma ocean to the base of the mantle. The composition and buoyancy of the rising thermo-chemical phase is tested with Stokes theory and modifications made for reduced drag and the added mass of conduit material indicate that rise times and differentiation during ascent are consistent with magmatism for proto-continental crust formation as well as later effusive volcanism from remnants of this reservoir.