Magnetic granulometry results for intrusive rock samples

Abstract

‘Magnetic granulometry’ techniques have been developed^1–4 to infer the domain state of magnetic minerals contained in rocks. Such techniques include study of the development of hysteresis of samples in fields of the order of 10 Oe (Rayleigh loops), study of hysteresis in high fields both at room temperature and at liquid nitrogen temperatures, and study of variation of magnetic susceptibility with temperature from −196°C to 700°C. These methods enable the magnetic properties to be quickly determined and so give an idea of the bulk magnetic behaviour of the rock sample. Although the usefulness of such methods for palaeomagnetic studies is limited because the remanent magnetic behaviour can be quite distinct from the bulk magnetic behaviour⁵, in general, the granulometry method is a good supplement to regular experimental methods in palaeomagnetism. The study of several thousands of basalt samples by this method^6,7 has revealed insights into the magnetic behaviour of rocks. However, as part of the interpretive procedure of the method, the presence of low-field hysteresis loops has been considered a diagnostic feature of predominant superparamag-netic bulk behaviour of the rock sample. As we report here, we have observed, for the first time, low-field hysteresis loops associated with confirmed multi-domain magnetic grains, a possibility originally suggested on theoretical grounds by Néel⁸.