Effect of cyclic loading at elevated temperatures on the magnetic susceptibility of a magnetite-bearing ore

Rocks are often subjected to dynamic stress that occurs during earthquakes, volcanic activity as well as human-induced activities. The aim of this study is to test if mechanical fatigue in rocks can be monitored by magnetic methods. For this purpose, the effect of cyclic-mechanical loading (150 + 30 MPa) on the magnetic susceptibility and its anisotropy of a magnetite-bearing ore with varying temperatures and environment was investigated. Our study shows that magnetic susceptibility decreases significantly (up to 23%) under air conditions and even in vacuum (up to 4 %) within the first ca. 1000 cycles. Further loading does not significantly affect the magnetic susceptibility which then remains more or less constant. However, a stronger decrease of susceptibility parameters is observed at higher temperatures. As magnetic susceptibility was measured after decompression of the loaded sample at room temperature, magnetostriction cannot be the reason for these changes. After cyclic loadings in air, the transformation of magnetite to hematite is the major mechanism affecting bulk magnetic susceptibility. The weak changes in magnetic susceptibility after vacuum loadings are related to the formation of damage and deformation structures observed on the surface of magnetite grains. We have shown that cyclic loading can change significantly the induced magnetization of a rock due to mineral transformation below < 1000 cycles and that mechanical fatigue, which is a precursor of the failure of a rock, is closely associated with these transformations. Therefore time-dependent magnetic susceptibility measurements can be used as a proxy parameter of mechanical fatigue.