Application of Oxygen Isotope Studies to Some New Zealand Geological Problems

Abstract

The derivation of the relationship between isotopic composition and temperature by statistical mechanics is outlined and the fundamentals of the method of determining temperatures from oxygen isotope compositions are explained. The experimental methods of both carbonate and silicate analyses are described in detail. The oxygen isotope technique is applied to the following five studies. (1) Otoliths of living fish were analysed to show that they were in isotopic equilibrium with the sea water in which they live and fossil otoliths were then used to determine Miocene deep sea temperatures in Wairarapa, New Zealand. (Published in Science, vol. 155: 1684-1685 (1967)). (2) Oxygen isotope paleotemperatures were calculated from the isotopic compositions of various kinds of fossils from two deep-water coral thickets in Wairarapa, New Zealand. Conflicting paleoecological conclusions on the thickets had been draw previously from the corals and from the associated foraminifera. The oxygen isotope paleotemperature agreed with the conclusions based on corals (Published in Paleogeography, Paleoclimatology, Paleoecology, vol. 3; 447-455 (1967)). (3) Tertiary macrofossils and microfossils from various part of New Zealand were analysed and a Tertiary paleoclimate curve was constructed. Results agreed to some extent with curves previously drawn from paleontological evidence but indicated that there had been several major temperature fluctuations during the Tertiary period. (Published in the New Zealand Journal of Science, vol. 10: 988-1011 (1967)). (4) A survey of metamorphic temperatures of the Otage greenschist facies rocks was carried out by measuring the oxygen isotope ratios of coexisting minerals. Calculated temperatures agree fairly well with those based on mineralogy and range from 250° to 550°c. When plotted on a map the maximum isotopic temperatures are offset from the axis of the Biotite Zone of Landis and Coombs (1967). The uniform isotopic composition throughout the region of the abundant mineral quartz indicates that there was a high rock/water ratio during metamorphism and supports the conclusion of Reed (1958) that little or no material was introduced into the schists during metamorphism. (5) Four analyses of quartz from the Ngawha deep bore No.1, together with the isotopic analyses of the Ngawha geothermal waters previously determined by McDonald (1966), gave a temperature of 230°C for the deep underground system. They also indicate that the system does not contain large amount of water.