Sulfur and oxygen isotopic variations of dissolved sulfate in Crater Lake, Mt. Ruapehu, New Zealand

Sulfur and oxygen isotopic compositions of dissolved bisulfate (HSO4-) were measured for samples collected from 1982 to 1995 at Crater Lake, Mt. Ruapehu, NZ, The delta(34)S(HSO4-) values are fairly constant at around +18 parts per thousand throughout the period, and are consistent with experimentally determined sulfur isotope fractionation between HSO4- and reduced sulfur (S degrees and H2S) during disproportionation react/on of gaseous SO2 with an assumed delta(34)S of 10 parts per thousand in a magma-hydrothermal system beneath Crater Lake. The HSO4- -rich hydrothermal fluid is mixed with downward-percolating lake water to form a lower temperature hot water region at the upper part of the magma-hydrothermal system. The temperature of the hot water system is 'estimated to be 140+/-30 degrees C based on O-18 fractionation between HSO4- and lake water. Oxygen isotopic covariance between HSO4- and lake water gives strong evidence that lake water circulates beneath the lake floor. The kinetics of sulfur isotopic equilibration between HSO4- and S degrees suggests that the residence time of HSO4- in the hot water system is 1-2 years.