| Abstrakt: |
Mercury 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. Plain Language Summary: The surface of Mercury is compositionally and morphologically diverse, suggesting multiple periods of volcanic activity during the planet's early history. The range of compositions on the surface of Mercury suggests that the volcanic source compositions in Mercury's interior are also diverse. After a magma ocean stage, Mercury may have had a layered mantle with distinct mineralogical horizons, with dense layers overlying less dense layers. Mixing of the layers is needed to produce the compositional variety seen on the surface today. A process called cumulate mantle overturn could have effectively mixed the mantle layers. This process involves the sinking of the denser material into less dense material. This study explores the possibility of mantle overturn by evaluating physically reasonable properties of Mercury's mantle (e.g., density, layer thickness, viscosity). The results suggest that mantle overturn was possible within 100 Myr of solidification of Mercury's magma ocean and can explain the chemical diversity seen on the surface of Mercury today. Key Points: We calculate the density structure of Mercury's hypothesized mantle, post‐magma ocean solidificationWe evaluate the density and thermal effects of heterogeneous distributions of sulfides in Mercury's mantleUsing scaling relations, we explore the rate and scale of cumulate overturn and its potential to produce diverse surface terranes [ABSTRACT FROM AUTHOR] |
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