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Sulfur speciation in silicate melts under varying pressure and redox conditions from first-principles simulations
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DOI：10.1016/j.gca.2026.07.041.png)
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Abstract
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Sulfur (S) in silicate melts exhibits complex behavior as a function of the oxygen fugacity and pressure, and its speciation and dissolution mechanisms are not fully understood. Here we report the results of first-principles molecular dynamics simulations of S-bearing MgSiO3 melts in the pressure range from ∼0 GPa (2000 K) to ∼17 GPa (3000 K) under varying redox conditions as imposed by adjusting the number of oxygen atoms in the supercell relative to sulfur-free composition ( ΔnO) from 4.2 to −12.5 % (used as redox state proxy). The simulation results and analysis show that sulfur forms dimer ( S2 ), trimer ( S3 ), and SO1,2,3 radicals, which tend to loosely bind with the cations in the oxidized melts for ΔnO≥0 . However, sulfur dissolves predominantly as sulfide ( S2- ) complexes by bonding with cations, preferably Mg2+ over Si4+ in the reduced melts with oxygen deficiency ( ΔnO<0 ). The calculated relative proportions of various sulfur-bearing species are strongly dependent on temperature and pressure. When the sulfur content of the melt is varied from 2.0 to 11.3 wt% under more reducing conditions corresponding to ΔnO=-2.1to-12.5% , we find that sulfur shows weak or no self-interactions and instead exclusively bonds with cations. This suggests that several wt.% of sulfur can dissolve in silicate melts as sulfide complexes under highly reducing conditions and the dissolved sulfur tends to suppress the melt’s density and oxide activities. These findings are expected to have significant implications for melt transport and phase relations during magmatic differentiation of sulfur-rich, silicate reservoirs in highly reduced planets, such as Mercury and exo-Mercuries.
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