ИСТИНА

During chemical transformations of minerals in the Earth's magnetic field, chemical remanent magnetization (CRM) can be formed, which interferes with the solution of paleomagnetic problems. The study of the properties of an artificially created CRM shows that there are conflicting data on the separation of CRM and TRM in rocks. [Maksimochkin et al., 2015; Gribov et al., 2017]. To develop criteria for the recognition of CRM in NRM, an experimental simulation of CRM on the basalts samples of the Red Sea P72-2 and P72-4 and the study of its properties were carried out. According to electron microscopy and thermomagnetic analysis, the magnetic mineral of these basalts is titanomagnetite with a magnetite consentration of 44.7% and 50.1% and Curie temperatures Tc=222±5°C and 258±10°C, respectively. The grains of the titanomagnetite of the dendritic structure in the initial state are homogeneous, the composition is close to the stoichiometric, the domain structure according to [Day et al., 1977] corresponds to the pseudo-single-domain state. Studies of samples annealed at temperatures Tan=290–535°C for 10 hours made it possible to distinguish two processes of transformation of the initial titanomagnetite. The first process was observed at annealing temperatures of 290–410 °C. It was characterized by an increase in the magnetic susceptibility and saturation magnetization by 1.5–1.6 times, an increase by 13–14% of the remanent saturation magnetization and a 10–15 % decrease in the magnetic stiffness, and also the inhomogeneity phase state, which was probably due to the heterogeneity of the oxidation of grains of different sizes. Annealing of the samples at 350 °C showed that saturation magnetization reached a maximum value after holding for 40.5 hours. With increasing dwell time the contribution to the saturation magnetization of the magnetic phase with a lower Curie temperature decreases. After annealing for t=40.5–110 hours, the thermomagnetic analysis showed the presence of one magnetic phase with Tc=490–510 °C. A distinctive feature of this process of transformation of titanomagnetite is its reversibility: heating of samples in an argon medium up to 600 °C led to homogenization of oxidized titanomagnetite. It is shown that an increase of the annealing temperature to T=460–535 °C, as well as an increase of annealing time up to 350 hours at T=350 °C, leads to changes in the magnetic properties of titanomagnetite, characteristic of oxy-decomposition. The restoration of the initial state of titanomagnetite after heating the samples to 600 °C in argon was not carried out in this case. It was found that CRM, measured at room temperature, increases with the annealing time by a law close to exponential, and reaches saturation at t=40 hours. The ratio of the partial thermoremanent magnetization (PTRM) formed in the temperature range from Tc to the annealing temperature decreases from 1.3–1.4 for t=4.5 hours to 0.93 for t=110 hours. Studies have shown that the properties of CRM and PTRM (Tc–Tan) approach each other with increasing oxidation state. With a high degree of oxidation of titanomagnetite, CRM can be identified as TRM. An indicator that the NRM in basalts in this case is CRM can be a significant decrease in the Curie temperature of the samples after heating them in an argon medium to 600 °C. In the middle stage of single-phase oxidation, the chemical and PTRM at T>Tan have practically identical spectra of blocking temperatures and differ significantly at T<Tan. Obviously, in this case, CRM and TRM in basalts can be recognized.