抄録
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 (PbMoO4) 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 fO2, and a higher Mo content (log fSO2 = -2.1, log fH2S = -5, log fO2 = -18.6, log fH2MoO4 = -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 fO2. The increase in fO2 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.
