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

Abstract

S U M M A R YMost 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.

A method for improving the measurement of low-field magnetic susceptibility anisotropy in weak samples

“…For example, with the AGICO instruments the susceptibility is measured in 15 directions, so the data quality and significance of the AMS can be estimated (Jelinek, 1977). Another way to determine the full susceptibility tensor utilizes the high-field slopes of hysteresis loops (Kelso et al, 2002). This method uses 24 different orientations of the sample to the field direction and yields the paramagnetic component of the anisotropy.…”

Magnetic fabric: methods and applications — an introduction

“…It has become necessary, in order to unravel the history of deformation and alteration of natural samples, to separate these individual magnetic subfabrics. Several techniques are now available for this purpose, based on measurements at different temperatures (Richter & van der Pluijm 1994;Hrouda et al 1997;Lfineburg et al 1999;Par6s & van der Pluijm 2002), measurements at different fields (Hrouda & Jelinek 1990;Martin-Hernfindez & Hirt 2001;Kelso et al 2002;Martin-Hern/mdez & Hirt 2004) or a combination (Rochette & Fillion 1988;Richter & van der Pluijm 1994 The separation of superposed magnetic subfabrics can also be achieved through measurements of the anisotropy of magnetic remanence (AMR) alone (reviewed by Potter, this volume). The AMR is a measurement of the anisotropy of ferromagnetic minerals that can be evaluated using different types of magnetization, including anhysteretic remanence (ARM) (McCabe et al 1985;Jackson et al 1988;Jackson 1991), isothermal remanence (IRM) (Daly & Zinsser 1973;Stephenson et al 1986;Borradaile & Dehls 1993;Jelinek 1993), thermoremanence (TRM) (Cogn~ 1987) and gyroremanence (GRM) (Stephenson 1981).…”

Magnetic fabrics in the Bjerkreim Sokndal Layered Intrusion, Rogaland, southern Norway: Mineral sources and geological significance

“…6 A number of separation techniques have been developed, including (1) statistical methods (Henry, 1983;Henry and Daly, 1983); (2) field-or temperature-dependence to either enhance or isolate specific components of the AMS (Ferré et al, 2004;Kelso et al, 2002;Martín-Hernández and Ferré, 2007;Hirt, 2001, 2004;Pares and van der Pluijm, 2002;Rochette and Fillion, 1988;Rochette et al, 1983;Schmidt et al, 2007); and (3) thermal enhancement . A different approach is to measure the anisotropy of magnetic remanence (Borradaile and Jackson, 2010;Daly and Zinsser, 1973;Jackson and Tauxe, 1991;Jackson, 1991;Potter, 2004).…”

“…Clark, 2014). High field AMS was determined by measuring hysteresis loops in 36 different orientations in three orthogonal planes (Ferré et al, 2004;Kelso et al, 2002). Cubes with 1 cm or 5 mm side length were used in order to minimize artefacts due to shape, and two cubes were measured for each specimen to assess reproducibility.…”