Paleomagnetic properties of laboratory TRM of natural titanomagnetite during it`s low-temperature oxidation

It is well known that low-temperature oxidation of rocks in the magnetic field could lead to the formation of secondary magnetization of chemical nature (CRM). This case is a common reason for errors in determining the strength and direction of the ancient magnetic field related to primarily thermoremanent magnetization (TRM). The absence of explicit criteria for separating these types of magnetization in rocks is a fundamental problem of paleomagnetic studies.

To try to recognize CRM and TRM components of magnetization in the basalt we have carried out a laboratory simulation of the low-temperature oxidation of the Red Sea basalt P72/4 by annealing samples in the magnetic field. The main magnetic mineral of carried basalt is homogeneous titanomagnetite with a median Curie temperature T_c=260°C and NRM=55 A/m. The Ti content in titanomagnetite grains varies in the range of 6.8-7.5 wt. %, lattice constant a=8.455A. Titanomagnetite magnetic grains are mostly in the PSD state and have a dendritic structure. Laboratory TRM was created when demagnetized samples were cooled from 400 °C to room T in an argon atmosphere and a magnetic field B=50 µT. The TRM value is about 85-90% of the NRM (TRM_mean=49.2 A/m). 

Annealing of samples in the air was carried out at T_an=260 °C in a magnetic field with induction B=50 µT parallel and perpendicular to the previously created TRM. As a result of annealing during 12.5-1300 hours, T_c increased by 40-170 °C, I_s increased from 2400 A/m to 3050 A/m, H_c decreased from 17.3 mT to 14.3 mT, H_cr – from 22.1 c to 19.2 mT. The oxidation state (Z) after annealing for 12.5, 100, 400, and 1300 hours were found to be 0.16, 0.31, 0.58, and 0.67, respectively.

Regardless of the field B direction, magnetization co-directed with the initial TRM and blocking temperatures above 400 °C occurs. The effect increases with the annealing time: particularly t=12.5 h, the proportion of magnetization with unblocking temperatures T>400 °C is about 15-20% of TRM (260-400 °C), while for t=400 hours it is already 35-40% of TRM (260-400 °C). Then again the magnetization obtained as a result of annealing in B⊥TRM, two components are detected by thermal cleaning: one co-directed with B (CRM component), the other in the direction of the original TRM. The thermal stability of the TRM component is significantly higher than that of the CRM: most (75-80%) of the blocking temperatures of the CRM are confined to a narrow range near T_an (260-340 °C), while the TRM is destroyed after heating to 340 °C only by 35-40%. Also, CRM and TRM components differ in their resistance to the influence of an alternating magnetic field. In the case of B∥TRM, only one component is diagnosed with magnetic and thermal cleaning. In this case, it is not possible to detect the presence of two superimposed magnetization components of different genesis.

This work was supported by the Russian Foundation for Basic Research, project 20-05-00573.