Trace element transport by volcanic gases at Vulcano (Sicily, Italy) – Speciation, deposition and fluxes

Abstract

The geochemistry of trace elements in volcanic gas emissions at Vulcano (Sicily, Italy) was investigated. Trace element concentrations in 94–412 °C fumarole gases span over 10 orders of magnitude, from ∼0.01 pmol/mol to ∼300 μmol/mol, with some metalloids (B, Si) being the most abundant, followed by alkali, alkaline earth, and certain transition metals, and rare earth elements typically displaying the lowest concentrations. Thermodynamic modeling predicts most trace elements to be transported as chloride, hydroxide, and mixed hydroxy-chloro gas species (LiCl, KCl, NaCl, RbCl and CsCl, Be(OH)₂, Mg(OH)₂, MgCl₂, CaCl₂, SrCl₂, CaCl(OH), TiOCl₂, VOCl, VOCl₂, VOCl₃, NbOCl₃, Cr(OH)₃, CrCl₃, Fe(OH)₂, FeCl₂ Co(OH)₂, CoCl₂, Ni(OH)₂ to NiCl₂, Cd(OH)₂, CdCl_2, Re(OH)₃, ReCl₃, ZnCl₂, AgCl, WO₂(OH)₂, Al(OH)₃, Si(OH)₄, B(OH)₄, TlO, GaCl₃, SbCl, MnCl₂, CuCl). Sulfide, hydrate, and elemental gas species are also important for some elements (Cd, AuS, Hg, PbS₂, BiS, Bi, AsS, As₂S₃, TeS, SeH, SeS). However, for many trace elements, speciation remains uncertain or unknown due to a lack of thermodynamic data. Upon cooling and decompression of the volcanic gas, most trace elements are predicted to reach gas-solid equilibrium, resulting in the formation of secondary minerals. At high temperatures (∼700–1000 °C), the mineral assemblage forming is dominated by quartz, Ca-Na-K feldspars, and Mg-pyroxene, containing minor concentrations of other alkali and alkaline earth metals. Further cooling and decompression leads to the formation of minerals including magnetite, pyrite, chalcocite, and chalcopyrite together with other less abundant oxides (V, Cr, Ga, W, and Sn) and sulfides (Zn, Pb, Ni, Co, Cd, Mo, Ag, As, and Bi), and eventually a range of sulfates and sulfosalts (Li, K, Na, Rb, Cs, Be, Mg, Ca, Sr, Bi, Mn, Fe, Zn, Pb, and Sn) at the lowest temperatures (∼100–300 °C). For most trace elements, fumarole emission concentrations reflect higher gas-solid equilibrium temperatures than those observed during sampling, suggesting gas-solid equilibria at high temperatures followed by incomplete re-equilibration upon further cooling near the surface.

Trace element fluxes span over eight orders of magnitude, ranging from >100 kg/day to ∼1·10⁻⁶ kg/day. Silica, Al, and B consistently exhibit the highest fluxes, followed by alkali and alkaline earth metals, various transition metals and metalloids, with rare earth elements and actinides displaying the lowest fluxes. Generally, the trace element fluxes are lower compared to neighboring Stromboli and Etna, except for Pb, Bi, B, As, Sb, and Te.