GC2022-PERM024

Suevites are clastic rocks of impact origin commonly considered as allogeneic breccia. They are formed both inside and outside the impact crater under specific physical conditions. Suevites are distinguished from other impactites by their significant magnetic viscosity (superparamagnetism, SPM) caused by the presence of nanoparticles of magnetic minerals. Magnetic viscosity can be detected in situ by the transient electromagnetic (TEM) method: in the presence of magnetically viscous rocks recorded transient processes decay according to the 1/t law.

In the Zhamanshin impact structure, suevites are found mainly on the eastern side of the crater, where these rocks occur at the surface. However, the TEM survey made it possible to identify areas with increased magnetic viscosity of rocks in the near-surface layer which are not directly related to outcrops. The anomalous magnetic viscosity of the Zhamanshin suevites has been confirmed in the laboratory by measurements of impulse response of the samples and frequency dependence of magnetic susceptibility. Measurement results in the frequency and time domains agree with each other. At the same time, estimates of the difference Δχ between the static and dynamic magnetic susceptibility of suevite samples (10^-3SI) are an order of magnitude higher than estimates obtained on the basis of the analysis of the TEM data (10^-4SI). This indicates a high sensitivity of the TEM method to the presence of SPM particles in near-surface layers. These particles were likely brought to the soil during the weathering of the suevite bedrock. Detecting of SPM associated with suevites directly in the field would, in principle, make it possible to argue the impact origin of geological structures prior to the high-precision laboratory measurements on rock samples.

Most probable ranges of characteristic size and magnetic characteristics of particles in various states have been theoretically evaluated using an approximation of lognormal particle volume distribution and the model of dipole-dipole interacting particles. Characteristics of superparamagnetic particles determined from theoretical modeling are in good agreement with the experimental data obtained using frequency dependence of magnetic susceptibility and granulometry methods.

The studies were supported by the RFBR grant 18-05-00626 and performed with the facilities provided by the Research park of St. Petersburg State University.