Reliability of palaeointensity methods using alternating field demagnetization and anhysteretic remanence

Abstract

Summary. Using natural volcanic rocks which acquired thermoremanence (TRM) in known fields, reliability of various palaeointensity methods using alternating field (AF) demagnetization were evaluated. Natural remanence (NRM), TRM and anhysteretic remanences (ARM's) before and after heating were stepwisely AF demagnetized following Shaw's method. It was found that the coercivity spectra of TRM and ARM in these samples are very similar, and that even when changes occurred during heating, the changes for two remanences are quite similar in many samples. Therefore, Shaw's method of palaeointensity determination, which incorporates ARM checks to the conventional comparison of NRM and TRM coercivity spectra, gives results as reliable as those obtained by the Thellier method. Many examples were demonstrated in which TRM and ARM intensities changed substantially by heating, but without changes in the shape of their coercivity spectra. Such changes cannot normally be detected and erroneous palaeointensities with apparent internal consistency would be obtained by usual AF demagnetization methods. Although ARM is quite similar to TRM, the rate of acquisition of ARM and TRM in weak fields varies by a factor of five among the present samples. To determine palaeointensities from the linear relation between ARM and TRM, it is necessary to determine experimentally the relative acquisition rate of these remanences. Therefore, methods based only on NRM‐ARM relations would not give palaeointensities with acceptable errors. Copyright © 1978, Wiley Blackwell. All rights reserved