Isotopic fractionation of copper during silicate melt evaporation: effects of sulfur, fO2, and temperature

Florin, G; Pangritz, P; Renggli, CJ; Gleißner,P; Berndt,J; Rohrbach, A; Becker, H; Klemme, S

Zusammenfassung

Moderately volatile trace elements (MVE) such as Cu are emitted in large quantities from volcanoes on Earth and are tracers and are important for volatility related processes on other planetary bodies such as the Moon. The evaporation of MVE from silicate melts is complex, as evaporation rates change with temperature, oxygen fugacity, and may also be influenced by the presence of ligand-forming elements such as S. While on Earth, MVE evaporation and their ligands can be evaluated by in situ sampling at volcanic vents and fumaroles, this is not possible on the Moon. Isotopic investigations of Apollo lunar picritic glasses provide evidence for evaporation and condensation processes on the lunar surface. Unlike Earth’s oxidized volcanic activity, evidence suggests that ancient lunar volcanism was more reduced. To study the effect of temperature, oxygen fugacity, and the influence of S on the degassing of Cu from silicate melts, we conducted experiments at 1150–1500 °C and different oxygen fugacities (logfO2) ranging from strongly oxidizing conditions (i.e., in air) to reducing conditions that correspond to a fO2 that is 6 log units lower than the Fayalite-Magnetite-Quartz buffer (FMQ). Experimental results show an increase of Cu volatility associated with strong isotopic fractionation with increasing temperature, and with increasingly reducing conditions for all starting materials. We also observe that the temperature and redox conditions have a competing effect on Cu evaporation and isotopic fractionation. In S-bearing systems, Cu content decreases together with increasing isotopic composition as the conditions become more reducing. The isotopic fractionation becomes less pronounced as the temperature increases from 1200 to 1500 °C while the Cu content continues to drastically decrease. As such, we observe a lower depletion in Cu content but a stronger enrichment of 65Cu in the residue at 1200 and 1300 °C compared to 1400 and 1500 °C, and a negative correlation between Cu content and δ65Cu under the most reducing conditions. We relate this to the behavior of Cu isotopes that are more fractionated in presence of S in the melt, and to Cu being more volatile than S under reducing conditions. We report α fractionation factors in the CuS system increasing from 0.9973 in oxidizing conditions to 0.9997 at high temperatures and reducing conditions (1400–1500 °C, ΔFMQ −2 to −6). These results suggest that the presence of the ligand-forming element S strongly influences Cu degassing at low temperatures and under oxidizing environments, such as terrestrial fumaroles. Its influence is more limited during degassing of hot magmas and under more reducing conditions, such as on the Moon. In agreement with these observations, we report lower Gibbs-free energies of formation for CuO compared to CuS, implying that less energy is required for Cu evaporation in the presence of S. Therefore, Cu evaporation and related isotopic fractionation under isothermal conditions is expected to be higher in S-dominated systems compared to S free experimental systems. This conclusion is in contrast with Heck et al. (2025) and highlights the significant role of Fe in stabilizing FeS molecules in the silicate melt.