Oxygen isotope studies of the geothermal system at Wairakei,New Zealand

Volcanic rocks in the Wairakei geothermal field have undergone extensive oxygen isotope exchange with the thermal waters, resulting in an O¹⁸-depletion averaging about 4%. A lower limit on the ratio of the mass of water to rock in the exchange system is 4·3, at least ten times greater than the corresponding figure for the Salton Sea geothermal system. Carbonates, present as alteration products in most samples, are found to be in equilibrium with waters at present-day temperatures in some wells, and to record higher ‘fossil’ temperatures in others. Quartz phenocrysts and xenocrysts remain unexchanged, and only new hydrothermal quartz is in isotopic equilibrium with geothermal solutions.