Earth's core formation aided by flow channelling instabilities induced by iron diapirs

Abstract

The core formation mechanism remains poorly known. An unstable gravitational configuration of a dense molten metallic layer overlying a cold chondritic protocore is predicted by most studies, which leads to the formation of a Rayleigh–Taylor (RT) instability. Recent results [Dahl, T.W., 2005. Turbulent mixing during planet accretion and core formation: Interpretation of the Hf/W chronometer and implications for the age of the Moon. M. Sc. Thesis, University of Copenhagen.] indicate that additionally, iron cores of predifferentiated planetesimals are also able to plunge mostly intact into the cold protocore and create large iron diapirs. For both scenarios we propose the application of the stress-induced melt channelling mechanism [Stevenson, D.J., 1989. Spontaneous small-scale melt segregation in partial melts undergoing deformation. Geophys. Res. Lett. 16, 1,067–1,070] in the region surrounding an incipient iron diapir. We therefore perform numerical experiments solving the two-phase, two composition flow equations within a 2D rectangular box with symmetrical boundary conditions. We apply the Compaction Boussinesq Approximation (CBA) and include a depth-dependent gravity. For simplicity we use a constant viscosity for the solid phase and a melt fraction dependent rheology for the partially molten region around the diapir. We investigate the physical conditions under which the melt channels can form and whether they are applicable to the early Earth. As a result, for sufficiently small melt retention numbers iron-rich melt channels develop within a region of approximately twice the diapir's size. This could lead to effective draining of the surrounding region and might initiate cascading daughter diapirs. The region of the protocore drained by this cascading mechanism is expected to significantly increase with depth, and thus indicates an effective mechanism to also extract iron melt from deeper parts of the initially chondritic protocore. This mechanism could effectively accelerate the process of core formation.