The effect of magnetic field direction during the low-temperature oxidation of titanomagnetite bearing TRM on paleointensity determinations

In this study, we have carried out a laboratory simulation of the low-temperature oxidation (below T_c) of titanomagnetite carrying a primary thermoremanent magnetization (TRM) created in B_lab=25-50 µT. The initial material was natural basalt P72/4 from the Red Sea rift zone containing unoxidized (degree of oxidation Z =0) titanomagnetite with ulvöspinel content x=0.5 (Fe_2.5Ti_0.5O₄).

Cubic basaltic samples containing titanomagnetite were annealed in air in weak field B_an=50-100μT at temperature T_an=260 Cfor maximum time t=1300 hours. One group of samples were annealed in the magnetic field perpendicular (B_an⊥TRM) and other parallel (B_an∥TRM) to the primary TRM. Thellier's-Coe double-heating method in argon atmosphere were conducted using all samples with different Z.

As for B_an∥TRM, the calculated field value (B_calc) obtained from Thellier-Coe’s procedure coincided with the laboratory field (B_lab) for all annealing times except t=1300 hours. This result indicates the applicability of the Thellier-Coe method of paleointensity determination on basalt containing titanomagnetite of low and medium degrees of oxidation (Z<0.5).

However, inadequate results were obtained from samples annealed in a magnetic field perpendicular to primary TRM(B_an⊥TRM). In this case, the calculated value of the B_calc field for t=12 hours are overestimated by ~40%, and for t=400 and 1300 hours is underestimated by ~20% relative to the TRM creation field.

We conclude that reliability of paleointensity data of oxidized titanomagnetite depends on the direction between magnetic field and TRM during the oxidation process.