The Pseudo-Thellier Method for Single-Domain Noninteracting Particles: Theory and Experiment

Abstract—The pseudo-Thellier method is numerically simulated based on a rigorous solution of kinetic equations for uniaxial randomly oriented noninteracting single-domain grains. Laboratory experiments have been performed to determine the relative paleointensity B_anc with thermoremanent magnetization (TRM) created in samples of igneous rocks in random fields B_rand; the domain structure of grains of these samples varies from single- to multi-domain. Both predicted and measured pseudo-Arai plots can be divided into two quasi linear segments, one in the relatively low-coercivity region B_c < 40–50 mT and the other at higher alternating field (AF) amplitudes. The determinations of relative paleointensity B_anc on TRM-bearing igneous rocks, estimated from low-coercivity segments of pseudo-Arai plots, give fairly good results with a linear correlation coefficient R = 0.8 between the true field B_rand and B_anc determined by the pseudo-Thellier method. It is shown that when thermal fluctuations for relatively small and magnetically soft particles (which corresponds to low blocking temperatures) are taken into account, a significant difference is observed between the coercive force B_cr of a grain and the actual field of its magnetization (demagnetization). The main conclusion of the study is that applying the pseudo-Thellier method to igneous rocks is promising. Further development of the methodological and practical aspects of this approach could yield interesting results, particularly when analyzing samples that are unstable to changes in magnetic minerals during the classical Thellier procedure.