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

Abstract

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 (fO2). 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 fO2 from FMQ+5 to FMQ−4.6 at the melt-gas interface (where FMQ is the fayalite-quartz-magnetite 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 °C and form sublimates along a temperature gradient (25–900 °C) in a suspended silica glass tube. Silica polymorphs are ubiquitous sublimates at all temperatures; halite is common below 600 °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, fO2 and ligand identity. The sublimation of metals is mainly controlled by T. Monovalent Rb, Cs, and Ag sublime in halides at 350 ± 50 °C with transport dominated by Cl. The effect of fO2 is only observed for Zn, As, Pb, Cd, Bi, Fe, and Ni below ∼FMQ−3, where they change their sublimation host from sulfates or sulfides at 500 ± 50 °C to halides at 350 ± 50 °C. Molybdates subliming at 600 ± 50 °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 fO2. Eruptions rich in Cl make halide sublimates a likely significant source of volatile trace metals to the environment.