Paleointensity determination using anhysteretic remanence and saturation isothermal remanence

Abstract

Determining the strength of ancient planetary magnetic field is pivotal to understanding the evolution of planets and asteroids in the solar system. While the Thellier‐type double heating technique provides the most faithful field strength information for rocks carrying a thermoremanent magnetization (TRM), many extraterrestrial rock samples respond unfavorably to heat treatment. The present study systematically examined the two well‐known normalization techniques that avoid any heating by comparing the ratios of TRM/anhysteretic remanent magnetization (ARM) and TRM/saturation isothermal remanent magnetization (SIRM). Both the ratios of TRM/ARM and TRM/SIRM are dependent on the grain size as well as the volume concentration of magnetite. The remanence ratios also were found to increase as the alternating field increased during demagnetization for fine‐grained magnetite. A new calibration relation of 2.60 ± 1.32 for the TRM/ARM and of (3.62 ± 1.28) × 10−2 for the TRM/SIRM was defined for an external field of 50 μT. The TRM/SIRM is superior to TRM/ARM because the former showed less dispersion in the grain size dependence, the volume concentration dependence, and the stability against AF demagnetization. In addition, the standard error of the mean for the TRM/SIRM ratio was smaller than that for the TRM/ARM ratio. Thus, whenever heating is inapplicable, the SIRM method seems to be a better choice than the ARM method. However, it should be emphasized that the uncertainty of TRM/ARM and TRM/SIRM is still nearly an order of magnitude larger than that of the high‐fidelity Thellier estimation and thus must be limited in use only when samples are irreversibly altered during heating. In practice, the best approach is to carry out both ARM and SIRM methods and check whether the two estimations agree within the uncertainties.