Abstract :
[en] Mercury, the closest planet to the Sun, has a significant sulfur content in its crust (0.5-3.5 wt% S). At depth, sulfide phases might be abundant in the mantle and a FeS layer could also be present at the top of the core. During the early history of the planet, sulfur is thought to have significantly influenced the planet’s differentiation with effect on core formation (Fe-Si-S system), mantle melting, and crustal production. In particular, S-bearing phases could be a reservoir for heat-producing elements (U, Th and K) as the latter may become chalcophile under the reducing conditions that prevailed during Mercury differentiation.
Here we present a series of experiments performed using an IHPV, a piston-cylinder and a multi-anvils apparatus in order to investigate phase equilibria in Mercury analogues and to constrain the distribution of heat producing elements between the silicate mantle, the metallic core, and sulfides. Experimental conditions covered a large range of pressures (1 kbar to 60 kbar), temperatures (1500-1700°C), and oxygen fugacities fO2 (IW -2 to IW -8). The starting materials consisted of mixed pure oxides representing the silicate portion of an average enstatite chondrite, considered to be a good proxy for the composition of Mercury’s mantle. We used a range of SiO2/Si ratio in the starting mixtures to control the fO2 and added FeS ± S ± CaS. Powders were doped with U, Th and K as well as with other minor and trace elements. All experiments contain a silicate melt phase, a metal melt phase (FeSi), and sulfides ((Ca,Mg)S and/or FeS). The phases were analyzed using electron microprobe and LA-ICP-MS. (Ca,Mg)S phases were rare and difficult to measure due to the small size of the grains. These phases usually contain several percent of Cr, Mn and Ti, and FeS incorporate a few percent of these as well. The Si content in metal varies between 5 and 25 wt%, depending on fO2. We also observe that U, Th and K present an increasing chalcophile behavior with decreasing fO2.
These new experimental results will be interpreted in terms of distribution of heat producing elements (U, Th and K) between the different reservoirs of Mercury. Implications for the volatile budget of the planet and its thermal evolution during planetary cooling will be discussed.
