Palaeosecular variation for 0.1-21 Ka from the Okataina Volcanic Centre, New Zealand

Abstract

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.

Palaeointensity determinations from historical and Holocene basalt lavas in Iceland

“…The mostly linear remanence direction of RH02‐6‐2 contrasts to that of RH02‐2‐2, indicating that this part of the lava flow was quite stable during cooling. Similar cases in which heterogeneous block movement occurred during cooling was also reported in some of the pyroclastic flows in Japan and New Zealand (Tanaka et al 2004; Tanaka et al 2009). In spite of these difficulties, site mean palaeodirection was successfully determined including demagnetization great circles from two samples, as shown in the equal area plot, which was analysed using the method of McFadden & McElhinny (1988).…”

“…4(a), high reversibility of heating and cooling curves are observed and those χ− T curves in the progressive measurements coincide almost completely. Usefulness of the χ− T curve to detect unsuitable samples for palaeointensity experiments was suggested by Tanaka et al (2009), in which it was claimed that magnetic susceptibility is more prone to thermal alteration. In fact, in the two fair results (c, d), poor reversibility of the heating and cooling runs is only recognized in the χ− T curves, although this might be due to the difference in atmospheres between M s − T and χ− T measurements or due to alteration of paramagnetic silicates which make no contribution to the remanence.…”

Paleointensities on 8 ka obsidian from Mayor Island, New Zealand

“…The uncorrected mean paleointensity of 57.0 ± 1.0 µT seems most reliable and can at least give an upper limit of the field strength 8 ka ago in New Zealand, as faster cooling in the laboratory compared to nature leads to an overestimate of paleointensity when dealing with SD remanence carriers (Halgedahl et al, 1980;Dodson and McClelland-Brown, 1980). Interestingly, a comparison with previous intensity data from the same region shows excellent agreement: Within error our Mayor Island paleointensity data for 6050 ± 70 BCE is identical to the field value of 58.1 ± 2.9 µT, which Tanaka et al (2009) obtained for a ∼500 year younger (5500 BCE) rhyolithic lava at Okataina Volcanic Centre (OVC). OVC is a rhyolitic eruptive centre within Taupo Volcanic Zone and just ∼100 km distant from Mayor Island.…”

“…Further studies will have to check whether the uncorrected or the corrected value is more likely. In their study on rhyolithic lava from OVC, Tanaka et al (2009) have also compared their data to other volcanic and lake sediment data and to the geomagnetic field model CALS7K by Korte and Constable (2005). While the paleointensity data itself agrees well with a world-wide trend with a moderate high at 7-8 ka (Yang et al, 2000), there is a very poor fit to CALS7K.…”

“…OVC is a rhyolitic eruptive centre within Taupo Volcanic Zone and just ∼100 km distant from Mayor Island. Tanaka et al (2009) did not apply any coolingrate correction. Normally one would expect different cooling rate dependencies.…”