Shock and static pressure demagnetization of pyrrhotite and implications for the Martian crust

Abstract

The absence of crustal magnetization around young impact basins suggests impact demagnetization of vast regions of the crust after the cessation of the Martian dynamo. Attempts to understand the impact demagnetization process and to infer the magnetic properties (e.g., the carrier phase) of the Martian crust have been based on the experimental pressure demagnetization of magnetic rocks and minerals. We investigate the magnitude of demagnetization and permanent changes in the intrinsic magnetic properties of single and multidomain natural pyrrhotite under hydrostatic pressures up to 1.8 GPa and shock pressures up to 12 GPa. Both static and dynamic pressures result in an irreversible loss of predominantly low coercivity magnetic remanence. The pressure demagnetization results can be divided into a low-pressure regime and a high-pressure regime. The transition between the two regimes roughly coincides with a ferri- to paramagnetic transition (between 1.2 and 4.5 GPa) and the Hugoniot Elastic Limit (~ 3.5 GPa) of pyrrhotite. The low-pressure regime is characterized by a decrease in remanence with increasing pressure in both static and shock experiments. The higher pressure regime, probed only by shock experiments, is characterized by a more complicated modification of remanence as a result of permanent changes in the intrinsic magnetic properties of the material. These changes include an increase in saturation remanence and a change in the coercivity distribution towards greater bulk coercivity. Samples that were only submitted to hydrostatic pressure up to 1.8 GPa do not show permanent changes in the magnetic properties. Demagnetization of pyrrhotite as a result of pressure is likely due to a combination of domain reordering (in multidomain grains) and magnetostrictive effects (in single-domain grains). Microfracturing of multidomain grains effectively reduces the domain-size leading to the observed increase in single-domain like behavior. Based on uncertain shock pressure contours around basin-sized craters, impact demagnetization appears to be efficient at only a few GPa. Impact cratering affects the magnetic remanence of crustal rocks on all extraterrestrial bodies. For Mars, the pressure demagnetization results for pyrrhotite indicate that it is a possible carrier of the magnetization in the crust.