ИСТИНА

In this work we present first results of the long-time annealing experiments carried out on laboratory magnetized basalt samples in air and in weak magnetic field. All selected samples are basalt P72/4 from the Red Sea rift zone. Natural magnetic characteristics varies in the small range: NRM=52.1-60.5 A/m, ko=(1.5-1.7)*10-2 SI units, Qn=75-105. TMA and EMA shows that the main carrier of NRM in basalt is homogeneous titanomagnetite (TM), characterized by a Curie temperature spectrum Tc=190-350°C. According to the hysteresis characteristics (Irs/Is=0.37 and Hcr/Hc=1.28), TM grains are in a PSD state. The initial magnetic state of all demagnetized samples was obtained after cooling in the magnetic field (B=50 µT) from 400°C to room T in the argon atmosphere to create primary TRM. The value of the TRM is average 85-90% of the NRM (TRM=49.2 A/m). Samples bearing part of the laboratory TRM with blocking temperatures 260-400°C were annealed at Tan=260°C in the magnetic field B=50 µT, directed in some cases parallel(∥) and in others perpendicular(⊥) to TRM. This pTRM keeps 55-60% of TRM value. As a result of 12.5, 100 and 400 hours annealing, Tc and Is increased from 40 to 150°C and from 2400 A/m to 2950 A/m, respectively. At the same time, Hc decreased from 17.2 mT to 15.6 mT, and Hcr from 22.1 to 20.4 mT. Heating of annealed samples in argon atmosphere up to 600°C resulted in the complete homogenization to the initial phase state, which indicates the single-phase TM oxidation. Analysis of the temperature cleaning of three-component IRM (Bx=18mT, By=50mT, Bz=1500mT) shows an increase in the width of the blocking temperature spectrum and it’s shift towards higher temperatures. In the case of B⊥TRM, 400 hours annealing reduces magnetization from 27A/m to 23A/m and provides appearance of the new CRM component (up to 25% of TRM). Using magnetic and thermal cleaning, two components of magnetization are confidently diagnosed: thermal stability of the TRM is significantly higher, and most of the CRM blocking temperatures are confined in a narrow range 260-300°C near Tan. The coercive force spectra of CRM and TRM are also do not coincide. Maximum deblocking temperature of the TRM component increases by 80-100°C after annealing. Otherwise when B∥TRM, the magnetization decreases from 31.5 A/m(t=0h) to 28.2 A/m(t=12h) and then slowly grows up to 31.7 A/m(t=400h). In this case, it’s not possible to diagnose CRM by TDA or AFD. RFBR supported this work (grant 20-05-00573)