We now come to the mechanism which provides the physical basis of the palaeomagnetic method as applied to igneous rocks, or other rocks heated without physical or chemical change in nature during metamorphism. Igneous rocks solidify, the magnetic minerals crystallize out and cool. At the Curie points of the magnetic constituents magnetic order sets in and, just below the Curie temperature, the spontaneous magnetization is weak. Anisotropy, from whatever source, is even weaker. Thermal energy is high compared to the barriers to magnetization change and the magnetic configuration in the system quickly reaches the thermal equilibrium corresponding to the presence or absence of an ambient magnetic field. This will be so regardless of the absolute value of the Curie temperature, so that the process would be equally effective for minerals or other materials with Curie points below room temperature, such as ulvöspinel. Although such minerals will not contribute to the natural remanent magnetization of rocks, they may acquire remanence in laboratory experiments aimed at understanding the processes believed to operate in nature, and the properties of the minerals themselves have intrinsic interest. The rapid approach to thermal equilibrium will also take place regardless of the grain size or other microstructural properties. Minerals in the monodomain state will be superparamagnetic regardless of their volume, K u v being less than kT because K u is small.
KeywordsDomain Wall Coercive Force Curie Point Mineral Magnetism Natural Remanent Magnetization
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