Revisiting a domain-state independent method of palaeointensity determination

The use of a domain-state independent method (the Wilson method) of acquiring absolute palaeointensities is tested using three sets of historical lavas from Mexico, Italy and Iceland. The Wilson method works by plotting continuous thermal demagnetisation curve data for the original natural remanent magnetisation against those for a laboratory-induced thermoremanence. By comparing the predicted intensities with the known fields, a new empirically derived rejection criterion is proposed that accounts for chemical alteration and improves the accuracy of the palaeointensity estimates. The Wilson palaeointensity estimates are compared with palaeointensity determinations made on sister samples using standard Thellier-type heating. Generally, the Wilson palaeointensity estimates compare favourably for samples that yielded technically correct Thellier-type determinations. However, at one locality the Wilson method did not yield any estimates due to severe chemical alteration, in contrast it was still possible to determine palaeointensity estimates from the lower temperature Thellier data. To assess the Wilson palaeointensity protocol's ability to return a correct estimate for a true multidomain system, a synthetic multidomain magnetite sample was imparted with a thermoremanent magnetisation. The Wilson protocol returned the correct palaeointensity within error, in contrast the Thellier-type protocol failed. As the Wilson method is significantly faster than Thellier-type methods, it is suggested that the Wilson method should be tried as a ‘first-approach’ in palaeointensity studies, or for samples dominated by multidomain material.