FROM MAGMA OCEAN TO CRUST: UNDERSTANDING MERCURY’S INTERNAL EVOLUTION AND SUBSEQUENT CRUSTAL FORMATION THROUGH EXPERIMENTS AND MODELS (Invited Presentation)

Mercury has a compositionally diverse surface with volcanic provinces that range in composition, age, and possible formation style (e.g., effusive volcanism, impact crater-related). The expansive smooth plains of the Borealis Planitia in the northern hemisphere of the planet are more Mg and Ca depleted than the older Heavily Cratered Terrain/Intercratered Plains that covers the majority of the planet, requiring multiple mantle sources to produce these volcanic compositions. To understand Mercury’s compositionally diverse present-day surface, the mantle structure must be constrained starting with understanding the planet’s magma ocean stage and subsequent mantle evolution. Viscosities of Mercury-relevant magma ocean liquids were measured via falling-sphere viscometry experiments at the Advanced Photon Source, Argonne National Laboratory at mantle relevant pressure and temperature conditions. These results, paired with solidification timescale, grain growth, and crystal suspension models, are used to predict the crystallization style in the cooling Mercurian magma ocean and the resultant mantle structure for magma oceans with and without a flotation crust. Without a graphite flotation crust, the solidification timescale is short (<100 yr) and may cause formation of a mineralogically uniform mantle. With a graphite flotation crust, the magma ocean cools slowly (over thousands of years) and is expected to be mineralogically stratified. A stratified mantle scenario suggest additional processes, such as mantle overturn, must occur to provide the correct source compositions for the different geochemical terrains on Mercury’s surface. We consider the effect that different sulfides may have on the proposed mantle structure and density stratification and how that could influence these additional geophysical processes. Based on different density stratifications in the mantle, overturn could have occurred in Mercury’s mantle resulting in cumulate mixing to produce different source compositions for the igneous provinces seen on the surface today. More precise age and compositional measurements of the geochemical terrains and a better understanding of the possible early graphite crust on the surface of Mercury will be critical to further constraining these mantle solidification models.