The crystallization of Mercury’s magma ocean and the formation of its primordial mantle structure

Early in its history, Mercury underwent a magma ocean stage; its crystallization produced a primordial mantle and a flotation crust, setting the stage for early volcanism, crustal production, and thermochemical evolution. Here, we performed crystallization experiments on reduced, sulfur-rich silicate melt compositions relevant to Mercury’s magma ocean and its solidification during cooling. Our approach aims to reconstruct the primordial mantle stratigraphy by combining a fractional crystallization model with phase equilibria experiments on a suite of residual melts at 1525–1125 ℃ and 1.5–0.5 GPa under low oxygen fugacity (-3.7 to -8.4 log units below iron-wüstite equilibrium) to investigate the crystallization sequence for two potential Bulk Silicate Mercury compositions: a low-Mg/Si melt in the enstatite stability field and a high-Mg/Si melt in the forsterite stability field. Residual melts become co-saturated in enstatite and forsterite, followed by the crystallization of clinopyroxene at melt fractions F = 0.40–0.35, quartz at F = 0.28–0.24, and plagioclase at F = 0.19–0.14. We define the evolution of the mantle cumulate pile and the thickness of the refractory and fertile reservoirs based on the appearance of clinopyroxene. We propose that Mercury’s volcanic crust resulted from partial melting of the fertile mantle. Density calculations indicate that sulfur reduced the density of the silicate magma ocean, causing sulfides to become denser than the magma ocean, ultimately being stored in the mantle. We illustrate the influence of the magma ocean bulk composition ± sulfides on the storage and spatial distribution of heat-producing elements in Mercury’s interior.