On the Potential for Cumulate Mantle Overturn in Mercury

Abstract

AbstractMercury has a compositionally diverse surface exhibiting geochemical terranes that represent different periods of igneous activity, suggesting diverse mantle source compositions. Mercury's juvenile mantle likely formed after fractional solidification of a magma ocean, which produced distinct mineralogical horizons with depth. To produce the diversity of observed volcanic terranes, dynamic mixing of materials from distinct mantle horizons is required. One process that could dynamically mix the juvenile cumulate pile is cumulate mantle overturn, where dense layers in shallow planetary mantles sink into deeper, less dense layers as Rayleigh‐Taylor instabilities. Gravitationally unstable density stratification is a requisite starting condition for overturn; solidification of the Mercurian magma ocean is likely to have produced such a density inversion, with a relatively dense clinopyroxene‐bearing pyroxenite layer atop lower density dunite and harzburgite layers. Sulfides are present in abundance on Mercury's surface and would be additional mantle phase(s) if they are indigenous to the planet's interior. Sulfides have variable densities; they could potentially enhance the formation of gravitational instabilities or prevent them from developing. Exploring physically reasonable mantle density and viscosity structures, we evaluate the potential for cumulate mantle overturn in Mercury and predict the possible timing, scale, and rate of overturn for plausible physical parameter combinations. Our analysis suggests that overturn is possible in Mercury's mantle within 100 Myr of magma ocean solidification, providing a mechanism for producing the mantle sources that would melt to form surface compositions on Mercury, and overturn may control the spatial scale of volcanic provinces observed on the surface today.