Sulphur and sulphate-oxygen isotopes in New Zealand geothermal systems and volcanic discharges

Sulphur and oxygen isotope geothermometry has been applied to many of the New Zealand geothermal systems and volcanoes. Isotopic analyses of SO2 and H2S from the discharges of the main fumaroles in the White Island volcano indicate maximum temperatures of 900 deg. C. Analyses made in geothermal systems show that the SO4-H2O geothermometer gives temperatures close to those measured down hole were the systems have been drilled and in new geothermal fields gives a better estimate of deep temperatures than other geothermometers. The SO4-H2 geothermometer generally gives much higher temperatures (>350 deg. C), probably indicating conditions at greater depth. Since the SO4 and H2S do not re-equilibrate isotopically in the geothermal reservoirs they cannot have been there for more than about 10 a. A minimum residence time of about 2 a can be calculated for most of the areas from the SO4-H2O system which equilibrates relatively rapidly. In the fumaroles of the White Island volcano the δ34S values of the total discharge vary with the outlet temperature. During hot periods (fumarole temperatures up to 800 deg. C) the δ34S values are between +2 and +6 per mille and during cooler periods (≤ 300 deg. C) the δ34S values are up to +9 per mille. The higher δ34S values are considered to be due to the removal of isotopically light elemental sulphur in and around the fumarole. The average value for the total sulphurs in the New Zealand volcanic environment is +4 per mille which is the same as the average δ34S value for the underlying greywacke/argillite sediments and significantly higher than the 0 per mille assumed for magmatic material. This sulphur is also the source of H2S in the geothermal systems. Most of it is thought to become fixed as sulphide minerals deep in the system. The remaining H2S moves towards the surface, where varying amounts of sulphate form from it. In areas with a low H2S/SO4 ratio the δ34S values of the H2S are lower and reflect some isotopic equilibration of the sulphate with the H2S. (author). 42 refs, 5 figs, 2 tabs