Sulfur isotopes are valuable tracers to constrain processes such as the outgassing and ingassing of the magma ocean during planet formation. During these processes, the magma ocean oxidation state controls sulfur speciation, and therefore its solubility, diffusivity, partitioning and isotopic fractionation between the silicate and the gas phase. In this study, we experimentally determine the effects of temperature and oxygen fugacity on sulfur behavior.

Experiments were designed to reproduce the evaporation process in a gas-mixing furnace, under reduced conditions representative of magma oceans. A silicate glass containing 5000 ppm S was melted at various temperatures (1400 to 1600°C), fO2 conditions (IW-2, IW, IW+2), and durations (5 to 90 minutes). Sulfur concentration and isotopic composition profiles were measured by Secondary Ion Mass Spectrometry (SIMS) after investigating the effects of reduced sulfur speciation on Instrumental Mass Fractionation (IMF) and the S content calibration curve, using 13 newly synthetized reduced reference materials. Based on comparison with values determined using oxidized MORBs (SO42-), such analytical effects are negligible. Sulfur concentration profiles and maps were also studied using the Electron Probe Micro-Analyzer (EPMA) after Johnson et al. (2024)'s method, which show perfect agreement with SIMS data.

The acquired maps suggest that diffusion is not the sole process controlling sulfur distribution. Convection seems to play a role in S transport for 5 minutes experiments which does not produce any S isotopic fractionation. Whereas under high fO₂ conditions S maps reveal diffusion patterns, at low fO₂, and for durations ≥20 minutes, samples appear homogeneous, with very low sulfur contents, implying more advanced and probably faster evaporation. At 1400°C, S diffusion from core to rim of the samples produced isotopic fractionation up to 4 ‰ at IW and up to 9 ‰ at IW+2.

Data modelling is ongoing to better characterize the S isotopes mobility during evaporation.