A New Method of Determining the Magnitude of the Palaeomagnetic Field: Application to five historic lavas and five archaeological samples

Abstract

Summary A new method for determining the magnitude of the palaeomagnetic field (palaeofield), By has been developed and applied to five historic lavas and five archaeological samples. The palaeofield was determined for four lavas. The fifth gave no result. The palaeofield was determined for all five archaeological samples. The Thellier method had previously been applied to three of these samples and the results are compared. A new method for determining the palaeofield, B*, has been developed. The method has been tested on five recent lavas, that had been extruded in the known geomagnetic field, and on five archaeomagnetic samples of known age. The palaeofield is usually determined by comparing the natural remanent magnetization (NRM) with a laboratory thermoremanent magnetization (TRM) (Thellier 8z Thellier 1959), produced in a known field (Blab). The palaeofield (B,,,), is given by equation (I), which is valid for small constant magnetic fields of up to T (Nagata 1943). Usually the TRM does not have the same coercive force spectrum as the NRM, because of changes that occur during the laboratory heating of the sample when producing the TRM. Therefore the direct comparison of the NRM and the TRM (equation (1)) can produce very large errors. In the new method described in this paper only that part of the coercive force spectrum which has not been altered by the (TRM) heating, is used to determine the palaeofield. Empirically, this always lies in the high coercive force region and is therefore not likely to be affected by viscous components of magnetization. 2. The method The method involves comparing two ARM'S created before and after heating. The comparison permits selection of a coercive force region within which the heating * The IAGA (Kyoto 1973) recommended that values of the geomagnetic field be expressed in terms of B. 1T = lo4 G.