Sulfur isotope effects during the SO2 disproportionation reaction to form elemental sulfur (3SO(2) + 3H(2)O --> 2HSO(4)(-) + S + 2H(+)) at 200-330 degrees 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 1000 ln alpha(HSO4)(-)-s = 6.21 x 10(6)/T-2 + 3.62. The rates at which the initial kinetic fractionation factors approach the equilibrium ones were evaluated at the experimental conditions. delta(34)S values of HSO4- and elemental sulfur were examined far active crater lakes including Noboribetsu and Niseko, (Hokkaido, Japan), Khloridnoe, Bannoe and Mary Semiachik (Kamchatka), Poas (Costa Rica), Ruapehu (New Zealand) and Kawah Ijen and Keli Mutu (Indonesia). Delta HSO4--S values are 28 parts per thousand for Keli Mutu, 26 parts per thousand for Kawah Ijen, 24 parts per thousand for Ruapehu, 23 parts per thousand for Poas, 22 parts per thousand for Maly Semiachik, 21 parts per thousand for Yugama, 13 parts per thousand for Bannoe, 9 parts per thousand for Niseko, 4 parts per thousand for Khloridonoe, and 0 parts per thousand 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 Delta HSO4--S values, whereas contribution of HSO4- produced through bacterial oxidation of reduced sulfur becomes progressively dominant for lakes with lower Delta HSO4--S values. Currently, Noboribetsu crater lake contains no HSO4 of magmatic origin. A 40-year period observation of delta(34)SHO(4)(-) and delta(34)S(s) 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 delta(34)HSO(4)(-) values. The delta(18)O 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 degrees C. (C) 2000 Elsevier Science B.V. All rights reserved.
