Stable isotope ratios of volcanic steam from White Island, New Zealand

Abstract

The D/H and¹⁸O/¹⁶O ratios of fumarole condensates from White Island, an andesite volcano in the Bay of Plenty, have been measured during the period 1965 to 1969 to determine the origin of the water and the changes which occur as the volcanic activity changes. The D/H and¹⁸O/¹⁶O ratios of all of the samples were correlated with a slope of −2. The δ¹⁸O values were proportional to the logarithm of the chloride concentration but with distinctly different relationships between the period from 1965 to 1967, and the period of tephra eruptions in 1968. In the latter period the chloride contents were close to that of sea water while in the former quieter period the contents were lower. The δD values follow a similar pattern but with a poorer correlation, indicating that a more variable process is controlling the deuterium results.

Possible hydrothermal models are: mixing of near surface water and magmatic water; progressive leaching of chloride from underground rocks and exchange of isotopes (Craig, 1966); and lastly, equilibrium evaporation at about 255°C of either sea water or local surface water.

The first of these models is considered most unlikely because of the incompatible logarithmic relationship between the chloride content and the oxygen isotope results. The second model has some merit, but the available evidence favours the last of the three models in which the 1965–67 samples derive from surface water or vapour from a boiling chloride water aquifer of sea water origin and the 1968 samples derive from the boiling chloride water, because of disturbance of the system during the tephra eruptions.