The geothermal system of Wairakei, New Zealand: physical processes and age estimates inferred from noble gases

Abstract

Atmospheric and non-atmospheric noble gases were identified by their isotopic composition in 31 samples from geothermal features from the Wairakei, Tauhara and Mokai geothermal fields, North Island, New Zealand. In the fumaroles, atmospheric noble gases (ANG) were found in concentrations that exceed the values in air-saturated recharge water (ASW) by a factor of up to 14. Significant concentrations of non-atmospheric (NA) 4He (He NA) and 40Ar (Ar NA) were observed, up to 4.3 × 10 −4and up to2.9 × 10 −4 cm 3 STP/g fluid, respectively. In contrast, the hot-spring waters revealed significant depletions in ANG (as low as 0.02 of the ASW concentrations) and low, but recognizable, concentrations of He NA and Ar NA. In geothermal wells, a wide spectrum of ANG concentrations was observed, from 0.02 to 30 times the concentrations in ASW. 4He NA and 40Ar NA varied accordingly. These observations lead to the simplified boiling model: meteoric water penetrated into the geothermal compartment, where it was heated and enriched by 4He NA and 40Ar NA of mantle and crustal origins. Boiling of ascending fluid, due to pressure decrease, results in formation of a noble-gas-enriched steam phase, issuing as fumaroles and drowned fumaroles, and a noble-gas-depleted residual liquid. The latter may cause secondary boiling of shallow groundwater, emerging in hot springs. Geothermal wells tap fluids at various degrees of steam separation. The depth of recharge of meteoric water into the geothermal system exceeds the depth of boiling, which is 600 m at the studied area. The ANG patterns, along with He NA/ANG ratios are applied to identify the following physical processes: (a) mixing of early separated steam with residual liquid; (b) mixing of a few per cent groundwater with residual liquid; (c) addition of air; (d) addition of excess Ne; and (e) encroachment of recent groundwater due to pressure decrease. Retention-enrichment values of the ANG were applied to reconstruct non-atmospheric initial (NAI) concentrations of He NAI, Ar NAI and the initial CO 2 in the geothermal water. Literature data on 3He/ 4He were used to separate the 4He NAI concentrations into crustal- and mantle-derived components. The crustal-radiogenic 4He concentration, calculated for the initial water, was applied to derive an age estimate of the geothermal water, assuming storage in rhyolitic rocks. Effective ages of 2 × 10 4, 1 × 10 5and2 × 10 5 a were obtained for the initial liquids feeding the Wairakei wells, fumaroles, and Mokai wells, respectively. Crustal Ar NA concentrations were estimated from those of crustal He and from the rhyolitic ( 4He/ 40Ar) production ratio. The calculated crustal 40Ar is≤ 0.1 of 40Ar NA, and ( 4He/ 40Ar) contribution is found at Wairakei to be ∼0.5, within the range of values measured by other workers in mantle-derived rocks.