Determination of absolute palaeointensity using a multi-specimen procedure

Abstract

THE determination of the absolute strength of the ancient geomagnetic field from igneous rocks and other fired materials requires the original thermoremanent magnetization (palaeo-TRM) to be compared with that artificially acquired (lab-TRM) in a known field. In the Thellier approach¹, in which several sets of heatings are involved, the progressive removal (unblocking) of the palaeo-TRM is compared to the progressive acquisition (blocking-in) of the lab-TRM. But serial thermal treatments can cause significant physicochemical alteration to the magnetic carriers in a specimen (see, for example, refs 1–5), and for this reason Shaw² proposed a method employing only one thermal treatment. The peak temperature of exposure during a Shaw determination, however, must be above the highest Curie point of the sample. Here we present a new approach to the determination of palaeointensity which attempts to minimize both the number of heatings and the peak temperatures involved. It is based in principle on the Thellier method¹, in that it involves step-heating to several temperatures, but it departs dramatically from past practices in that several specimens are used for each determination. A comparison of the two methods shows that the new, multi-specimen methods may give reliable results for samples for which the Thellier method fails.