Physics of the Earth and Planetary Interiors 125 (2001) 111–117 Detecting multidomain magnetic grains in Thellier palaeointensity experiments Peter Riisager a,∗ , Janna Riisager b a Danish Lithosphere Centre, Øster Voldgade 10, DK-1350 Copenhagen K, Denmark b Geological Museum, Øster Voldgade 5-7, DK-1350 Copenhagen K, Denmark Received 21 December 2000; received in revised form 18 June 2001; accepted 19 June 2001 Abstract 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. © 2001 Elsevier Science B.V. All rights reserved. Keywords: Thellier–Thellier method; Palaeointensity 1. Introduction While cooling down in the weak magnetic field of the Earth (of the order of 10 A/m), igneous rocks acquire a thermoremanent magnetisation (TRM) that is proportional in intensity to the strength of the field. In theory, the linear relationship between TRM in- tensity and the inducing field allows one to estimate the strength of the palaeomagnetic field by compar- ing the intensity of the TRM acquired by an igneous rock during its initial cooling with the intensity of a TRM lab created in the laboratory in a known field. ∗ Corresponding author. Tel.: +45-38-142638; fax: +45-33-110878. E-mail address: pri@dlc.ku.dk (P. Riisager). In practice, it is found that laboratory heating of ig- neous rocks to temperatures above their Curie point, in most cases alters the magnetic mineralogy and thereby breaks down the relationship between field strength and TRM intensity. Furthermore, the natural remanent magnetisation (NRM) of rocks older than a few thousand years, in most cases contain secondary components in addition to the primary TRM. To over- come these difficulties, Thellier and Thellier (1959) devised a method to study partial TRMs (pTRMs), allowing palaeointensity estimates to be obtained from parts of the blocking/unblocking temperature spectra not affected by secondary non-TRM com- ponents or laboratory-induced magnetomineralogical alterations. A unique advantage of the Thellier tech- nique is that magnetomineralogical alteration, as the 0031-9201/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved. PII:S0031-9201(01)00236-9