[16]  As noted above, partial self-reversal of TRM was already observed during multiphase oxidation of the given titanomagnetite samples [Gapeev and Gribov, 2002b]. Based on the evident dependence of this effect on the level of internal elastic stresses, the authors stated that, in this case, the cooling-related anomalous behavior of the thermal curve Jrt(T) can be accounted for by magnetostatic interaction between sample volumes differing in the level of stresses accommodating elastic deformations of the crystal lattice in the junction plane of phases that form during oxidation, the rhombohedral phase segregated as a network of ilmenite lamellae and the spinel phase forming a system of cells whose composition continuously varies from almost pure magnetite in the direct neighborhood of a lamella to the initial titanomagnetite far from it. A possible method of this phenomenon of TRM self-reversal is considered in detail in [Shcherbakov et al., 1976] and is based on the well-known fact of diffuse ferri-paramagnet phase transition in structurally heterogeneous ferrimagnets [Belov, 1959]. According to [Shcherbakov et al., 1976], we have a situation in which stressed (deformed) and unstressed volumes of ferrimagnetic material play a role of physically different phases (cf. [Verhoogen, 1959]) that have somewhat different Curie points. In this case, the cooling-related self-reversal of TRM can be effected due to magnetostatic interaction (magnetically les rigid unstressed regions of the ferrimagnet are magnetized in the demagnetizing field of higher-temperature stressed regions) and different temperature behavior of spontaneous magnetizations ( JS ) of the given regions (we emphasize that, according to [Shcherbakov et al., 1976], smaller JS(T0) correspond to higher TC, which is very favorable for the validity of self-reversal conditions).

[17]  In the framework of the given model, the total TRM experiencing partial self-reversal during thermal demagnetization of the multiphase-oxidized titanomagnetite fraction of the samples under study [Gapeev and Gribov, 2002b] can be tentatively subdivided into three components: (a) a magnetically rigid or high-temperature positive component whose carriers are spinel regions adjacent to lamellae, (b) negative component acquired under the action of component (a) and related to titanomagnetite volumes directly contacting with stressed zones, and (c) positive magnetically soft component whose carriers are unstressed titanomagnetite regions that are not involved in the self-reversal process.

[18]  A gradual attenuation and disappearance of a self-reversal in the spinel phase recorded in laboratory experiments [Gapeev and Gribov, 2002b] is evidently related to a decrease in the volume of this phase during the subsequent oxidation of a sample. In this case, the volume of the rhombohedral phase increases and a self-reversal should be expected in it due to the interaction between stressed and unstressed volumes. This is a characteristic feature of the magnetostatic self-reversal of TRM of both, low and high temperature types [Gapeev and Gribov, 2002b] during multiphase oxidation of titanomagnetite samples in which a self-reversal is discovered in the present work in curves of cooling and subsequent thermal demagnetization Jrc(T, H = 0).

[19]  In principle, the aforediscussed causes of the TRM self-reversal can be similar to those of the CRM self-reversal. Actually, in the case of CRM formation, the Curie points of forming material move away from the chemical reaction temperature Tr (on the geological time scales, even at relatively low temperatures of the medium), with the magnetic moments of the ferrimagnet being blocked along the direction of the external magnetizing field. In other words, with a fixed temperature of reaction, the Curie point of the newly formed material, rather than the cooling temperature (as in the case of the pTRM formation), is a variable value. This means that chemical magnetization accompanied by a rise in TC with fixed Tr is actually similar to the thermal magnetization in the analogous temperature interval TC - Tr of stable ferrimagnets. On the other hand, as was expected, anomalous behavior of curves of cooling and heating Jrc(T, H = 0) was fixed in our experiments just at the stage of titanomagnetite multiphase oxidation associated with development of significant elastic stresses in a sample (cf. [Gapeev and Gribov, 2002b]). In accordance with these facts, it seems therefore quite natural to apply the aforementioned considerations to the treatment of the CRM self-reversal with appropriate modifications: during chemical magnetization, the magnetization order in an oxidized structurally heterogeneous grain will be determined not by Curie points but by the succession of blocking temperatures of segregations (in the form of different phases or even separate pseudo-single-domain regions of a higher coercivity). Although in the present experiments we failed to obtain negative values of the resulting magnetization, it is important that the values of Jrc+rpt measured at 20oC were invariably smaller than the corresponding values of the "pure'' magnetization Jrpt (500o C, 20oC) (cf. Figures 2b and 5). Evidently, the complete CRM self-reversal requires that the earlier forming resulting Jrc must be less intense and less rigid than the magnetization forming later.

[20]  The physical mechanism of the self-reversal of remanent magnetizations (both TRM and CRM) considered here and previously proposed for the explanation of a particular case of the TRM self-reversal in titanomagnetites at the stage of coarsening of titanomagnetite spinodal decomposition structures [Melnikov and Khisina, 1976] appears to be of a more general significance. This is also indicated by results derived from studies of the TRM and CRM self-reversals at the spinodal stage (the formation stage of a modulated structure) of the natural titanomagnetite decomposition process [Gapeev and Gribov, 2008a], (A. K. Gapeev and S. K. Gribov, in press, 2008b).

[21]  In particular, as was established in [Gapeev and Gribov, 2008a], a stressed state can be preserved in a ferrimagnetic fraction representing finely disperse magnetite-ulvospinel segregations of the primary solid solution in ~250-Ma trap formations. Moreover, it is significant that samples of this doleritic collection generally had one or two antipodal components of the NRM vector. However, thermal treatment of the given natural samples revealed a partial self-reversal of TRM only in samples at the stage of semicoherent interfaces between segregations and, accordingly, at a relatively low level of stresses in the structure. In this case, laboratory-induced homogenization led to diffusional redistribution of cations and, as a result, generation of additional stresses in a nonequilibrium solid solution; i.e. conditions required for the realization of TRM self-reversal were created in the experiment. On the other hand, samples stabilized under in situ conditions at earlier decomposition stages (before the coherence loss in segregations) did not reveal the Jrt self-reversal effect because of rapid homogenization during thermal treatment of the samples. Therefore, we have every reason to suggest that the Jn self-reversal property can be lost at some stages of the titanomagnetite evolution but the reversed remanent magnetization is preserved. As a result, an undiscovered neglected self-reversal will lead to incorrect paleomagnetic and geochronological correlations, inadequate geostructural interpretations, and so on.

[22]  Thus, the comparison of results obtained in [Gapeev and Gribov, 2002b, 2008a; Melnikov and Khisina, 1976]; (A. K. Gapeev and S. K. Gribov, in press, 2008b) and in this study shows that the presence of concentration heterogeneities of the short-range layering type and the presence of significant inhomogeneous deformations of a crystal lattice are, on the one hand, features characteristic of products of titanomagnetite decomposition of both spinodal and oxidative types and, on the other hand, factors responsible for anomalous behavior of TRM and CRM.

[23]  Results of this work imply that the NRM self-reversal effect is not a unique case but a general phenomenon caused by physicochemical processes post-crystallization transformations (decomposition and oxidation) of a primary titanomagnetite solid solution.


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