The primary magnetic mineral in the newly constructed crust at the mid-ocean spreading centres is titanomagnetite. This becomes progressively maghemitized under the physical and chemical conditions which prevail in the submarine crust and it is believed that it is this process which is largely responsible for the striking fall in intensity of the remanent magnetization of sea-floor basalts with increasing age, and the initial rise and later fall in the stability of remanence with increasing age. In this paper we apply the data which has now accumulated on synthetic analogues of the titanomagnetites and titanomaghemites, together with ideas about the magnetic state of fine particles, to produce physical models for the characteristic magnetic properties of sea-floor basalts. By considering physically plausible models for the evolution of the cation distribution, as maghemitization makes the spinel structure more and more non-stoichiometric, the variation is spontaneous magnetization (Ms) with degree of maghemitization (z) can be inferred. It appears that this compositional change cannot bring about a big enough fall in Ms to account for the large fall in remanence observed in basalts. A microstructural change must therefore be invoked. In the heterogeneously maghemitized particles of the submarine crust, the more maghemitized outer layer is stretched over the less maghemitized core. As maghemitization progresses, the stressed outer layer grows in volume. We consider how the magnetoelastic energy of a model spherical monodomain titanomagnetite particle increases as maghemitization proceeds and show that, especially for particles near the maximum size for the monodomain state (unstressed), a transformation from the coherent to an incoherent spin structure becomes energetically favourable. This reduces the moment of the particle and allows the moment to fall subsequently to a very small value. This effect seems the most plausible cause of the fall in remanence of sea-floor basalts. Data for the observed temperature dependence of coercive force of synthetic titanomagnetites is reviewed, together with ideas about the anisotropy of Fe2+ ions, to produce a semi-empirical relation between coercive force, temperature and degree of maghemitization (z). Complex behaviour arises because as z increases the concentration of the anisotropic Fe2+ falls but Curie point temperature rises. Thus at room temperature the model predicts first a rise in magnetic hardness as maghemitization develops followed by a fall, but cannot in itself account for the observed increase in stability with age of sea-floor basalts. The model represents the case of constant stress, σ, so essentially shows the variation with z of (λMs), where λ is the composition and temperature dependent magnetostriction coefficient. The effect of the growing stress in the heterogeneously maghemitized particle is incorporated into the model, which then provides a plausible fit to the rise and fall of the magnetic stability of the sea-floor basalts. Thus the microstructural consequences of maghemitization play as great a role as the consequences of compositional change in accounting for the magnetic characteristics of sea-floor basalts.
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Jan 28, 2002
A M Bratkovsky + 1
Open Access