Magnetic and structural behaviour of magnetite subjected to cyclic loading under temperature conditions relevant for the upper crust

Magnetic properties of rocks are used to qualitatively describe the dynamics of tectonic-related deformation phenomena within the Earth’s crust. Up to now modifications of magnetic properties are mainly described from static or dynamic shock-related deformations (e.g. Reznik et al. 2016). However, it is generally accepted that tectonic stress accumulates non-uniformly, but remains quite low until a rapid stress variation occurs shortly before and after earthquake propagation or volcano eruption. Rapid stress increase may be especially favorable if an intermediate to high-temperature tectonic loading exhibits a cyclic character. This raises the question whether the magnetic properties of rocks are sensitive to fatigue-assisted deformation under increased temperature.
In order to reveal fundamental changes in magnetite exposed to cyclic loading, samples of a banded magnetite-quartz ore were compared with samples prepared from a natural magnetite single crystal. Cyclic experiments have been carried out using a GABO Eplexor Dynamic Mechanical Analysis (DMA) system allowing controlled force loading with a precision better than 0.1 mN (Klumbach and Schilling, 2013). ... mehr40 cycles in 1 K steps from 303 to 623 K have been conducted under a static load of 98 N and a dynamic load of 46 N for about 3 h which corresponds to a maximum pressure of about 10 to 15 MPa. Magnetic properties were studied using the temperature dependence of magnetic susceptibility (induced magnetization). Changes in microstructure were characterized by reflected light microscopy, Raman spectroscopy, X-ray powder diffraction and thermogravimetric analysis.
The most obvious change is a decrease in the sharpness of the Verwey transition of magnetite at 120 K suggesting a decrease of magnetic domain size, and / or a phase transition into hematite via maghemite. The formation of hematite along magnetite grain boundaries on the (001) surface of single crystals is documented by reflected light microscopy. We hypothesize that oxidation and recrystallization in magnetite are two competing deformation mechanisms controlling changes in magnetic behaviour of magnetite. Our study focuses on the question to what extent these competing mechanisms contribute to the magnetic property changes. Therefore, high-resolution transmission electron microscopy is underway to control the deformation of magnetite subjected tocyclic loading.

References
Klumbach, S., and F. R. Schilling (2013), Elastic and anelastic properties of a- and b-quartz single crystals, Eur. J. Miner., (1889).
Reznik, B., A. Kontny, J. Fritz, and U. Gerhards (2016), Shock-induced deformation phenomena in magnetite and their consequences on
magnetic properties, Geochemistry Geophys. Geosystems, 17, 1–20.