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

We report 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 65Cu 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  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, DFMQ -2 to -6).