A comparison of archaeointensity results from Chinese ceramics using microwave and conventional Thellier's and Shaw's methods

Abstract

Palaeo- and archaeointensity determinations of the geomagnetic field are limited by mineralogical alteration that may occur when samples are heated in the laboratory to produce a TRM (Thellier and Thellier 1959; Shaw 1974). By using microwave excitation of the magnetic grains we have been able to create a TRM without significantly heating the bulk samples, thereby avoiding thermal alteration (Walton et al. 1993; Shaw et al. 1996). When applied to Peruvian ceramics in a manner analogous to the standard Thellier method, the microwave technique was found to reduce the scatter dramatically in the archaeointensity results (Shaw et al. 1996). In the present study we apply this microwave technique to a collection of Chinese ceramics covering the time interval 2700-7500 yr BP. These ceramics have already been investigated using adaptations of both Thellier's (Coe 1967) and Shaw's (Rolph and Shaw 1985) palaeointensity methods (Yang et al. 1993a). Although an acceptable agreement was found between those two methods, the equivalent virtual axial dipole moments (VADMs) were significantly lower than for the global model of McElhinny and Senanayake (1982). The present study, using the microwave technique with cooling-rate correction, has provided more consistent VADMs, and is much closer to the global model.