Detecting multidomain magnetic grains in Thellier palaeointensity experiments

The Thellier technique is the most commonly used method for absolute palaeo intensity studies. It is based on Thellier's laws stating that partial ther moremanent magnetisations (pTRMs) are independent and additive, which howev er only are true for non-interacting single domain (SD) grains. For multido main (MD) grains, individual pTRMs are not independent and Thellier palaeoi ntensity experiments on rocks with a significant proportion of MD grains ma y 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 roc ks. The usefulness of the pTRM-tail check is exemplified by Thellier palaeo intensity experiments performed on two palaeomagnetic sample collections wi th different magnetomineralogy. Miocene baked sediments from French Massif Central are shown to be ideal for Thellier studies with the remanent magnet isation residing in thermally stable SD grains. For Palaeocene-Eocene basal tic lavas from Faeroe Islands, 64% of otherwise acceptable palaeointensity estimates are discarded due to the failure of pTRM-tail checks indicating t he presence of significant concentrations of MD grains. (C) 2001 Elsevier S cience B.V. All rights reserved.