Anisotropy of Out-of-Phase Magnetic Susceptibility: A Non-Standard Approach for Magnetic Subfabrics Determination in Variscan Granites of Iberian Massif

Abstract

The magnetic susceptibility measured in an alternating field is made up of in-phase and out-of-phase components. The in-phase Anisotropy of Magnetic Susceptibility (ipAMS) measures the bulk response of all minerals in a sample; however, out-of-phase AMS (opAMS) is sensitive to only select ferromagnetic minerals such as hematite, titanomagnetite, and ultrafine magnetite. The opAMS can be harnessed as a tool for the direct determination of magnetic subfabrics defined by ferromagnetic minerals. This work focuses on the following three Portuguese plutons: Lamas de Olo, Lavadores-Madalena, and Santa Eulália. The results show that the magnetic susceptibility is lower in opAMS, the degree of magnetic anisotropy is much higher in the opAMS, and the ellipsoid shape parameter has no significant differences. The ipAMS and opAMS tensors are, in general, coaxial, which indicates that the standard AMS fabric is parallel to the subfabric of minerals such as hematite, titanomagnetite, and ultrafine magnetite.

Keywords

• anisotropy of out-of-phase magnetic susceptibility
• anisotropy of in-phase magnetic susceptibility
• rock and mineral magnetism