Search

—The experiments have been carried out on the acquisition of thermochemical remanent magnetization (TCRM) in basalt samples containing titanomagnetite (TM) with the Curie temperature Тс ~200°C by their rapid heating to maximum temperatures Т* from 450 to 530°C followed by slow cooling in the laboratory magnetic field Blab. At different stages of the preliminary thermal treatment of the initial samples, a set of magnetomineralogical studies including electron microscopy, X-ray diffraction and thermomagnetic analyzes, and measurements of magnetic hysteresis parameters were performed. It is shown that as early as the very beginning of the cooling process, all samples demonstrate explosive growth of TCRM corresponding to the stage of rapid single-phase oxidation of the initial titanomagnetite fraction of basalt, and that TCRM is acquired by the increase of Тс and volume of single-phase oxidized parts of TM grains as well as by the growth of the volume of Ti-depleted (relative to the initial TM) cells of microstructure of the subsequent oxidative exsolution. The Arai–Nagata diagrams for the samples carrying TCRM have a form of a broken line consisting of two linear segments. The low-temperature interval T < Т* corresponds to a mixture of thermochemical and thermoremanent (TRM) magnetizations and gives a slightly overestimated Blab because of the effect of a low cooling rate during the acquisition of TCRM and TRM. The high-temperature interval corresponds to pure TCRM and the Blab strength determined from this interval is underestimated by 20–27%. It is recommended to reject samples whose Araii–Nagata diagram has two or more linear segments against the background single-component NRM.

—The experiments on the determination of paleointensity Banc of the Earth’s magnetic field by Thellier method on basalts of the Berd (Jurassic) and Paravakar (Cretaceous) collections sampled in the northeastern Armenia in 2006 revealed an anomalous behavior of the Arai–Nagata diagrams. The anomaly manifests itself by a sharp drop in the intensity of natural remanent magnetization (NRM) under heating of samples to about 400°C, which is accompanied by a very weak acquisition of partial thermoremanent magnetizations (pTRMs) in this temperature interval. The further increase in temperature leads to the opposite phenomenon—an unexpectedly sharp rise in pTRMs intensity with almost no decrease in NRM, which creates the L-shape of the Arai–Nagata diagram. A similar phenomenon of a steep decline during thermal demagnetization was also established for remanent saturation magnetization Mrs(T). We carried out a number of magnetomineralogical experiments from which we conclude that, consistent with the hypothesis suggested by (Kosterov and Prevot, 1998), the sharp drop in Mrs(T) and NRM(T) curves is caused by the transitions of the domain state from a metastable configuration to a more stable one. These transitions are initiated by the processes of single- and heterophase oxidations of primary titanomagnetites under the laboratory heating of samples to moderate temperatures.

Laboratory modeling of the CRM formation on basaltic rocks from the Reykjanes submarine ridge and the Red Sea rift zone containing titanomagnetites (TM) with Tc ≈ 200 °C was carried out by annealing at different temperatures Tan for different time intervals up to 200-hour exposure in the external field BCRM = 50 μT. Monitoring the CRM value against annealing time revealed a strong correlation between the CRM intensity and a strong field magnetization Ms. The existence of this correlation implies that the process of CRM acquisition actually takes place only in a relatively short initial time interval of annealing, so that the further CRM changes in time basically follow variations of Ms due to the process of oxidation of TM taking place during the annealing. It is shown that at Tan = 350 and 400 °C TM grains are subjected to high degree of single-phase oxidation while annealing at high Tan = 450 and 500 °C leads to the oxyexsolution. The Arai-Nagata diagrams for samples bearing the isolated induced CRM can be approximated by two linear segments with a break point at T = (560–570) °C. For the low temperature interval (Tan, 560 °C), the calculated field Bcalc underestimates the true field BCRM by 33–56% at Tan = 350 and 400 °C but by 29–42% at Tan = 500 °C. Surprisingly, all specimens annealed at 450 °C yield Bcalc which is very close to the true field BCRM. The observed error in the determination of BCRM tends to decrease with increasing Tan probably due to the relative narrowness of the blocking temperature interval, centered near Tc.