Origin, speciation, and fluxes of trace-element gases at Augustine volcano, Alaska: Insights into magma degassing and fumarolic processes

Abstract Thermochemical modeling predicts that trace elements in the Augustine gas are transported from near-surface magma as simple chloride (NaCl, KCl, FeCl 2 , ZnCl 2 , PbCl 2 , CuCl, SbCl 3 , LiCl, MnCl 2 , NiCl 2 , BiCl, SrCl 2 ), oxychloride (MoO 2 Cl 2 ), sulfide (AsS), and elemental (Cd) gas species. However, Si, Ca, Al, Mg, Ti, V, and Cr are actually more concentrated in solids, beta-quartz (SiO 2 ), wollastonite (CaSiO 3 ), anorthite (CaAl 2 Si 2 O 8 ), diopside (CaMgSi 2 O 6 ), sphene (CaTiSiO 5 ), V 2 O 3 (c), and Cr 2 O 3 (c), respectively, than in their most abundant gaseous species, SiF 4 , CaCl 2 , AlF 2 O, MgCl 2 TiCl 4 , VOCl 3 , and CrO 2 Cl 2 . These computed solids are not degassing products, but reflect contaminants in our gas condensates or possible problems with our modeling due to “missing” gas species in the thermochemical data base. Using the calculated distribution of gas species and the COSPEC SO 2 fluxes, we have estimated the emission rates for many species (e.g., COS, NaCl, KCl, HBr, AsS, CuCl). Such forecasts could be useful to evaluate the effects of these trace species on atmospheric chemistry. Because of the high volatility of metal chlorides (e.g., FeCl 2 , NaCl, KCl, MnCl 2 , CuCl), the extremely HCl-rich Augustine volcanic gases are favorable for transporting metals from magma. Thermochemical modeling shows that equilibrium degassing of magma near 870°C can account for the concentrations of Fe, Na, K, Mn, Cu, Ni and part of the Mg in the gases escaping from the dome fumaroles on the 1986 lava dome. These calculations also explain why gases escaping from the lower temperature but highly oxidized moat vents on the 1976 lava dome should transport less Fe, Na, K, Mn and Ni, but more Cu; oxidation may also account for the larger concentrations of Zn and Mo in the moat gases. Nonvolatile elements (e.g., Al, Ca, Ti, Si) in the gas condensates came from eroded rock particles that dissolved in our samples or, for Si, from contamination from the silica sampling tube. Only a very small amount of rock contamination occurred (water/rock ratios between 10 4 and 10 6 ). Erosion is more prevalent in the pyroclastic flow fumaroles than in the summit vents, reflecting physical differences in the fumarole walls: ash vs. lava. Trace element contents of volcanic gases show enormous variability because of differences in the intensive parameters of degassing magma and variable amounts of wall rock erosion in volcanic fumaroles.