Sulfur isotope effects in the exchange systems (1) graphic and (2) graphic have been studied at various temperatures. It has been shown that in the presence of water vapor sulfur isotope exchange proceeds at an appreciable rate in (1) above 150 °C and in (2) above 300 °C.The isotopic exchange constant for (1) approaches unity in the temperature range from 200 to 400 °C and the exchange constants for (2) at 400 °C and 450 °C equal 1.0095 and 1.0090 respectively favoring 34S in the SO2.The kinetic isotope effect in the chemical reduction of SO2 to S0 in consequence of its interaction with H2S has been determined at 25 °C, 200 °C, and 280 °C. At these temperatures the 32SO2 molecules react 1.6% faster than the 34SO2 molecules.It is concluded that the spread in isotopic composition of various forms of sulfur in volcanic gases is caused not only by isotope exchange reactions between oxidized and reduced forms of sulfur but also by kinetic isotope effects in unidirectional reactions which result from major temperature changes at the outflow.

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