Other remanence-inducing mechanisms

Abstract

The TRM mechanism of Chapter 5 may account for the stable palaeomagnetic record carried by igneous rocks or the pre-existing country rock heated during igneous activity. Many sedimentary rock types also carry a stable though weak remanence which provides a useful contribution to the record of the history of the Earth’s crust and the ancient field. Mechanisms believed to operate in sediments and to be responsible for their stable remanence are those which lead to the acquisition of chemical remanent magnetization (CRM) and depositional remanent magnetization (DRM). Neither is quite so straightforward or simple as the TRM acquisition process which requires only a change in temperature. A simple reheating may reverse or reproduce the process in the ideal system. A CRM- or DRM-carrying system is unlikely to allow this in practice. CRM is not necessarily confined to sediments. Apart from minerals crystallizing in a solidifying magma above their Curie points and eventually acquiring TRM, an assemblage of grains produced during alteration at temperatures lower than their Curie point may become part-blocked isothermally at the alteration temperature by the CRM mechanism and, conceivably, part-blocked during cooling to ambient temperature producing additional TRM. A sediment which does not fluctuate significantly in temperature is unlikely to acquire TRM. Although some igneous rocks have features in common with sediments, the temperatures at which mineral grains separate out and are deposited within a semi-molten body are invariably above the highest Curie temperatures of materials and DRM is not acquired.