Earth Planets Space, Vol. 61 (No. 1), pp. 213-225, 2009
Hidefumi Tanaka1, Naoyuki Komuro1, and Gillian M. Turner2
1Faculty of Education, Kochi University, Kochi 780-8520, Japan
2School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington, New Zealand
(Received September 27, 2007; Revised March 25, 2008; Accepted March 30, 2008; Online published January 23, 2009)
Studies of palaeodirections and palaeointensities were carried out on mainly rhyolitic lavas and pyroclastics from the Okataina Volcanic Centre, New Zealand, which has erupted during the past 32 kyr. Of the 17 sites studied, 14, spanning the period 0.1-21 Ka yielded good mean palaeodirections, while three carried unstable natural remanent magnetizations. Of 49 specimens from 7 sites, on which Thellier palaeointensity experiments were carried out, 21 specimens gave successful results, yielding 3 site mean palaeointensities for 1886 AD, 5 Ka, and 7.5 Ka. When the new palaeodirections, together with previously reported archaeo and volcanic data, were compared with sedimentary records from New Zealand and eastern Australia for the last 10 kyr, good agreement was obtained in inclination but discrepancies were observed in declination. The new 7.5 Ka and 5 Ka palaeointensities are moderately high and relatively low, respectively, and are concordant with the global trend. The mean palaeointensity obtained for 1886 AD is, 11% higher than the IGRF1900. The difference is scarcely significant, but might indicate a small bias toward high values. Although the sedimentary directional curves show excellent agreement with the prediction from CALS7K, the fit of the palaeointensity data to model values was relatively poor over the wider Pacific region. Further reliable palaeointensity data are needed to solve the discrepancy.
Key words: Palaeosecular variation, Okataina, rhyolite lava, palaeointensity, Thellier method.