The Shaw paleointensity method: Can the ARM simulate the TRM alteration?

The effectiveness of the ARM correction in Shaw’s paleointensity method was investigated for four kinds of volcanic samples with different remanence characteristics from single domain to multidomain. Samples were heated in air and vacuum for successively longer times of 10, 20, 50, 100, 200, and 500 min and changes in the AF demagnetization curves of the ARM and TRM were observed. Drastic changes of the ARM and TRM were observed in three cases from two samples in which changes of the remanence magnitude were coherent between the ARM and TRM for a wide range of the AF demagnetization. However, the coherent change of the ARM and TRM due to alteration was not completely proportional, giving a moderate to large error in the ARM correction. This suggests that the double heating methodology should be practiced for the Shaw method to ascertain the effectiveness of the ARM correction. In two cases of andesite lava with a multidomain nature, the ARM-corrected Shaw plots with marginally linear data points were obtained from a heavily altered sample, indicating the insensibility of the coercivity spectra to changes in the blocking temperature. This fact suggests that strict linearity of the NRM to the corrected TRM should be posed in the analysis of Shaw plots.