Previous investigations reported on the volcanic production
of halocarbons including chlorofluorocarbons (CFCs). It
has been suggested that this natural source could account
for a significant atmospheric CFC background concentration,
but no quantitative assessment of its source strength
has yet been presented. The synthetic mechanism for their
volcanic formation has neither been clarified. Fumarole
and lava gas samples from four volcanoes (Kuju, Satsuma
Iwojima, Mt. Etna, Vulcano) have been studied using gas
chromatography/ion trap−mass spectrometry. More than 300
organic substances were detected, among which 5
fluorinated, 100 chlorinated, 25 brominated, and 4 iodinated
compounds have been identified. The most abundant
organohalogen species were chlorinated methanes,
unsaturated C2-chlorohydrocarbons, and chlorobenzene,
suggesting a synthetic course that includes the thermolytic
formation of acetylene from hydrothermal methane,
condensation reactions, and synchronous catalytic
halogenation in the presence of highly activated surfaces
of cooling magma or juvenile ash. The only CFC compound
found was CFCl3 (CFC-11), which was detected in some
samples at concentrations of up to 1 ppbv. A conservative
estimate of the upper limit of global CFC emissions by
volcanoes clearly shows that this source is negligible as
compared to the atmospheric burden by anthropogenic
activities.
The Satsuma-Iwojima volcano has been emitting continuously high temperature (600• to 900• C) gases for at least 800 years. We identified the minerals that form in response to closed-system cooling of these gases and from airmixing reactions. Major differences compared with the sublimates observed at other volcanoes are the occurrence of wulfenite (PbMoO 4 ) and several mixed chlorides. This is the first report of wulfenite in fumarolic deposits. Thermochemical modeling shows that wulfenite precipitates between 540• and 490• C from a gas with lower sulfur content and/or higher f O 2 , and a higher Mo content (log f SO 2 = −2.1, log f H 2 S = −5, log f O 2 = −18.6, log f H 2 MoO 4 = −4.5, T = 500• C) than the previously reported gas composition. The occurrence of abundant K, Pb, Fe, Zn, Rb and Cs mixed chlorides may be promoted by the low S/Cl of the Satsuma-Iwojima high temperature gases. Natural sublimates of metallic elements (molybdenite, wulfenite, anglesite, Tl-Pb and Tl-Bi sulfides, Mo oxydes and Pb oxides) are deposited along the fumarolic conduit and on the ground under conditions of variable temperatures and f O 2 . The increase in f O 2 due to the mixing of the gases with the atmosphere reduces the volatility of several elements (As, Sn, Na, K and Pb) by promoting their condensation at higher temperatures. As air mixes with volcanic gas in the fumarolic plume, we can expect to find these metals as aerosols.
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