Palaeointensity study of the Oshima 1986 lava in Japan: implications for the reliability of the Thellier and LTD-DHT Shaw methods

Abstract

We have performed a palaeointensity study of the Oshima 1986 lava in order to examine the reliability of the Thellier and LTD-DHT Shaw methods with special reference to high-temperature oxidation states of magnetic grains. Coe’s version of the Thellier method was applied to fifteen specimens from five block samples. The palaeointensities determined by this Thellier method ranged from 44.8 to 59.0 μT with an average of 51.0 ± 4.1 μT ( N = 15). They are systematically higher than the expected intensity (45.5 μT from the DGRF 1985). Since the in situ magnetic survey at the sampling site indicates no strong local magnetic anomaly, these high palaeointensities are overestimation, probably due to some rock magnetic property and/or some thermal alteration during the experiments. Microscopic observations show that the samples, where some titanomagnetites surrounding vesicles were classified with intermediate oxidation indices of III–V, gave 10–30% higher palaeointensity values. Therefore, the erroneously high Thellier palaeointensities have a relation to the high-temperature oxidation state. For eight of the fifteen specimens used in the Thellier experiments, the applied DC field was perpendicular to the primary NRM component for quantitative evaluation of laboratory-induced chemical remanent magnetisation (CRM). Directional changes of NRM thermally demagnetised during the Thellier experiments indicate that most of the samples more or less acquired CRM due to laboratory heating suggesting that some magnetic phase with high blocking temperatures was produced. For the specimens of intermediate oxidation level, the CRM appears to have been gradually acquired at relatively low temperatures (≤400 °C). It would appear to be difficult to detect this gradual thermal alteration with the standard pTRM check, weak-field susceptibility measurements and hysteresis parameter changes. We also applied the double heating technique of the Shaw method combined with low temperature demagnetisation (LTD-DHT Shaw method) to seven specimens from the same block samples. The palaeointensities determined by the LTD-DHT Shaw method yielded an average of 46.4 ± 4.7 μT ( N = 6), which agrees with the expected intensity indicating no dependence on the high-temperature oxidation states.