Temperature and frequency-dependent magnetic susceptibility parameters: improving the reliability of archaeointensity in burnt clay ceramics

Analysis of magnetic mineralogy alteration parameters has been used to understand mineralogy transformation at sample heating. While many studies used the reversibility of low-field magnetic susceptibility vs. temperature κ(T) curves in a qualitative sense to select material for paleomagnetic studies, few parameters have been developed to assess a quantitative description. This work aims to correlate the magnetic mineral properties deduced by susceptibility experiments of archaeological ceramics (burnt clay) during the heating steps of the Thellier-Thellier intensity method. Eight ceramics from a Mexican archaeological site were examined in cyclical experiments of κ(T) curves and susceptibility vs. frequency-dependence, respectively. We found that no degree in the reversibility of κ(T)-curves determine the successful samples for the archaeointensity estimation in advance, neither in single nor incremental temperature cycles. However, a complete analysis including more than seven cycles with an estimation of magnetic grain properties constrains the most useful samples for the archaeointensity experiment. We propose a new parameter (modIPT) to evaluate the apparent reversibility for cyclical κ(T)-curves. We found a significant correlation of this parameter with archaeointensity statistical values that infer remanence alteration or directional deviation produced by mineral transformations after heating steps. We realized that particular burnt clay material as archaeological ceramic samples are suitable to register a reliable geomagnetic intensity and consequently an accurate archaeological dating, even though similar selection filters used in preceding works could have underestimated these materials.