A. K. Gapeev and S. K. Gribov
The Borok Geophysical Observatory, Russian Academy of Sciences
The titanomagnetites (TM) being the main carriers of the rock magnetism are not stable on the earth surface from the point of view of thermodynamics. Influenced by different physical-chemical processes, oxidation and decomposition being the main, they suffer the mineralogical transformations, which lead not only to a failure of the primary magnetization (for the volcanogenic rocks this is of a thermoremanent origin, as a rule), that carries information about an ancient geomagnetic field, but also to generation of a chemical remanent magnetization (CRM or JCRM ), which distorts, in its turn, the primary magnetization and, consequently, affects the paleomagnetic safety. Therefore, a great interest to generation of the CRM and to the study of its properties is quite natural [Bailey and Hale, 1981; Beske-Diehl, 1990; Borisova and Sholpo, 1991; Gapeev et al., 1991; Gromme and Mankinen, 1976; Haigh, 1958; Hall, 1976, 1977; Heider and Dunlop, 1987; Hunt et al., 1986; Johnson and Hall, 1976, 1978; Johnson and Merrill, 1972, 1973, 1974; Kobayashi, 1959; Markov and Shcherbakov, 1987; Marshall and Cox, 1971, 1972; Merrill, 1975; Nguyen and Pechersky, 1987; Nishitani and Kono, 1989; Özdemir and Dunlop, 1985, 1988, 1989; Prevot et al., 1981; Raymond and La Brecque, 1987; Smith and Banerjee, 1985; Soppa, 1989; Soroka and Beske-Diehl, 1984].
Experiments on creation of the CRM were conducted both on the synthetic objects [Johnson and Merrill, 1972; Nishitani and Kono, 1989; Özdemir and Dunlop, 1985] and on the natural objects [Bailey and Hale, 1981; Johnson and Merrill, 1972, 1973; Marshall and Cox, 1971]. The generalized results which were obtained may be presented in the following way.
1. At a single-phase oxidation of TM the CRM inherits the direction of the primary remanent magnetization, when the ferrimagnetic grains are in a single-domain (SD) or in a pseudosingle-domain (PSD) state. However, some authors [for example, Johnson and Merrill, 1973; Nishitani and Kono, 1989] show the possibility of the CRM to generate in the direction of the applied external magnetic field. Moreover, the given CRM in the highly oxidized TM proved to be comparable, by its intensity and stability to the alternating magnetic field ( H ), with the corresponding characteristics of the primary thermoremanent magnetization (TRM) residue retained in the course of maghemitization [Nishitani and Kono, 1989].
2. At a single-phase oxidation of a multi-domain (MD) TM, as well as at the heterophase changes of the TM grains in the SD, PSD and MD states, the CRM being generated, reflects according to all the data available, the direction of the external magnetizing field.
Investigations of the vector of the natural remanent magnetization ( Jn ) of the oceanic basalts have shown that generation of the field controlled JCRM component is also possible at a single-phase oxidation of the titanomagnetite small and coarse fractions [Gromme and Mankinen, 1976; Hall, 1977; Johnson and Hall, 1976; Prevot et al., 1981].
The ambiguity which exists in the concepts of direction of the JCRM vector has induced us to study thoroughly the rules of generation and properties of the chemical remanent magnetization at a single-phase oxidation and a subsequent decomposition of the TM grains. Along with this the following tasks have been posed: 1) study the change of the JCRM value depending on the time and temperature of its formation; 2) find out the relation of the mineralogical transformations and magnetic characteristics of the TM with changes of the JCRM value and stability; 3) compare the JCRM value and stability with the corresponding characteristics of other types of the remanent magnetization; 4) estimate the influence of the single-phase oxidation and the subsequent decomposition of the TM on safety of the primary (thermoremanent) magnetization.
The experiments on creation of the chemical remanent magnetization were carried out on the cube samples 111 cm in size which were a mixture of kaolin and 1% by weight of the synthetic TM powder whose composition was Fe3-xTixO4 (where x = 0.2, 0.4 and 0.6) with the 0.5-0.8 m m ferrimagnetic particles. The samples which had been demagnetized first ( H = 2000 V m -1 ) along three orthogonal directions were subjected to oxidation in a thermomagnetometer with a temperature range DT =153-518oC in a constant magnetic field H=H0=1 V m-1 and with the isothermal time period t from 2 minutes up to 700 hours. The CRM which was generated at this time was measured directly at the temperature of its formation in the thermomagnetometer shields. Cooling of samples down to a room temperature ( T0 20oC) was also there with the field being switched off.
A partial thermoremanent magnetization JpTRM(TCRM-T0 ) was created when the sample was cooled in the H0 field. The anhysteretic remanent magnetization JARM(H = 1 V m-1, H = 2000 V m-1 ) was created at a room temperature. The remanent magnetization of different types was alternating field (AF) demagnetized also at a room temperature T0. All the remaining magnetic characteristics, which were used in this work, were measured according to standard procedures of the rock magnetism.
Examination of the results allowed to connect the original growth of JCRM(t) with the single-phase oxidation of the initial TM, to connect the subsequent relative drop of JCRM(t) with the beginning of decomposition of the cation-deficient TM when the neogenic magnetite phase does not contribute to the overall CRM because of a small size of nuclei at the initial stage of decomposition. The new and a more considerable growth of JCRM(t) with its subsequent gradual drop (which is clearly fixed at sufficiently high annealing temperatures) may be connected with the increase of the magnetite phase volume and its subsequent oxidation. Naturally, only original growth of the chemical remanent magnetization is observed at relatively low temperatures as the TM transformation process in this case stops at the stage of generation of the cation-deficient spinel and becomes much extended in time at the given temperatures. Finally, let us also note, that when the Curie temperature ( Tc ) of the single-phase oxidized TM was below the heat treatment temperature, no CRM formation occurred.
Along with investigation of the CRM generated in the direction of an external magnetizing field, the magnetic measurements of all samples were made in projections orthogonal to H0. The results show that the JCRM is firmly single-component throughout the studied time interval of the TM transformations and is generated towards the external magnetic field ( JCRM r| H0 ).
The experiment has shown that the value of CRM formed on a titanium-rich TM ( x = 0.6) appeared to be greater than for the titanium-poor TM ( x = 0.2 and x = 0.4) both at the single-phase oxidation and at the stage of a subsequent decomposition of TM. This difference is expressed distinctly at the single-phase oxidation stage (Figure 1).
Experiments on estimation of changes of the viscous magnetic properties of the TM during their single-phase oxidation were carried out in the following way. After the alternating magnetic field destroyed the CRM generated in the course of oxidation of the TM samples (originally x = 0.4) to different Z -states (with the isothermal period from 2 minutes up to 370 hours at a fixed temperature from 153 to 288oC) the samples were subjected to a repeated holdup under similar temperature conditions and also at 20oC; the time changes of magnetization ( JVRM(t) ) after the action of the constant magnetic field H0 on the samples were recorded. The dependencies obtained in coordinates DJVRM-D log t were described well by a straight line that allowed the magnetic viscosity coefficient Sv (calculated according to equation Sv=dJVRM/d ln t by a change of the viscous magnetization JVRM with time t under action of the constant magnetic field) to be calculated by it s inclination. The results are presented in the Table 1 (columns 2, 3, 4 and 5). The analysis of the data shows that velocity of the viscous magnetization is greater on the samples with a high oxidation parameter of the titanomagnetite grains than on the weakly oxidized TM and along with this the temperature rise leads to an increase of Sv. However, with the T rise, influence of the parameter of the TM single-phase oxidation on speed of their viscous magnetization reduces.
Thus, investigation of the magnetic properties of the single-phase oxidized TM samples (initial x = 0.4) has shown that at the oxidation parameters Z<0.5 intensity of the occurring CRM is proportional to Z; at higher values of Z the viscous magnetization of samples becomes apparent and the magnetic viscosity coefficient becomes greater both with the oxidation temperature rise and with the rise of the TM oxidation level.
As to the separated components of the CRM resistance to the alternating magnetic field effect it has been found experimentally that AF stability of the complete CRM is higher than stability of its viscous component but it is less than resistance to AF cleaning of a "pure'' chemical CRM component.
All experiments on creation of the CRM described earlier were performed on the samples which had been demagnetized first. Under the natural conditions, however, as has been mentioned before, the process of generation of the CRM is of an superimposed nature and a primary role here belongs to magnetization of the igneous rocks, which are of a thermoremanent origin, as a rule. This brings up a point about influence of the secondary transformations of the TM on safety of the TRM of the parent phase. The following experiments were carried out to estimate this influence.
During the last experiment with the TM samples with the initial x = 0.4, stability of the newly generated CRM and leavings of the undecayed initial TRM was also compared. The results of a simultaneous stepwise demagnetization of mutually perpendicular types of the remanent magnetization by an alternating magnetic field are shown in Figure 11. It follows from this that at the single-phase stage ( T 378oC) of the TM oxidation by small H -fields the chemical remanent magnetization is destroyed somewhat more strongly and the TRM is destroyed by greater - fields. This may be explained, on one hand, by influence of a viscous component of the CRM and on the other hand, by occurrence of stresses and by an increase of the magnetic rigidity of TM at their single-phase oxidation. Attention is attracted on the same figure to a 45-degree slope of straight portions of the curves, which characterizes the Jr coordinates of the heterophase modified TM after a corresponding H -cleaning of its TRM and CRM components thus indicating to their (components) similar resistance to this type of action. This complies with the results of the investigation of stability of the above presented pTRM and CRM. Thus, separation of the CRM-component and the retained primary TRM-component of Jr (just as pTRM-component) by their magnetic field stability becomes impossible because of overlapping of the coercive spectra. This condition, of course, complicates the use of the H -cleaning method in the practice of the paleomagnetic investigations for separation of the characteristic (thermoremanent) magnetization for the rocks with the transformed TM.
This investigation has shown that the chemical remanent magnetization, reflecting a direction of the external magnetizing field, occurs not only upon the heterophase change of TM (which accepted unconditionally by all the researchers) but also in the course of a single-phase oxidation of the stochiometric single-domain TM; along with this destruction of the primary thermoremanent magnetization takes place. On the other hand, according to the kinetic investigations of the TM transformation processes [Gapeev and Gribov, 1990, 1997], the single-phase oxidation of TM and decomposition of the cation-deficient TM processes on the earth surface may be very much extended in the geological time scale. All this taken together means that the chemical magnetization of the TM may be formed and reflect the terrestrial magnetic field of different geological epochs and during the geomagnetic field inversions as well. Therefore, a possibility of a partial remagnetization of the rock as a result of occurrence of the secondary JCRM components of different age becomes realizable not only due to decomposition of the cation-deficient TM, but also in the course of the preceding titanomaghemitization.
The investigation has shown that only high-oxidized TM may play an important role in generation of Jn at the stage of the single-phase oxidation due to a considerable contribution to the their remanent magnetization of the chemical component and its viscous constituent. By assuming that regularities of growth of the viscous magnetization during the geological time are exactly the same as those estimated in the laboratory experiment (see Table 1), then the JVRM taken by the surface basalts (at 20oC), similar to the TM composition ( x = 0.4) under investigation, will make 0.00277 G/g at Z 0.53, 0.00469 G/g at Z 0.73 and 0.00723 G/g at Z 0.95 for 1 million years and 0.00295 G/g, 0.00527 G/g and 0.00831 G/g for 100 million years respectively. It may be seen from comparison of these findings with the thermomagnetization that the viscous component will make no more than 1.5% at Z<0.5. However, at a more deep stages of the single-phase oxidation when, according to our results, the initial TRM reduces by about 6 times, while the chemical magnetization component proper increases by 95% of the TRM remainder and velocity of the viscous magnetization becomes 6 times greater only on account of the oxidation level rise, the viscous magnetization on the earth surface may amount to approximately 20% of the nonfailed thermomagnetization after 1 million years, provided the geomagnetic field is constant. It is obvious, that the viscous component may contribute even more to the natural magnetization of the rocks due to a rise of the magnetic viscosity along with the temperature rise (see Table 1, column 8). In case of a frequent inversion of the geomagnetic field the total magnetization will, evidently, be completely determined by a contribution of the nonfailed portion of the TRM.
The established regularities regarding influence of the single-phase oxidation and a subsequent decomposition of the single-domain TM upon safety of the primary (thermoremanent) magnetization allow us to draw a conclusion that it is not possible to use the multi-domain and also SD grains of TM for the paleomagnetism purposes both with a high single-phase oxidation level and also with the signs of decomposition due to a considerable distortion of their original magnetization. At the same time, since the CRM value at the initial stages of the single-phase oxidation of TM, according to this investigation, still does not exceed 10% of the primary TRM and deviation of the resultant vector of the remanent magnetization from the primary TRM direction is not more than 10o, one might suppose that the single-phase oxidation cannot be a serious obstacle for the use of a weakly oxidized ( Z<0.5 ) titanomagnetite SD fraction in the paleomagnetics practice.
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