The effects of explosive-driven shocks on the natural remanent magnetization and the magnetic properties of rocks

Abstract

The effects of shock waves on the natural remanent magnetization and the intrinsic magnetic properties of geological materials remain poorly known. Still, hypervelocity impact phenomena are of primary importance in the evolution of many extraterrestrial bodies and of Earth. We present new experiments in which four rocks with different lithology and magnetic mineralogy (titanomagnetite, magnetite, monoclinic pyrrhotite, titanohematite) were shocked using a high-order explosive (penthrite) that provided maximum pressure of about 30 GPa. The shock wave was modelled numerically and we studied the effects on the natural remanent magnetization as well as on the intrinsic magnetic properties of the shocked rocks as a function of the distance to the explosion. The intrinsic magnetic properties of the rock are permanently modified by the shock wave. The shock wave was able to superimpose a new fabric (with a minimum susceptibility axis parallel to the direction of shock) to the original magnetic fabric of the rock. Magnetite-, titanomagnetite- and pyrrhotite-bearing rocks show a noticeable increase of coercivity for pressure above 10 GPa. These changes are attributed to fracturing and/or dislocations of the ferromagnetic grains. These results show that the magnetic properties of meteorites, which are commonly shocked to pressures well above 10 GPa (e.g. Martian meteorites), may not be representative of the magnetic properties of their parent body. Natural remanent magnetization appears to be much more resistant to shock than isothermal remanent magnetization, probably because it is carried by grains with higher coercivity. For titanomagnetite-bearing rocks, we observe both a partial shock demagnetization of the original thermoremanent magnetization and the appearance of a shock magnetization. The demagnetizing effect of the shock wave depends closely on the coercivity spectrum of the grains carrying the original remanence, and the shock-remagnetizing effect depends on the presence of low-coercivity magnetic grains. With an impact occurring in an ambient magnetic field of similar intensity to the original magnetizing field, the post-shock magnetization may be higher or lower than the pre-shock magnetization depending on these two factors. The magnetic anomalies observed above impact basins are often used as a proxy to the presence or absence of an active dynamo at the time of impact. Our results show that the magnetic anomalies associated to shocked rocks can be misleading, so that the only decisive clue to the presence or absence of a dynamo at the time of impact is the possible thermoremanence carried by the volume of rock heated above blocking temperatures during the impact.