Variations of 3He/4He Isotope Ratios Within the Broadlands Geothermal Field, New Zealand

Abstract

The Broadlands-Ohaaki geothermal field is located 20 km NE of Wairakei on he Central Volcanic Zone of New Zealand. It falls within a resistivity low extending 4 km by 3 km. A study of 'He/'He ratios within this field has shown R/RA ratios close to 6.0 in the Ohaaki production area (NW). In contrast he production area to the SE on the east bank of he Waikato River has R& values close to 3.5. Studies of carbon and hydrogen isotopes in the methane and carbon dioxide gases, however, show no significant variation across the field. The geothermal temperatures and production of the wells is also similar in both groups of production wells but differences in chemical ratios reported previously are found to correlate with the 'Hel'He measurements. A tentative interpretation of the results indicate that there is a contribution of 'He from the mantle and that the variations in the 'He/4He ratios are probably related to differences in the geochemistry of the rocks through which the geothermal fluids flow to the surface. Preliminary measurements of the argon isotopes also show a contribution from radiogenic rocks at depth.