Experimental study of the sublimation behaviour of volatile trace metals during volcanism

Date

2020-08-27

Authors

Scholtysik, Rebecca Ann

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Abstract

Volcanoes are a key component of the Earth system, with volcanic activity reaching from deep in the Earth’s mantle and extending to interactions with volcanic gases and the atmosphere. Volatile trace metals degas from volcanic eruptions and at fumaroles, but their behaviour is poorly understood. I designed and built a benchtop fumarole, from which I degassed a silicate melt with trace metals, to simulate the volatilization and sublimation of trace metals from volcanic gases. I collected sublimates along a temperature gradient to examine the behaviour of the trace metals. The experimental sublimates were analysed for their chemical composition and phase identification. Lithium, Cu, As, Rb, Mo, Ag, Cd, Cs, W, Pt, Tl, Pb and Bi were found to be volatile and sublimed in elevated concentrations at various temperatures between 250-600°C. Compared to natural fumarole studies, similar volatile behaviour is seen for Cu, As, Ag and Tl. Variability between the experimental and natural fumarole sublimates is proposed to be from a lack of ligands in the experiments. Ligands can complex with trace metals, to transport and sublime mineralogical phases. Given the importance of ligands to metal complexation, I proceeded to examine the importance of chloride as a ligand in volatile transport and sublimation of trace metals. I degassed a silicate melt with trace metals and variable concentrations of Cl-, up to 2 wt% Cl-, in air. Sublimates produced from these experiments were analysed for mineralogical and chemical information. Raman spectroscopy and scanning electron microscopy helped to determine that silica polymorphs occur at all temperatures and that halite forms below 600°C. Additional phases, including hydrated phases transporting Mo, Cu and Pb also formed as sublimates. These hydrated phases are suggested to be hydrated post-experiment or are Cl--bearing analogues. The addition of Cl- to the experiments increases the concentration of Li, Rb, Cs, Ag, Cr, Cu, Mo and W in the sublimates compared to Cl-free experiments and Cl-bearing phases are likely hosts of volatile trace metals. Volcanic gases in nature do not have the oxygen fugacity of air and contain considerable S. To conduct sublimation experiments at various lower oxygen fugacities and with S as it is a redox sensitive ligand, I adapted my original benchtop fumarole design to a gas-mixing furnace, in which I degassed silicate melts containing S, Cl and trace metals. Substantial loss of S and Zn, Sn, As, Bi, Pb and Cd occurred from the starting material melt in the most reduced experiment at 4.6 log units below the FMQ buffer. This loss corresponded to increased concentrations of the same elements in the sublimates of the same experiment. These trace elements are likely hosted as sulfide minerals, as the fO2 conditions are in the sulfide stability field. This agrees with thermodynamic calculations that determine that sulfides should be stable in similar conditions to this experiment. Chlorides are sublimed in experiments from ~200-650°C and are likely subliming as a NaCl-KCl-FeCl3 solid solution. Halite is calculated to form at all temperatures in the experiments, based on modelling. These chlorides are probably hosting Cu, Cd, Bi, Li, Rb and Ag in the experiments. In nature, if these metals are in soluble salts, when leached they provide a source of metals to the environment where they are deposited. Overall, I demonstrated that trace metal behaviour in the sublimates from volcanic gases will be affected by available ligands and the oxygen fugacity of the melt and the gas. Chlorides are a likely phase to host trace metals and are ubiquitous in experiments, even with variable melt compositions, fO2 conditions and across a wide temperature range.

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Keywords

Volcanology, Trace metals, Volatility, Fumarole, Geology, Experimental Petrology, Geochemistry

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