Abstract
SUMMARY Remagnetization experiments, involving the acquisition of secondary CRM, VRM and PTRM by reheating in controlled laboratory fields of 0-75pT, have been carried out on three different basic igneous rocks. The initial magnetic phases in all three rocks are titanomagnetites modified by varying degrees of deuteric- and low-temperature oxidation. Laboratory reheating results in further modification of the parent magnetic mineral phases, and the formation of new cubic and/or rhombohedra1 phases as daughter products. The induced CRM precludes identification of primary remanence directions, despite maximum reheating temperatures well below the original unblocking temperatures. The breakdown of a cation-deficient SD/PSD cubic parent phase to haematite in a sample from a basaltic lava flow leads to a directionally stable CRM intermediate between the NRM and external field directions, while a similar process in a dolerite leads to CRM directions parallel to the external field. Chemical alterations within the cubic system in an alkaline dyke sample with slightly larger (PSD) magnetic ‘grain sizes’ also lead to CRMs controlled by the external field. The intensities of all secondary remanence components show a linear dependence on laboratory field strength.
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