Sulfur isotopic effects in the disproportionation reaction of sulfur dioxide in hydrothermal fluids: implications for the δ34S variations of dissolved bisulfate and elemental sulfur from active crater lakes

Abstract

Sulfur isotope effects during the SO2 disproportionation reaction to form elemental sulfur (3SO2+3H2O→2HSO4−+S+2H+) at 200–330°C and saturated water vapor pressures were experimentally determined. Initially, a large kinetic isotopic fractionation takes place between HSO4− and S, followed by a slow approach to equilibrium. The equilibrium fractionation factors, estimated from the longest run results, are expressed by 1000lnαHSO4−–S=6.21×106/T2+3.62. The rates at which the initial kinetic fractionation factors approach the equilibrium ones were evaluated at the experimental conditions.δ34S values of HSO4− and elemental sulfur were examined for active crater lakes including Noboribetsu and Niseko, (Hokkaido, Japan), Khloridnoe, Bannoe and Maly Semiachik (Kamchatka), Poás (Costa Rica), Ruapehu (New Zealand) and Kawah Ijen and Keli Mutu (Indonesia). ΔHSO4−–S values are 28‰ for Keli Mutu, 26‰ for Kawah Ijen, 24‰ for Ruapehu, 23‰ for Poás, 22‰ for Maly Semiachik, 21‰ for Yugama, 13‰ for Bannoe, 9‰ for Niseko, 4‰ for Khloridonoe, and 0‰ for Noboribetsu, in the decreasing order. The SO2 disproportionation reaction in the magmatic hydrothermal system below crater lakes where magmatic gases condense is responsible for high ΔHSO4−–S values, whereas contribution of HSO4− produced through bacterial oxidation of reduced sulfur becomes progressively dominant for lakes with lower ΔHSO4−–S values. Currently, Noboribetsu crater lake contains no HSO4− of magmatic origin. A 40-year period observation of δ34SHSO4− and δ34SS values at Yugama indicated that the isotopic variations reflect changes in the supply rate of SO2 to the magmatic hydrothermal system. This implies a possibility of volcano monitoring by continuous observation of δ34SHSO4− values. The δ18O values of HSO4− and lake water from the studied lakes covary, indicating oxygen isotopic equilibration between them. The covariance gives strong evidence that lake water circulates through the sublimnic zone at temperatures of 140±30°C.