The effect of grain size on the field-dependence of ac susceptibility in titanomagnetites
Abstract
Low-field magnetic susceptibility is one of the mostly adopted rock physical properties in geoscience. It provides useful information about variations in composition and concentration of magnetic minerals. An effect not routinely considered is the dependence of some magnetic minerals on the field amplitude of the applied alternating field. Titanomagnetites (tm) generally exhibit increasing values of field-dependence as the magnetite content decreases. Rock magnetic investigations of basaltic lava from different subaerial and submarine stages of the Hawaiian shield volcano revealed different types of magnetic behavior related to variable degrees of high-temperature oxidation or cooling rates (Kontny et al. in press). Titanomagnetite is the main Fe-Ti oxide, additionally ilmenite, and in some samples chromian spinel and titanohematite occur. The primary tm is homogeneous and shows low Curie temperatures in the range of 70 - 200 °C (TM50-70). TM of this compositional range shows a strong dependence on the applied field amplitude. Generally, field-dependence in tm is related to composition whereas in pyrrhotite a correlation with grain size was observed. It will be shown that in addition to compositional effects variations in grain size also contribute to the field-dependence of magnetic susceptibility of tm.
- Publication:
-
EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA....11454V