[2] Self-reversal (the phenomenon orienting the remanent magnetization in the direction antiparallel to an external magnetizing field) is a phenomenon alternative to an Earth's magnetic field reversal as a factor responsible for a reversed direction of the natural remanent magnetization (NRM or Jn ) in rocks. In this respect, an anomalous magnetization of rocks can lead to an incorrect idea concerning the variation of the ancient geomagnetic field and, therefore, an inadequate interpretation of magnetic data obtained from paleomagnetic studies of the rocks. This is why elucidation of the origin of the NRM self-reversal effect and prediction of this phenomenon are of great importance for solving paleomagnetic problems (in particular, the identification of spurious paleomagnetic zones).
[3] L. Néel [Néel, 1951] was the first who substantiated theoretically the possibility of self-reversal with reference to thermoremanent magnetization (TRM or Jrt ) and this undoubtedly stimulated the interest of paleomagnetologists in experimental studies of this phenomenon. Cases of the TRM self-reversal were discovered in rocks the ore fraction of which is represented by assemblages of minerals belonging to isomorphous series and to series of the solid solutions hematite-ilmenite (hemoilmenite) [Bina et al., 1999; Hoffmann and Fehr, 1996; Kennedy, 1981; Minibaev et al., 1965; Nagata, 1961; Ozima and Funaki, 2001; Ozima et al., 2003; Prevot et al., 2001] and magnetite-ulvospinel (titanomagnetite) [Gapeev and Gribov, 2008a; Havard and Lewis, 1965; Heller, 1980; Hoffman, 1982; Ozima and Larson, 1968; Trukhin et al., 2004; Westcott-Lewis and Parry, 1971; Zhilyaeva et al., 1970, 1971]. The effect of anomalous behavior in Jrt(T) was also observed on synthesized analogues of these ferromagnets [Bugaev et al., 1972; Gapeev and Gribov, 2002b; Havard and Lewis, 1965; Lewis, 1968; Petersen and Bleil, 1973; Tucker and O'Reilly, 1980; Trukhin et al., 1997; Zvegintsev and Grankin, 1973]. However, until recently the self-reversal effect has been very poorly studied in relation to the chemical remanent magnetization (CRM or Jrc ) directly associated with magnetomineralogical alterations of rock-forming minerals at various stages of their evolution. In general, one may state that CRM is the least studied and difficultly diagnosable NRM component mainly because many stages of its formation (particularly at relatively low temperatures of reactions) cannot be reproduced on usual laboratory time scales.
[4] At present, the reality of the total CRM self-reversal has been demonstrated experimentally only in two case studies of the maghemite-hematite transformation [Hedley, 1968; McClelland and Goss, 1993]. Under the conditions of lacking information on the recurrence extent of the CRM self-reversal, it is desirable and appropriate to conduct long-term laboratory experiments designed for the creation of CRM and examination of its possible self-reversal using real types of reactions involving magnetomineralogical alterations of main magnetic carries in rocks.
[5] This work continues our cycle of investigations devoted to experimental simulation of titanomagnetite multiphase transformation processes (oxidation and decomposition) and the study of properties of related remanence types in newly formed phases. In particular, experiments on multiphase oxidation of synthetic titanomagnetites (Fe2.4Ti0.6O4 ) revealed the possibility of the partial TRM self-reversal in two temperature intervals. The partial TRM self-reversal in a high temperature interval is realized in the spinel phase due to magnetostatic interaction between regions of different magnetic properties because of both heterogeneity of composition and the presence of stresses, while a low temperature self-reversal (near the Curie point TC of newly formed hemoilmenite) is caused by interphase interaction between spinel and rhombohedral phases: during cooling of titanomagnetites subjected to multiphase oxidation, lamellae of the low temperature hemoilmenite phase are thermally magnetized in the demagnetizing field of the higher temperature spinel phase (however, in discussing the effect, we did not exclude the possibility of anomalous TRM behavior in the rhombohedral phase itself, as concentration and continental heterogeneities develop during its decomposition). We should also emphasize here that in both cases, as was shown experimentally [Gapeev and Gribov, 2002b], the magnetic rigidity of the component directed along an external magnetizing field is always higher compared to the reverse-polarity TRM component. On the other hand, our earlier studies of CRM properties [Gapeev and Gribov, 1996, 1997, 1998, 1999, 2002a] also showed that, regardless of the titanomagnetite transformation, the acquired CRM is always magnetically more rigid than the corresponding pTRM. On the whole, these patterns suggest that the CRM self-reversal effect is even more pronounced during cooling of titanomagnetites, at the earlier stages of their multiphase oxidation, when the anomalous behavior of TRM (and pTRM) is best expressed [Gapeev and Gribov, 2002b]. Results of direct experimental verification of this suggestion are described in the present work.
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