Mercury in vapor-gas fumarole jets and products of their sedimentation at the Mutnovsky volcano

Abstract

High Hg content was measured in acid condensates of fumarolic gases and in dry volcanic gas (without H2O vapor), up to 2 mg/kg and ~ 10-6 mol%, respectively. Mercury concentration in sediments from boiling mud pools reaches values of 3.5x10-2 wt% which is close to values found in some Hg ore deposits. The Hg speciation in mud pools is represented by small cinnabar aggregates (< 20 μ) or grains of 3-5μ. The Vulcannaya stream that drains all crater springs, pools and fumarolic fields transports mercury (cinnabar) outside the crater to distances up to 12 km. The cinnabar structures were investigated using the MS-46 “Cameca” instrument. We have studied the temperature dependence of the Hg releasing from the mud pool sediments using heating of dry mud samples to >600°C with the continuous measuring of Hg using the atomic absorption techniques (with assistance by I.I.Stepanov). Three temperature maximums of the Hg releasing were established: 190-220°C, 380-410°C and >600°C. The medium and high temperature peaks are attributed to the breakdown of cinnabar and metacinnabar (380-410°C) and Hg-bearing pyrite (>600°C). The low-temperature peak is most probably related to the thermo-dissociation of Hg chlorides. Their formation in the Mutnovsky pools is quite probable taking into account that the pool solutions are very acid and chloride enriched. The Hg speciation in acid solutions with compositions similar to those of acid pools of Mutnovsky was also studied using thermochemical computer codes. It was found that at pH~0 in a chloride-bearing solution with a low sulfide the stable species of Hg are HgCl-4 , HgCl-3 and HgClo2. With increasing pH to 1.3, elemental Hg becomes also relatively stable. A strong correlation was observed between Hg concentrations in Mutnovsky fluids and the volcanic tremor activity of Gorely volcano (15 km to NW). These studies were conducted in collaboration with V.A.Shirokov and they showed a regional character of the mercury degassing that most probably origins from the underlying mantle wedge. High concentrations of mercury observed in fumaroles and thermal mud pools of the Mutnovsky crater make the local air pollution by Hg essentially higher than the health hazard limits. This may affect volcanologists and tourists visiting the crater of Mutnovsky volcano.