The source of halogens in geothermal fluids from the Taupo Volcanic Zone, North Island, New Zealand

摘要

Forty-seven samples of deep geothermal well fluids and hot springs from 14 geothermal sites in the Taupo Volcanic Zone(TVZ), North Island, New Zealand, were analyzed for Cl, Br and Li concentrations and δ~(37)Cl, δ~7Li, δ~(18)O and dD values. The main purpose of this study was to identify the sources of salinity in the TVZ fluids and to assess the processes that control stable chlorine isotopic fractionation in the geothermal waters. The samples were obtained from deep wells in the Mokai, Ohaaki, Wairakei, Rotokawa, Kawerau, Ngatamariki and Tauhara geothermal fields and from the hot springs at Rotorua, Waiotapu, Lake Taupo, Waimangu, Waikite, Orakei Korako and Tokaanu. Two main meteoric recharge areas were identified in the central TVZ, one located to the NE with δ~(18)O and dD values of -5.5‰ and -35‰, respectively and the other to the SW with a δ~(18)O value of -6.7‰ and dD value of -43‰. Water rock interactions in most of the geothermal fluids resulted in a δ~(18)O shift to more positive values; up to 1‰ for Mokai and over 3‰for Kawerau. The δ~7Li values range from -2.9‰to 2.0‰and suggest that the deep geothermal fluids have interacted with rocks with low δ~7Li values. The highest δ~7Li values (1.4‰) in the reservoir fluids were found at Kawerau where the low quartz content of the greywacke may result in the depletion of 6Li. The δ~(37)Cl values of the samples ranged from -1.0‰ to 0.8‰. The δ~(37)Cl values of 0.2‰, 0.1‰ and 0.0‰ were measured in samples from Orakei Korako, Waiotapu and Waimangu respectively, and these are interpreted to have been affected by surface evaporation and distillation processes. Excluding these data, most waters with high Cl/Br ratios had positive δ~(37)Cl values. The Cl/Br molar ratios in well samples ranged from 659 to 1664 and the δ~(37)Cl values from -1.0‰ to 0.8‰. The hot springs had Cl/Br molar ratios ranging from 858 to 1611and δ~(37)Cl values from -0.8‰ to 0.7‰. Overall, the Cl/Br molar ratios of the fluids are comparable with published data on basaltic magmas, and these magmas are inferred to be the main source of halogens in the geothermal fluids. However, the geothermal fluids can be separated into two main groups: one associated with rhyolitic magmas characterized by positive δ~(37)Cl values and high Cl/Br molar ratios, and other related to andesitic magmas with negative δ~(37)Cl values and lower Cl/Br molar ratios. We suggest therefore, that the isotopic fractionation of the stable Cl isotopes is related to the difference in water contents of andesitic and rhyolitic magmas and that the fluid phase is enriched in 37Cl during magmatic evolution. There is no evidence of isotopic fractionation during the transit of the geothermal fluids from the reservoir to the surface, but evaporation and distillation processes at the surface can affect the δ~(37)Cl values.