Effect of Stress on the Magnetic Memory of Induced Magnetic Anisotropy of Rocks and Its Mathematical Model

When re-heated to temperatures below the Curie temperature and subsequently cooled in a constant magnetic field (H _T), rock samples which contain magnetic minerals can acquire an induced magnetic anisotropy (IMA). As the result of acquiring the IMA, a constriction develops in the hysteresis loop of the magnetization of these rocks at the values of the magnetizing field close or equal to the HT. Thus the IMA is capable of retaining the information on the palaeointensity of the geomagnetic field, i.e., if IMA was created in a rock in the geomagnetic field in a past geological epoch, it preserves the information on the intensity of that field. Investigations have shown, that when IMA is created in a rock under external stress, the stress has an impact on the magnetic memory. Here we also deal with the issue of how stress affects the magnetic memory of IMA. A mathematical model for the effect of stress on magnetic memory phenomena related to induced magnetic anisotropy in rocks containing multidomain magnetite and titanomagnetite grains is proposed herewith. The effect of temperature on the magnetic memory of rocks is discussed also.     

Effects of and methods for determination of induced magnetic anisotropy of igneous and metamorphosed rocks

The phenomena of the magnetic memory of rocks associated with paleointensity, paleotemperature, and paleostress are reviewed and discussed here. The methods for the determination of the paleointensity and the paleoconditions are described and discussed in terms of their sensitivity and applicability. The determination of paleoconditions (stress and temperature) is essential for understanding rock mineral formation and rock history. Such knowledge is applicable also in ore deposit geology and geophysics. The phenomena of the magnetic memory manifest themselves through the constriction and asymmetry in the hysteresis loops, as well as through a nonlinear pattern in the anhysteretic magnetization curve, both being due to the induced magnetic anisotropy. The pros and cons of some applied methods are reviewed. The superposition of several paleotemperatures (re-heatings of the rock) and/or paleostresses is studied also. Under certain conditions, a rock can remember information on several paleotemperatures (paleo-heating events), as well as the respective intensities of the geomagnetic field of the past.