Investigation of the effect of Cl on the transport and sublimation of volatile trace metals in volcanic gases using benchtop fumarole experiments

Volatile trace metals emitted from volcanoes are frequently complexed with ligands and sublime from gases as halides, sulfides, sulfates and oxides. We investigate the effect of CI as a ligand on the sublimation process by degassing broadly phonolitic composition melts with added trace metals (V, Cu, Zn, As, Y, Mo, Cd, Sn, Yb, Pb and Bi) and Cl in a benchtop fumarole apparatus at 100 kPa. Sublimates forming from the gas phase at 25-900 degrees C along a silica glass tube suspended above degassed melts are characterized by electron microscopy, Raman spectroscopy and solution nebulization inductively coupled plasma mass spectrometry (SN ICP-MS). Silica polymorphs are ubiquitous at all temperatures and halite forms below 600 degrees C. Szenicsite (Cu-3(MoO4)(OH)(4)) and molybdofomacite (CuPb2(MoO4)(AsO4)(OH)) occur at similar to 500 degrees C. The latter phases may have formed from post-experiment hydration of original anhydrous molybates or arsenates, or alternatively are their Cr-bearing analogues. Arsenate and vanadate minerals such as bradaczekite (NaCu4(AsO4)(3)) and leningradite (PbCu3 (VO4)(2)Cl-2), known in natural fumaroles, also occur in the experiments. Chloride sublimates can host substantial Pb, TI, As, Cu, Bi and Zn. The addition of CI to the melt in the experiments increases the concentration of Li, Rb, Cs, Ag, Cr, Cu, Mo and W in the sublimates compared to Cl-free experiments. In Cl-bearing volcanic gases, molybdates and chlorides are the most likely hosts of volatile trace metals, and when adhered to ash partides, are potential sources of trace metals to the atmosphere and oceans. (C) 2020 Elsevier B.V. All rights reserved.