Mercury-related projects
Experimental Investigation of the Role of Sulfur
in Highly Reduced Silicate Glasses and Melts
A. Pommier, M.J. Tauber, H. Pirotte, G. D. Cody, A. Steele, E. S. Bullock, B. Charlier, and B. O. Mysen (2023, GCA).
Elucidating sulfur speciation and bonding at the atomic scale is required to understand transport properties in S-bearing melts such as diffusivity, viscosity, and electrical conductivity at high temperature and pressure. These properties are fundamental to modeling the evolution of terrestrial planets and moons. Despite several investigations of sulfur speciation in glasses formed by temperature-quenching of their melts, questions remain regarding the structural role of S and its effect on transport properties under highly reducing conditions such as S-rich lava on Mercury.
We studied the role of sulfur in silicate glasses formed by quenching of melts at 1573-1673 K synthesized in evacuated silica tubes under highly reducing conditions (Delta IW=-5.8 and -6.4, with IW the iron-wüstite oxygen fugacity buffer). The compositions reproduce the silicate portion of enstatite chondrites, representative of the northern volcanic plains at the surface of Mercury. Major cations in the silicate glasses included Al, Mg, Ca, Na, and K; S was varied from 0 to ~5 wt.%. The samples were characterized with impedance spectroscopy performed at 2 and 4 GPa and from 475 up to 1738 K using a multi-anvil press and the 4-electrode technique, 29Si MAS NMR spectroscopy, Raman spectroscopy, and electron microscopy.
We observe that conductivity generally increases with the S content of the glass, though no systematic correlation is observed. Electrical activation energy Ea below the glass transition temperature ranges from 0.56 to 1.10 eV, in agreement with sodium being the main charge carrier in all samples. The glass transition is located at 650-750 K based on impedance measurements. Above Tg, Ea decreases (0.35-0.68 eV) and the conductivities of the samples are comparable (~5-8.10-3S/m) until 973 K. At T>1600 K, the melt fraction is 50-70% and melt conductivity varies from 0.8 to 2.4 S/m, with the melt containing 5 wt.% S being the most conductive. 29Si NMR results reveal that a portion of S bonds with silicon by substitution for oxygen, as previously observed in Na2S-SiO2 glasses (Asahi et al., 1998), affecting slightly the degree of polymerization of the glass. This result is in accord with a study on viscosity of S-bearing melts (Mouser et al., 2022). Raman spectra show that some S is isolated from the glass-network structure and combines with divalent cations, Ca2+and Mg2+, to form sulfide clusters in the glass, consistent with previous work (Namur et al., 2016). Our results also indicate that CaS and MgS are not as strong network modifiers as CaO and MgO. Our spectroscopic measurements do not reveal direct interactions between sulfur and sodium, which is consistent with only a moderate effect of S on conductivity. This study not only illuminates the role of sulfur in highly reduced complex silicate glasses and melts, but also exemplifies a multi-disciplinary approach that would be useful for the investigation of other geomaterials.
The multi-anvil cell assemblies used for electrical measurements are available to the scientific community via ASU.
Back-scattered electron image of an S-bearing glass sample containing spherical sulfides retrieved from an electrical experiment at 2 GPa and quenched at 1738 K.
Understanding core cooling processes
Pommier et al. (2019, EPSL); Davies et al. (in prep.).
The thermal state and chemistry of the metallic core of terrestrial planets and moons governs their evolution and in particular, the intrinsic magnetic field. The weak intrinsic magnetic field of Mercury is intimately tied to the structure and cooling history of its metallic core. Recent constraints about the planet’s internal structure are consistent with the presence of a FeS layer overlying a silicon-bearing core. In Pommier et al. (2019), we performed 4-electrode resistivity experiments on core analogues up to 10 GPa and over wide temperature ranges in order to investigate the insulating properties of core materials. Our results show that the FeS layer is liquid and insulating, and that the electrical resistivity of a miscible Fe-Si(-S) core is comparable to the one of an immiscible Fe-S, Fe-Si core. The difference in electrical resistivity between the FeS-rich layer and the underlying Fe-Si or Fe-Si-S core is at least 1 log unit at pressure and temperature conditions relevant to Mercury’s interior. If present, a thick (>40 km) FeS-rich shell is expected to maintain high temperatures across the core, and if temperature in this layer departs from an adiabat, then this might affect the core cooling rate. The presence of a liquid and insulating shell is not inconsistent with a thermally stratified core in Mercury and is likely to impact the generation and sustainability of a magnetic field.