Anisotropy of out-of-phase magnetic susceptibility in titanomagnetite-bearing rocks due to weak field hysteresis

Magnetic susceptibility (both in-phase, ipMS, and out-of-phase, opMS, components) and its anisotropy were investigated in artificial specimens with pure synthetic magnetite and in rocks with titanomagnetite with variable Ti-content reaching X_Usp = 0.6 of ulvospinel end member. The opMS of pure magnetite is in its absolute values three orders lower than the ipMS. The phase angle is then lower than 0.2° in absolute values, which is mostly lower than the sensitivity of phase angle determination. Consequently, the opMS can be considered effective zero. In low-Ti titanomagnetite (X_Usp~0.2), the intensity of the ipMS variation with field is very low, hardly reaching 1% of the initial value. In high-Ti titanomagnetite (X_Usp~0.6), the intensity of ipMS variation is relatively strong reaching 50% of the initial value and that of opMS variation is even much stronger reaching multiples of the initial value.

In artificial specimens with magnetite, the opMS is isotropic from the statistical point of view as indicated by the values of the F-statistics and by confidence angles. In titanomagnetite-bearing rocks, the opAMS ellipsoids resemble the ipAMS ellipsoids, the degree of opAMS being significantly higher than that of ipAMS. The principal directions of ipAMS and opAMS are related closely in specimens with high-Ti titanomagnetites and only poorly in specimens with low-Ti titanomagnetites. In specimens with high-Ti titanomagnetites, there is a linear and very strong (R² = 0.9433) correlation between the degree of ipAMS and root squared degree of opAMS.

The results of the present research are mainly applicable to the rocks whose magnetism is dominantly carried by titanomagnetite and pure magnetite. The opAMS indicates solely the titanomagnetite fabric unaffected by magnetite whose fabric may often originate in different conditions or in different time.