Abstract
The study of palaeomagnetism has had a great impact on the earth sciences, providing the first conclusive proof that continental drift has occurred. Although it is generally accepted that the primary magnetisation of basic igneous rocks is a thermal remanent magnetisation (TRM) carried by opaque iron-titanium oxide minerals, particularly those of the magnetite-ulvospinel series, there is still some doubt as to the exact magnetic state of the grains responsible for this palaeomagnetic TRM. The great stability of the TRM in many igneous rocks suggests a population of non-interacting particles, each consisting of a single magnetic domain, but it has been argued that the grain size of magnetic minerals in rocks is much greater than the critical size for this behaviour. For magnetite this critical size is about 0. 05 μm for equidimensional particles, rising to about 1 μm for elongated grains with an axial ratio of 10 : 1 (Evans, 1972). Thus single-domain particles would not be detected by conventional (optical) petrographic techniques.
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Schedule a callMore From: Proceedings, annual meeting, Electron Microscopy Society of America
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