A new method for the separation of paramagnetic and ferromagnetic susceptibility anisotropy using low field and high field methods

Abstract

SUMMARY Most rocks contain both ferromagnetic and paramagnetic minerals that contribute to their bulk magnetic susceptibility and the anisotropy of magnetic susceptibility. Anisotropy of magnetic susceptibility techniques typically measure the net susceptibility and are not able to separate paramagnetic and ferromagnetic contributions. Since different minerals may form at various times and/or under different conditions, examination of their individual contributions provides unique information related to the rock’s formation and evolution. By subjecting a sample to high magnetic fields, the ferromagnetic minerals become saturated and the contribution of the paramagnetic minerals can be evaluated (the slope of the line at high field values, on a field vs magnetization plot). Using this approach, we developed a new technique that separates the ferromagnetic and paramagnetic components of standard 1 inch cylindrical samples using a Vibrating Sample Magnetometer. This separation is tested by artificially combining separate samples with known paramagnetic-only and ferromagnetic-only behaviour. By comparing the high-field results of a combined paramagnetic and ferromagnetic signal to the classic low field alternating current susceptibility of the paramagnetic-only signal, we demonstrate that the high field anisotropy is the result solely of the paramagnetic fabric even when the low field anisotropy of magnetic susceptibility is dominated by the ferromagnetic minerals. A ferromagnetic-only fabric is calculated for the combined paramagnetic and ferromagnetic rock, by tensor subtraction of the high field (paramagnetic-only) and low field (paramagnetic plus ferromagnetic) measurements on the same sample. Application of this technique to natural samples of combined paramagnetic and ferromagnetic behaviour is discussed.