K-Ar Ages and Paleosecular Variation of Direction and Intensity from Quaternary Lava Sequences in the Ruapehu Volcano, New Zealand

Abstract

Andesite lava sequences of the Quaternary Ruapehu Volcano, New Zealand were studied to obtain paleosecular variations, both direction and intensity. K-Ar dating was performed on 7 lavas of the younger Whakapapa Formation and 22 lavas of the older Te Herenga Formation. Initial experiments by the isotope dilution method using an ³⁸Ar spike were unsuccessful on the younger lavas, while an amplitude method in which the abundance of Ar is estimated from a peak height of mass spectrum gave consistent results of 32 ± 12 ka after the correction of the Ar isotopic ratios for mass dependent fractionation. The latter method also gave a more accurate result of 229 ± 35 ka for the older group. No significant relationship was observed in age versus stratigraphy for the 250 m thick older lava section. This suggests a short time span, less than the errors of dating, for the lava accumulation. Primary paleodirections were easily obtained, after cleaning by both AF and thermal demagnetization, from all lavas except two in which a possible effect of lightning was suspected. Paleointensity experiments, using mainly the Shaw method, were made on 15 lavas which showed good stability to thermal demagnetization. In the analysis of the Shaw results, the correction method of Kono (1978) was applied, since most samples showed changes in ARM capacity after heating. The double heating technique of Tsunakawa and Shaw (1994) was also applied to some lavas, and was useful to discriminate erroneous data when inconsistent results were obtained. Serial correlation was not detected in the paleodirections of any series of the lavas. In spite of the random appearance of the data, the dipole direction was not obtained from the older section which included 20 data.