The Chemical and Isotopic Position of the Ohaaki Field Within the Taupo Volcanic Zone

Abstract

Relative Rb, Cs and contents of waters discharged from geothermal systems over the Taupo Volcanic Zone are very uniform. They are thought to reflect the composition of a common deep brine formed through absorption of magmatic vapors into deep ground water followed by rock dissolution and double diffusive mixing with more shallow ground water. Relative B contents are uniform over the backarc basin, but increase rapidly towards the eastern boundary, the volcanic arc. These marked differences between arc and backarc systems are reflected in the much higher contributions from magmatic to gas discharges along the SE boundary of the TVZ. The increased discharge of volatile species there is ascribed to recent injection of heat by intrusion of andesitic to dacitic magma. Associated stimulation of convective flow is likely to be responsible for the establishment of a series of geothermal systems (Rotokawa, Ohaaki, Waiotapu, Kawerau) of which Ohaaki East is probably the youngest, too young to have produced major surface manifestations.