The effect of magnetite particle size on paleointensity determinations of the geomagnetic field

The Thellier method for paleointensity determinations has been applied to prepared samples containing magnetites whose mean particle sizes range from single domain, SD, to multidomain, MD. Linear (ideal) PNRM-PTRM curves are obtained for samples containing SD and submicron magnetite particles. However, for MD particles non-linear (concave-up) PNRM-PTRM curves are observed such that a linear approximation to the lower blocking-temperature data leads to apparent paleointensities that are higher than the actual paleofield; however, the ratio of the end-points, NRM/TRM, yields the correct (laboratory) intensity. The non-linear (concave-up) PNRM-PTRM curves for the MD particles are explained in terms of the lack of symmetry of the domain-wall movements during the two heatings of the Thellier experiment. Low stabilities with respect to alternating fields and with respect to temperature cycles below magnetite's isotropic temperature are diagnostic in detecting samples most likely to exhibit non-linearities due to the MD effect.