Shock-Induced Magnetic and Structural Changes in Magnetite: New Insights Towards Strain Memory Mechanisms

Abstract:

Shock recovery experiments using an air gun (5 GPa) and high-explosive set-up (10, 20, 30 GPa) were done from natural stoichiometric magnetite ore samples consisting mainly of multidomain magnetite and quartz. The aim of this study is to investigate the potential of changes in magnetic transition temperatures in magnetite as a geobarometer for extreme conditions like those observed in a meteorite impact on Earth material or in meteorites. We used the temperature dependence of magnetic susceptibility along with XANES and X-ray diffraction for monitoring magnetic and structural changes. We will demonstrate that the shocked samples show a shift in the Verwey transition temperature (Fig. 1a and b) compared to the unshocked magnetite. Although the Curie temperature itself is very similar for all investigated samples, the shape as well as the amplitude of the heating and cooling curves are nearly reversible for the unshocked “0 GPa” sample but irreversible for the shocked samples. While the amplitude changes before the Curie temperature and above the Verwey transition temperature (Fig. 1a and c) are related to reduction in magnetic domain sizes due to fragmentation, the shift in the Verwey transition temperature and the irreversibility of Curie temperature cannot be explained by this mechanism and we suspect that chemical (Fe²⁺/Fe³⁺ ratio in Fe₃O₄) or structural (lattice distortion) changes occur. These findings help to constrain data for a possible strain memory of magnetic transition temperatures in magnetite and help to explore the potential use of changes in magnetic transition temperatures as a strain memory as earlier suggested by Carporzen and Gilder (2010).

Carporzen, L., Gilder, S.A., 2010: Strain memory of the Verwey transition, J. Geophys. Res., 115, B05103, doi: 10.1029/2009JB006813.