Experiments on metal-sulphur-silicate melts equlibria under the reducing conditions of Mercury

Abstract

Mercury has a very high bulk density, with an iron core that makes up for 80% of the planet's radius. Yet it has a very low FeO content on the surface. Very high sulphur content is observed at the surface (up to 4 wt%), making Mercury a surprisingly volatile rich planet. The planet was formed under highly reducing conditions, creating unique conditions that change the behaviour of elements. This study aimed at understanding the stability field of FeS and (Ca,Mg)S sulfides in these conditions. The element partitioning between the silicate melt, the metallic melt and the sulfide melt was also studied. A piston-cylinder was used to produce high temperatures (1300-1700°C) and high pressures (~1.2 GPa) conditions relevant for crystallization conditions during the magma ocean stage. The first aspect of the project was to setup the apparatus and established an appropriate protocol to use it. Several days of testing and adjustments were necessary to obtain a fully functional device. The composition used for the experiment was similar to enstatite chondrite, as their silicate composition is close to the bulk silicate composition of Mercury. Sulphur (20 wt% and 10 wt%) was added to the powders to saturate the silicate melt in sulphur. FeS (10 wt%) was also added in order to obtain a metallic phase (FeSi) and to measure the oxygen fugacity in the samples. The oxygen fugacity was controlled using silica metal in the starting compositions. Experimental products contain a silicate and a metallic (FeSi) melt. SiO2 under high quartz form is present in most of experiments. Some low temperature samples show enstatite grains, and some experiments produced Si metal. FeS globules were formed in four samples, which implies that only four experiments reached sulphur saturation. Our experiments failed at producing (Ca,Mg)S under the investigated conditions. The composition for major elements of the silicate, the metallic and the sulphide melts were acquired using the electron microprobe at the University of Hannover, Germany. Partition coefficient between the silicate melt and the metallic melt were concordant with other studies; Si become siderophile at high reducing conditions, and Mn and Ti also shows a siderophile behaviour at low oxygen fugacity, which is in good agreement with other studies. Temperature also has an influence on the behaviour of elements; at higher temperatures, Mn, Ti and Na show increasing siderophile behaviours. Phosphorus on the other hand become less siderophile with temperature. Concerning the partition coefficient between the silicate melt and the sulphide melt, Ti becomes increasingly chalcophile with decreasing fO2. Several hypotheses can explain this lack of (Ca,Mg)S in our experiment. First of all, it appears clearly that a large amount of sulphur is lacking in the samples; approximately 23 wt% S were added to the powders (from 20 wt% S + 10 wt% FeS) and in non saturated experiments, only ~5% S is left in the silicate melt. At high temperature, sulphur escaped and saturation was not obtained in most experiments. This does not explain the absence of (Ca,Mg)S sulphides in the saturated experiments. Temperature, pressure and oxygen fugacity ranges are similar to other studies that produced these sulphides, the only difference being the amount of FeS they added; we added significantly less FeS than them and too much S pure. We thus propose that sulphur fugacity (fS2), which is the only parameter that was not directly controlled in our experiment, controls the apparition of these phases. The addition of FeS in experiments could control the fS2 via the iron-troilite buffer. The siderophile behaviour of Si at low oxygen fugacity could imply that significant amount of silicon partitioned into the core during the planet differentiation. This result has been confirmed by numerous studies. A bit of Mn and Ti could also segregate into the core. We propose that the chalcophile behaviour of Ti could form TiS (wassonite) as discovered recently in an enstatite chondrite. The stability of this mineral would have to be defined in future experiments, but, as the author of the discovery proposed, TiS could be the residue of evaporating Ti-bearing troilite. As space-weathering is an important phenomenon on Mercury, it would be interesting to study the partitioning of Ti in FeS and the evolution of FeS once exposed at the surface.