Static stress demagnetization of single and multidomain magnetite with implications for meteorite impacts

Stress demagnetization effects on ferromagnetic minerals are poorly known, especially above 1 GPa, and when initially magnetized under pressure and then subjected to further stress. Our experiments on pure magnetite under quasi-hydrostatic loads in the presence of a small (Earth's) field show that stress demagnetization depends on domain state and stress history. Viewed globally, the results follow a simple law where the percentage loss in magnetic moment is the inverse of pressure (e.g., 50% loss in moment at 1 GPa, 67% loss at 2 GPa, etc.). Our experiments also quantify the effect of demagnetization upon stress release, where the moment upon full decompression is two-thirds less than the moment when decompression first began. Given the magnitude of the stress demagnetization effect, we conclude that the presence or absence of a planetary magnetic field cannot be deduced from the magnetic fields measured over meteorite craters, such as those on Mars