The Magma Ocean of Mercury and the Role of Sulfur on the Early Evolution of the Innermost Terrestrial Planet

Planet Mercury exhibits unique geochemical features and physical properties among the planets in our solar system. Mercury is the smallest terrestrial planet and is characterized by a large metallic core. As the planet accreted, the heat originated from early accretional impacts, core-mantle differentiation, and radiogenic decay was sufficient to melt Mercury. This early period, known as the “Magma Ocean” stage, has been first invoked for the Moon and is thought to have occurred in most terrestrial planets. As the silicate and metal portions separated, the crystallization of the silicate part resulted in the formation of the primordial mantle. What occurs during this critical period sets the stage for all subsequent events that shaped Mercury as we see it today. This includes mantle remelting and the production of a secondary magmatic crust, as well as the stability and storage of volatile elements that are crucial for planetary dynamics and building of an atmosphere. The data returned by NASA’s MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) spacecraft revealed an unusually high concentration of sulfur of surface volcanic units coupled with the paucity of iron. This indicates highly reduced conditions (or low oxygen availability) of formation of surface lavas. Sulfur is thus a major mantle volatile, and its occurrence has profound implications for melting temperatures, phase equilibria and, consequently, the vertical structure of the mantle. The goal of this doctoral thesis is to investigate the structure of the primordial mantle of Mercury as the direct result of the crystallization of its magma ocean, to provide a revised standard model for the evolution of the planet in its early history. Piston-cylinder experiments were performed on compositions that span Mercury’s magma ocean’s evolving silicate liquid, over a vast range of temperature (1125 – 1950 ℃), pressures (0.5 – 3.0 GPa), and oxygen fugacity (from IW-3.0 to IW-8.5, with IW being the iron-wüstite solid buffer). Experimental results (phase equilibria, major element distribution) were then combined with petrologic modelling to simulate the fractional crystallization of Mercury’s magma ocean (MMO), to reconstruct the stratigraphic sequence of Mercury’s primordial mantle. Our results show that the structure of the primordial mantle of Mercury relies heavily on the Bulk Silicate Mercury (BSMe) composition and the presence of sulfur. Starting compositions with a low Mg/Si plus sulfur favour enstatite over forsterite, stabilize quartz, and delay the formation of clinopyroxene and plagioclase. In addition, Mg-, Ca-rich sulfides may segregate from the magma ocean, and their abundance will depend on the initial S content of BSMe. The mantle of Mercury is dominated by either orthopyroxene or olivine depending on the BSMe composition. Based on our modelled crystallization sequences, we identify both refractory and fertile mantle reservoirs. The former feature either olivine, orthopyroxene, or both coexisting phases in the lower cumulus, while the latter are defined by the appearance of clinopyroxene. Sulfur strongly influences the composition of the residual MMO liquid, leading to SiO2 enrichment in S-bearing melts, whereas S-free liquids become SiO2-depleted. We demonstrate that sulfur behaves like other common volatiles, as it lowers the liquidus temperature and the density of Mercury-like silicate melts. Finally, we show that the storage of heat-producing elements (HPE) in Mercury’s mantle is affected by sulfur. Indeed, the presence of sulfides segregated from the magma ocean, coupled with trapped liquid in the early cumulates will enhance HPE storage in the upper primordial mantle of Mercury, which is hypothesized to host a Moon-like KREEP layer.