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.