The effects of S, Cl and oxygen fugacity on the sublimation of volatile trace metals degassed from silicate melts with implications for volcanic emissions

Volatile trace metals emitted to the atmosphere by volcanism can be complexed with ligands in a gas phase that undergoes large changes in temperature (T) and oxygen fugacity (fO(2)). To study this effect, a modified vertical tube gas-mixing furnace was used to investigate the role of S and Cl on the degassing and sublimation of volatile trace metals from silicate melt at 100 kPa over a range of fO(2) from FMQ+5 to FMQ-4.6 at the melt-gas interface (where FMQ is the fayalite-quartzmagnetite buffer). Analogue phonolitic melt compositions in the Na2O-Fe2O3-Al2O3-SiO2 systems containing trace metals (Li, V, Fe, Cu, Ni, Zn, As, Y, Mo, Cd, Sn, Yb, Rb, Cs, Ag, Pb and Bi) are degassed at 950 degrees C and form sublimates along a temperature gradient (25-900 degrees C) in a suspended silica glass tube. Silica polymorphs are ubiquitous sublimates at all temperatures; halite is common below 600 degrees C with other less-easily identified oxides, sulfides, sulfates, chlorides and molybdates in trace quantities. The trace metal concentrations in the sublimates show trends used to infer their transport and deposition with temperature, fO(2) and ligand identity. The sublimation of metals is mainly controlled by T. Monovalent Rb, Cs, and Ag sublime in halides at 350 +/- 50 degrees C with transport dominated by Cl. The effect of fO(2) is only observed for Zn, As, Pb, Cd, Bi, Fe, and Ni below similar to FMQ-3, where they change their sublimation host from sulfates or sulfides at 500 +/- 50 degrees C to halides at 350 +/- 50 degrees C. Molybdates subliming at 600 +/- 50 degrees C host Li and Mo. No discernible host phase for Cu and Sn is observed with decreasing temperature. The experiments suggest different suites of trace metals in volcanic systems are transported with either S (Zn, As, Pb, +/- Cd, +/- Bi) or Cl (Rb, Cs, Ag, Fe, Ni, +/- Cd, +/- Bi) depending on T and fO(2). Eruptions rich in Cl make halide sublimates a likely significant source of volatile trace metals to the environment. (C) 2021 Elsevier Ltd. All rights reserved.

Automated summary

The study investigated the sublimation of volatile trace metals during volcanic activity, revealing that temperature plays a dominant role in metal sublimation, while chlorine significantly influences the transport of Rb, Cs, and Ag. The effect of fO(2) is observed for some metals under specific conditions, determining their sublimation host. The transportation of different metals in volcanic systems depends on temperature and oxygen fugacity.