Magnetic Mineral Phases Research Articles

Directional properties of alteration CRM in basic igneous rocks

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.

A cautionary tale for palaeomagnetists: A spurious apparent single component remanence due to overlap of blocking‐temperature spectra of two components

The magnetic properties of the upper Cretaceous red‐limestones from the Montsec thrust sheet in the Pyrenees reveal an unusual arrangement of the unblocking temperatures and coercivities of the magnetic mineral phases, which leads to an apparently single‐component remanence being isolated on thermal demagnetization. That this is an artefact of the almost complete overlap of the blocking temperature spectra of the two real components, can be deduced from alternating field and thermal demagnetization of a composite isothermal remanent magnetization (IRM). When two IRM components are imparted to the sample, at right angles to each other and each affecting grains with different coercivities, thermal demagnetizations of the composite IRM does not isolate the components but alternating field demagnetization does, illustrating the superiority of the latter treatment in this case.

The effect of uniaxial compression on the initial susceptibility of rocks as a function of grain size and composition of their constituent titanomagnetites

An ac bridge method has been used to study the effect of uniaxial compression on the initial magnetic susceptibility of rocks and separated titanomagnetites. The samples used were basalts containing titanomagnetite with varying grain size and morphology and dispersions of magnetite and titanomagnetite powders in epoxy. The results indicate that, (l) the effect of pressure on susceptibility decreases with decreasing grain size, not in a continuous manner but rather depending upon whether the dominant magnetic grain size is multidomain, pseudo single-domain or single-domain, (2) the pressure response increases with the composition parameter x of the titanomagnetite in the solid solution series xFe2TiO4 (1−x)Fe2O4, and (3) the morphology of the grains influences the pressure response. The results are interpreted in terms of the behavior of the multidomain, pseudo single-domain and single-domain material. Changes in thermoremanent magnetization and the acquisition of pressure remanent magnetization under uniaxial compression, which were observed in conjunction with the susceptibility studies, suggest that rocks containing coarse grain titanomagnetites as the dominant magnetic mineral phase are the most efficient stress transducers. Hence a definitive seismomagnetic experiment would be easiest in a seismically active region in which such rocks are prominent.

Post-depositional remanent magnetization in a re-deposited deep-sea sediment

Re-deposition of some deep-sea clay material in the earth's magnetic field is described. At the time of an induced reversal of the ambient field declination, calcium carbonate layers were introduced for identification purposes. NRM and bulk susceptibility measurements along the still moist sediment cores showed the presence of a time lag of around 10 days between a change in the ambient field direction and a corresponding change in the stable magnetization. AF-demagnetization of discs sampled from a dried core suggested that this time lag was caused by a post-depositional remanent magnetization mechanism. Magnetomineralogical investigations show the presence of the thermally unstable magnetic mineral phase, probably titano-maghemite. A positive correlation between NRM-intensities and water temperature during deposition is discussed. It is suggested that consolidation-rate is a major factor influencing the NRM intensity in deep-sea sediments.