The Field‐Dependence of AC Susceptibility in Titanomagnetites: Implications for the Anisotropy of Magnetic Susceptibility

In a case study on titanomagnetite‐bearing rocks from the Riedheim dike of (Hegau volcanics, SW‐Germany), samples of varying composition (Curie temperatures of 128 to 440 °C) have been tested for their ac field‐dependence of low‐field susceptibility. The measurements were performed in ac fields of 30 and 300 A/m. In order to quantify the field‐dependency of titanomagnetites, the parameter χ Hd [%] = [(χ 300A/m ‐ χ 30A/m ) / χ 300A/m ] × 100 is introduced, which ranges from 4.1 to 26.5. A liear relationship has been determined between the Curie temperature of the titanomagnetites and the log χ Hd parameter. The anisotropy factor (P′) of the anisotropy of magnetic susceptibility (AMS) ellipsoid is significantly enhanced in the higher ac field, while the shape factor (T) appears to be insensitive to the field amplitude. However, despite these variations, the principal axes of the AMS‐ellipsoids remain unchanged in orientation.