Production and Preservation of Sulfide Layering in the Mercury's Mantle

A striking feature of the Mercury's surface is the high S and low FeO contents of the surface currently best explained by very reducing conditions compared to other terrestrial bodies. Recent analysis of the data of the Gamma-Ray spectrometer (GRS) and X-Ray Spectrometer (XRS) onboard of the MESSENGER spacecraft have provided maps of Mercury's surface composition. For all geochemical terrains identified, the correlation between Ca/Si and S/Si suggests the presence of oldhamite and other sulfide minerals as observed in very reduced chondrites. This work investigates the effects of primordial sulfide layering on the first 750 Myr of Mercury's mantle dynamics. It is argued that sulfide layering could have been produced by fractional solidification in the highly reduced Mercury magma ocean (MMO). Such chemical layering implies mantle sources with variable sulfur contents that might have played an important role in early mercurian magmatism. Our models investigate the production of sulfide-rich layers and their preservation during post-MO solid-state mantle dynamics. An intriguing question is the role of these sulfide rich layers on mantle dynamics as they may incorporate substantial amount of radioactive heat producing elements (U, Th and K) and might be the low fO₂ equivalent of the lunar KREEP layers.

We use experimentally determined sulfur solubility in silicate melts to predict the depth at which sulfide precipitate during MMO crystallization. The model produces primordial sulfide layers whose thickness and locations depend upon the oxygen fugacity (fO₂) and initial sulfur content (S_init) of the MMO. A geodynamic model is then used to test under which conditions a primordial mineralogical layering would be mixed by mantle convection. Several geodynamic regimes have been identified in the fO₂-S_init space. This study shows that oxygen fugacity, bulk sulfur content and sulfide segregation are key for the early thermochemical evolution of Mercury.