MINERALOGICAL CHANGE AND SELF-REVERSED MAGNETIZATIONS IN PYRRHOTITE RESULTING FROM PARTIAL OXIDATION - GEOPHYSICAL IMPLICATIONS

Low-field thermomagnetic studies performed on natural and synthetic pyrrhotite samples showed that a very low oxygen fugacity is already sufficient to start the transformation into magnetite at 500 degrees C. The quantity of magnetite produced is a function of the oxidation time during which the temperature is maintained. The remanent magnetization acquired by the magnetite during its formation in the applied field is a chemical remanent magnetization (CRM). In the case of partial oxidation, the close coexistence of pyrrhotite and magnetite in a single grain may produce a self-reversed thermoremanent magnetization in pyrrhotite upon cooling in zero field. If the initial pyrrhotite is hexagonal, oxidation is necessary to obtain the transformation into the monoclinic variety, which may also show the self-reversed TRM. In contrast to the natural samples, in synthetic pyrrhotite, the magnetite produced by oxidation carries a CRM, but shows no self-reversal upon cooling to room temperature. A model for a self-reversal and structural change processes following oxidation is presented in which the size and the shape of pyrrhotite grains play an important role. The consequences for palaeomagnetism and magnetic anomaly interpretation are discussed.