Condensation behaviour of volatile trace metals in laboratory benchtop fumarole experiments

Volatile trace metals emitted from volcanoes condense in gas plumes and fumaroles. The transient nature of natural eruptions makes it challenging to isolate key variables affecting trace metal condensation in this complex natural system. To emulate these conditions in a controlled setting, we design a laboratory 'benchtop' fumarole to experimentally measure volatilization and condensation behaviour of volatile trace metals from magma. Synthetic silicate melt compositions in the Na2O-Al2O3-SiO2 and Na2O-Fe2O3-SiO2 systems doped with dissolved trace metals (V, Cu, Zn, As, Y, Mo, Cd, Sn, Tb, Pb and Bi) are degassed in a furnace at 900 °C over periods of days to weeks. The condensates from the gas phase form on a silica glass tube along a thermal gradient from 725 to 125 °C, and are examined by electron microscopy and chemical analysis. We observe variable crystallinity of condensates as functions of temperature. The concentrations of Li, Cu, As, Rb, Mo, Ag, Cd, Cs, W, Pt, Tl, Pb and Bi in leachates of the condensates change by orders of magnitude along the glass tube, and show maxima at various temperatures between 250 and 600 °C. A comparison of the experiments with natural examples shows similarities in the condensation pattern for Cu, As, Ag, and Tl. The enrichment of certain metals in the experimental gas condensates (e.g. Mo) is similar to that observed in natural systems, but differs greatly for other elements (e.g. Bi and Cd) likely due to lack of Cl, S or other complexing agents for metals in the experiments. Our experimental design is a starting point to investigate the role of these and other variables on trace metal condensation behaviour in natural volcanic emissions.