On partial thermoremanent magnetization tail checks in Thellier paleointensity determination

Abstract

Coarse magnetite grains, of multidomain (MD) or large pseudo‐single‐domain (PSD) size (≥10 μm approximately), cause nonlinearity and other problems in paleomagnetic field intensity determination and can lead to spurious paleointensity values. One suggested method of detecting their presence is to carry out an additional zero‐field heating step following the acquisition of partial thermoremanent magnetization (pTRM) to check for complete demagnetization of the pTRM. Residual undemagnetized pTRMs (“pTRM tails”) are characteristic of MD and large PSD grains and cause nonideal behavior in Thellier‐type paleointensity experiments. We have measured pTRM tails in synthetic and natural magnetites of many sizes and domain states (including PSD and MD), starting from two different initial states. First, a thermally demagnetized sample was given a pTRM by cooling in a field H from the blocking temperature TB. The pTRM tail is the remanence ΔMptr remaining after zero‐field reheating to TB. ΔMptr is most prominent for large grain sizes and when TB ≥ 500°C. We next measured pTRM tails ΔMptr* produced in a Thellier paleointensity experiment. The initial state in this case is a total thermoremanent magnetization gradually reduced by double heatings (zero‐field followed by in‐field) to a set of increasing TB. For all synthetic and natural samples containing large PSD and MD magnetites, regardless of grain size or lithologic differences, ΔMptr is significant but ΔMptr* is negligible. That is, the pTRM tail checks are always zero in our model Thellier experiments. However, in practical paleointensity studies, the pTRM tail check method has been shown to be successful in detecting MD grains. The reason for these contrasting results is that our experiments were carried out with the field Hlab that produced pTRM equal in intensity and parallel to the field H that produced natural remanent magnetization (NRM). In this special situation the pTRM tail is completely masked. For the pTRM tail check method to be most effective, either Hlab should be applied at a large angle to the NRM direction or Hlab should be larger than (twice or more) the paleofield H. This will highlight the pTRM tail; in the first case by deflecting the apparent NRM direction after the tail‐check step, and in the second case by increasing the apparent NRM intensity.