On the efficiency of shock magnetization processes

The acquisition of remanent magnetization caused by hypervelocity impacts may be a major contribution to the remanent magnetization of planetary surfaces and meteorites. In this paper, we investigate the properties of shock remanent magnetization (SRM) and in particular the relative efficiency of this phenomenon with respect to thermoremanent magnetization (TRM), a parameter of interest for the understanding of the magnetic signatures of impact basins on extraterrestrial bodies. We imparted shock remanent magnetization on basalt and microdiorite samples using laser shock in controlled magnetic field. Shock modeling indicates a pressure range of 0.6-1.5 GPa for our experiments. The results confirm that the SRM is parallel to the ambient magnetic field. SRM increases linearly with the ambient field in the 1-2500 mu T range. The intensity of SRM is independent of the angle between the shock direction and the direction of the ambient field. The coercivity spectrum of SRM is shifted towards lower values compared to the coercivity spectra of TRM and isothermal remanent magnetization. For the two studied lithologies, the efficiency of SRM versus TRM acquisition is >= 17% for the basalt and is 36% for the microdiorite. This efficiency depends on the magnetic mineralogy and increases with decreasing coercivity. The rather high efficiency values that we obtained show that a significant remanent magnetization can be acquired by the crust during an hypervelocity impact on a planetary surface in the presence of an ambient field. (C) 2007 Elsevier B.V. All rights reserved.