Detecting multidomain magnetic grains in Thellier palaeointensity experiments

The Thellier technique is the most commonly used method for absolute palaeointensity studies. It is based on Thellier's laws stating that partial thermoremanent magnetisations (pTRMs) are independent and additive, which however only are true for non-interacting single domain (SD) grains. For multidomain (MD) grains, individual pTRMs are not independent and Thellier palaeointensity experiments on rocks with a significant proportion of MD grains may therefore lead to erroneous palaeofield estimates. By introducing a pTRM-tail check in the Coe version of the Thellier technique [J. Geophys. Res. 72 (1967) 3247], we present a simple method to detect and discard such rocks. The usefulness of the pTRM-tail check is exemplified by Thellier palaeointensity experiments performed on two palaeomagnetic sample collections with different magnetomineralogy. Miocene baked sediments from French Massif Central are shown to be ideal for Thellier studies with the remanent magnetisation residing in thermally stable SD grains. For Palaeocene-Eocene basaltic lavas from Faeroe Islands, 64% of otherwise acceptable palaeointensity estimates are discarded due to the failure of pTRM-tail checks indicating the presence of significant concentrations of MD grains. (C) 2001 Elsevier Science B.V. All rights reserved.