VARIATION IN ROCK MAGNETIC PROPERTIES DUE TO SHOCK WAVES: A CASE STUDY FROM LONAR CRATER, INDIA
Shock waves propagating through the target rocks interact with the ferri-/para-magnetic minerals and affect the rock magnetic properties, such as the anisotropy of magnetic susceptibility (AMS), magnetic mineralogy (temperature dependent susceptibility). We have selected a relatively young and pristine crater, Lonar crater in India, for the investigations. Magnetic properties of the rocks from the crater rim were compared with the properties of the rocks taken far away from the crater rim.
Two separate NRM components are identified in the Lonar basalts. The low coercivity component, observed in magnetically soft samples, demonstrates present day magnetic field orientation and may be attributed to either post-impact viscous remanent magnetization and/or chemical remanent magnetization (cf. Louzada et al., 2008). On the other hand, rocks with higher percentage of single domain titanomagnetite show only a high coercivity component. The magnetic foliations and the orientation of magnetic axes, k1 or k3, in the crater rim are similar to that in the unshocked rocks far away from the rim. The AMS study, thus, implies that, the shock waves were not strong enough to reorient either the NRM vectors or k1 or k3 axis in the Lonar crater (Agarwal et al., 2014). On the other hand, the Verwey transition temperature (TV) and the Curie temperature (TC) have conspicuous relationship with distance from the crater centre. Further detailed investigations are required to understand the processes that cause variation in TV and TC, but do not affect the principal magnetic susceptibility axes, such as k1 or k3.
References
Agarwal, A., Kontny, A., Srivastava, D., Greiling, R.O., 2014. Shock pressure estimates in target basalts of a pristine crater: a case study in the Lonar crater, India. Geol. Soc. Am. Bull. doi:10.1130/B31172.1
Louzada, K.L., Weiss, B.P., Maloof, A.C., Stewart, S.T., Swanson-Hysell, N.L., Soule, S.A., 2008. Paleomagnetism of Lonar impact crater, India. Earth Planet. Sci. Lett. 275, 308–319. doi:10.1016/j.epsl.2008.08.025