Experimental investigation of Zn isotope fractionation during sulfur-bearing magma degassing

Moderately volatile metal elements like Zn in may be efficiently transported into the gas phase during magma ascending, and during vaporization may fractionate elements and their isotopes between gas and liquid [1-2]. The effect of major volatile elements like S in the melt on the volatilization process of Zn has not yet been investigated systematically. Here we present an experimental approach to test the effect of S on the evaporation of Zn by comparing the chemical and isotopic compositions of silicate glasses that were formed from both, S-bearing and S-free melts in a f(O2)-controlled gas mixing system at 1 bar.

Two series of evaporation experiments (S-bearing and S-free) were conducted with a temperature gradient of 1200-1400 °C, a log f(O2) = -13.2 to -0.68 (at air), and a time duration from 10 to 30 min. The results indicate that the volatility of Zn increased with decreasing f(O2) and increasing temperature during evaporation, while the isotope fractionation factor remained constant. Besides, the presence of S in the silicate melt seems to have a strong effect on the volatilization of Zn, resulting in less Zn remaining and larger Zn isotopic fractionations under highly reduced conditions. Our experiments reveal that the presence of major volatile elements such as S or Cl may have critical impact on the degassing of moderately volatile metal elements, resulting in a poor correlation with their 50% condensation temperatures.

[1] Sossi et al., 2020 GCA 288, 316–340. [2] Renggli and Klemme (2020) JVGR 400, 106929.