Dynamics of heat producing elements rich domains in rocky planets

The isotopic compositions of lavas from mantle plumes provide evidence for deep mantle heterogeneities and have been associated with primordial mantle material. However, little is understood about how such material formed during the early stages of planetary evolution. Its origin is typically linked to processes such as the sedimentation of iron-rich phases and crystallization in a primordial magma ocean, or, alternatively, to impacts during the later stages of planetary formation. These processes operated under varying temperature and pressure conditions, likely leading to a depth-dependent composition. Regardless of how it originated, this primordial material is thought to contain higher concentrations of radioactive elements compared to the upper mantle. We aim to address a critical question: how does a compositionally stratified mantle evolve over time under convective motions. These motions reshape the boundaries of chemically distinct domains and promote mixing. Therefore, it is crucial to understand the conditions that allow primordial material to persist at the mantle's base over long timescales, particularly in relation to differences in density and heat production between various mantle components.

To investigate this question, we conducted an in-depth experimental study of convection in a stratified system consisting of two fluids with distinct intrinsic densities and heat production rates. We derived scaling laws that connect the dynamical characteristics of convection to the key dimensionless numbers. These scaling laws, coupled with plausible physical parameters, are then applied to extrapolate the results to planetary mantle convection. We illustrate our approach with a diagram relating the effective partitioning coefficients of iron and that of heat producing elements to the lifetime of the stratified mantle.