Anisotropy of out-of-phase magnetic susceptibility as a potential tool for distinguishing geologically and physically controlled inverse magnetic fabrics in volcanic dykes

Abstract

Among the mechanisms of the mass transport within the Earth's crust and the upper mantle the magma movement within the dykes is very important. In some dykes, the magma movement is represented by almost free flow infilling more or less open fractures in the host rocks, while it has character of forceful injection of the host rocks in the other dykes. The magnetic fabric originated by the free flow is characterized by the magnetic foliation and magnetic lineation approximately parallel to the dyke plane (normal fabric) whereas the forceful injection gives rise to the magnetic foliation perpendicular to the dyke plane defining the intermediate or inverse magnetic fabric. It is known, however, that the inverse magnetic fabric can also originate through the free flow of magma in the case when the minerals carrying the AMS are represented by single-domain (SD) grains. In order to distinguish between the two mechanisms controlling the intermediate/inverse fabric, the anisotropy of magnetic remanence (AMR) is often used, because it indicates the shape preferred orientation of ferromagnetic minerals regardless of their domain state. We show that not only AMR, but also the anisotropy of out-of-phase magnetic susceptibility (opAMS) is conformable with particle shape regardless of the magnetic state of the particle. Consequently, the opAMS, which is measured automatically with the standard AMS using the KLY-5 Kappabridge, may substitute the AMR at least in some cases of volcanic dykes.