Helium isotope studies of geothermal fields in the Taupo Volcanic Zone, New Zealand

We report new measurements of 3He, 4He and Ne on 69 samples of gas from geothermal fields in the central section of the NE-SW trending Taupo Volcanic Zone (TVZ). We find 3He4He ratios in the range 5.9-7.3 times the atmospheric ratio (5.9-7.3RA), typical of subduction zone systems. In some areas near the boundaries of the volcanic zone the ratio is much lower (2–4RA). The values in the Wairakei field are extremely uniform and vary by only a few per cent from 7.2 RA. In contrast, the wells in the Ohaaki-Broadlands field show a striking factor of two variation, with the ‘West bank’ section characterised by values near 6 RA while the ‘East bank’ section has values as low as 3.0 RA. This suggests that mixing of two types of geothermal source fluids may be occurring at Ohaaki-Broadlands. An inverse relationship has been found between the 3He4He ratio of the gases and the BCl ratios in the associated fluids. Thus the helium isotope ratios appear to be affected both by the flux of mantle helium associated with mantle subduction and also by the presence of sedimentary rocks beneath the geothermal field. Leaching, by the hot ascending fluid, of radiogenic 4He together with boron from the underlying high-boron, low-chloride Mesozoic greywacke basement, is thought to be the cause of this inverse relationship. The CO23He ratio in the TVZ exceeds the MORB ratio by a factor of up to 100, with higher values corresponding to higher BCl fluid ratios, suggesting that the greywacke may also be the major source of the CO2. We have also measured argon concentrations and isotopes on 19 samples in the Ohaaki-Broadlands area in order to further elucidate the nature of the striking helium isotope variations. The argon isotope ratios are relatively close to atmospheric indicating that the majority of the argon comes from recirculated groundwater. The simplified boiling model of Mazor et al. (1990) has been used to calculate the 3Heheat ratio in the Ohaaki-Broadlands and Wairakei fields. The values obtained fall within the range previously reported for mid-ocean ridge systems. This model has been extended and has led us to the conclusion that, compared to the 3He4He ratio, the (3He36Ar)ASW ratio is a much better indicator of the potential geothermal heat. The (4He40Ar)NA (where NA = non-atmospheric) ratio of both the high (mantle-derived) and low (crustal-derived) 3He4He end-members of the analyses reported here is about 3.6, but for samples with low 3He36Ar ratio, this ratio falls to values below 0.5. The value of 3.6 is in the range which is expected for the radiogenic component and although considerably lower than that often found in subduction type mantle gases it is still within the accepted range of values.