Characteristics of Orthogonal and Arai–Nagata Diagrams on Titanomagnetite-Bearing Basalts with Laboratory-Induced Mutually Perpendicular Thermal and Chemical Remanent Magnetizations

Abstract—The paper evaluates the influence of secondary magnetization on the determination of characteristic component and paleointensity in basalt rocks whose magnetic remanence is carried by titanomagnetite with varying degrees of oxidation. To this end, laboratory experiments on magnetization acquisition were performed on basalt samples from the submerged Reykjanes Ridge, North Atlantic. The “primary” total thermoremanent magnetization TRM was imparted to rock samples by their exposure at temperature of 600°C followed by cooling in a magnetic field of 50 µT. The “secondary” chemical remanent magnetization CRM, perpendicular to TRM, was overprinted by subsequent exposure of samples at 350°C for 200 h in the same field. Directly at a temperature of 350°C, temporal monitoring of the changes in the TRM and CRM components was carried out. The samples with laboratory-induced total remanent magnetizations were subjected to the Thellier-type paleointensity experiments, and the results were used to construct the Arai–Nagata and Zijderveld diagrams. It is shown that CRM is formed on the new ferrimagnetic phases that resulted from the oxidation of an unstable fraction of titanomagnetite, and the CRM intensity critically depends on the degree of oxidation of the initial material. All the constructed diagrams have linear segments with different slopes, which are identified in the low-temperature (LT) range from 20–350 to 450°C, medium-temperature (MT) range from 450–475 to 500–530°C, and high-temperature (HT) range from 500–530 to 560–600°C. It is shown that the presence of a secondary component leads to errors in both paleodirection and paleointensity determinations from the primary component, and the errors are greater the greater the secondary CRM relative to the primary TRM. A new criterion of reliability of paleomagnetic data is proposed. This criterion applies to both paleointensity and paleodirection and states that the primary component should be several times as large as the secondary component.