A Comparison of Palaeointensity Results Obtained Using the Microwave Technique and LTD- DHT Shaw Method on Basalts From the Auckland Volcanic Field, New Zealand

Samples from five monogenetic volcanoes in the Quaternary Auckland volcanic field that record the same geomagnetic excursion have been investigated using the microwave palaeointensity technique. Both the perpendicular applied field method and Coe version of the Thellier technique were carried out, with sister sub samples being run in many cases to check for consistency. The experiments were performed using the Liverpool microwave systems operating at 14 GHz. A total of 58 samples gave results ranging from 1 to 24 μT with the mean palaeointensity per volcano ranging from 7 to 17 μT. Mochizuki et al (2006 Phys. Earth Planet. Int., 154, 168-179) previously studied samples from three of the five volcanoes using the LTD-DHT Shaw palaeointensity method. Their results from 17 samples showed greater consistency both between and within volcanoes. Possible reasons for the differences between results from the microwave method and LTD-DHT Shaw method will be discussed. Despite the differences, both overall mean palaeointensities are statistically indistinguishable and show that the geomagnetic dipole moment was reduced to about 2 x 1022 Am2 for this Auckland excursion.