Geothermal waters from the Taupo Volcanic Zone, New Zealand: Li, B and Sr isotopes characterization

Romain Millot (1) , Aimee Hegan (1, 2) , Philippe Négrel (1)

1 BRGM - Bureau de Recherches Géologiques et Minières (BRGM)
2 SEAES - School of Earth, Atmospheric and Environmental Sciences [Manchester]

In this study, we report chemical and isotope data for 23 geothermal water samples collected in New Zealand within the Taupo Volcanic Zone (TVZ). We analyzed major and trace elements including Li, B and Sr and their isotopic compositions (delta7Li, delta11B, 87Sr/86Sr) in high temperature geothermal waters collected from deep boreholes in different geothermal fields (Ohaaki, Wairakei, Mokai, Kawerau and Rotokawa geothermal systems). Lithium concentrations are high (from 4.5 to 19.9 mg/L) and lithium isotopic compositions (delta7Li) are very homogeneous, being comprised between -0.5 and +1.4‰. In particular, it is noteworthy that, except the samples from the Kawerau geothermal field having slightly higher delta7Li values (+1.4%), the other geothermal waters have a very constant delta7Li signature around a mean value of 0‰ ± 0.6 (2s, n=21). Boron concentrations are also high and relatively homogeneous for the geothermal samples, falling between 17.5 and 82.1 mg/L. Boron isotopic compositions (delta11B) are all negative, and display a range between -6.7 and -1.9‰. These B isotope compositions are in agreement with those of the Ngawha geothermal field in New Zealand. Li and B isotope signatures are in a good agreement with a fluid signature mainly derived from water/rock interactions involving magmatic rocks with no evidence of seawater input. On the other hand, strontium concentrations are lower and more heterogeneous and fall between 2 and 165 µg/L. 87Sr/86Sr ratios range from 0.70549 to 0.70961. These Sr isotope compositions overlap those of the Rotorua geothermal field in New Zealand, also confirming that some geothermal waters (with more radiogenic strontium) have interacted with bedrocks from the metasedimentary basement. Each of these isotope systems on their own reveals important information about particular aspects of either water source or water/rock interaction processes, but, considered together, provide a more integrated understanding of the geothermal systems from the TVZ in New Zealand.

Keywords: Geothermal waters; New Zealand; Lithium isotopes; Boron isotopes; Strontium isotopes

Domains: Geochemistry; Global Changes

Romain Millot, Aimee Hegan, Philippe Négrel. Geothermal waters from the Taupo Volcanic Zone, New Zealand: Li, B and Sr isotopes characterization. Applied Geochemistry, 2012, 27 (3), pp.677-688. ⟨10.1016/j.apgeochem.2011.12.015⟩. ⟨hal-00653418⟩