The Tongariro Geothermal System -- Stable Isotope Chemistry

Abstract

Stable isotope and gas chemistry are consistent with the existence of a vapor dominate geothermal field on the Mount Tongariro volcanic complex. The main outflow of heat is at Ketetahi hot springs, 4 km from minor fumaroles at Red Crater. The steam from ketetahi fumaroles ahs a range of stable isotopic compositions showing that some fumaroles have been affected by partial condensation of the steam. Central Crater steam is highly fractionated and may be related to that at Red Crater by condensation evaporation mechanisms. The Ketetahi steam has an oxygen 18 shift of about 3°/oo compared to local precipitation, implying that an underlying deep chloride water would have an oxygen 18 shift of 4.8°/oo. The gas chemistry and isotopes show minor variations indicating some difference in flow (e.g. contaminate) or fractionation between Ketetahi and the other fumaroles. Deuterium fractionation between hydrogen and water indicated reservoir temperatures near 200 °C. The 13 C fractionation between CH4 and CO2 indicates a temperature of 420°C which is higher than most other New Zealand geothermal fields.