Magnetic Field Surveys of Thin Sections

Abstract

SUMMARYScanning magnetic microscopy (SMM) maps the magnetic field generated by petrologic thin sections or polished rock surfaces at the sub-mm scale, creating the possibility to resolve the contribution of individual grains and mineral phases to bulk magnetic signal. The instrument developed at NTNU has the capability to measure both remanence properties and, uniquely, the magnetic response in a field of known magnitude and direction. A series of case studies are presented to examine the capabilities of the microscope and to describe applications to the study of crustal magnetism. The use of well-characterized samples allows signals in magnetic scans to be attributed to grains of known composition, including the discrimination of signal associated with discrete oxides and assemblages of exsolved oxides in silicates, which can be observed to have differing directions of magnetisation. The ability of this instrument to image in fields mimicking those experienced in the crust allows the observation of the induced magnetic response, both qualitatively in the form of changes in magnetization direction, and quantitatively by varying experimental conditions. This capability gives SMM the ability to map both remanent response and generate a map of susceptibility, and provides a powerful new tool for the examination of bulk magnetic response in the Earth's crust.

Mapping and Modeling Sources of Natural Remanent Magnetization in the Microcline–Sillimanite Gneiss, Northwest Adirondack Mountains: Implications for Crustal Magnetism

“…Using scanning magnetic microscopy (SMM), we further investigated the sources of this large and stable NRM, and the local effect of mineral microstructures on the magnetization. SMM is well suited to this aim because it allows for mapping of magnetic anomalies created by variations of magnetization at the mineral scale (Church & McEnroe, 2018; Egli & Heller, 2000; Hankard et al., 2009; Lima & Weiss, 2016; Oda et al., 2011; Pastore et al., 2018, 2019; Weiss et al., 2002). The focus of this paper is sample AD34‐6b which has a susceptibility value of 3 × 10 4 (SI), NRM of 8.2 A/m, and a Q value of 598 (McEnroe & Brown, 2000).…”

Multistep Parametric Inversion of Scanning Magnetic Microscopy Data for Modeling Magnetization of Multidomain Magnetite

“…Magnetic scans of a 30 μm thick section were made with a scanning magnetic tunnel junction (MTJ) device at the NTNU Rock and Paleomagnetism laboratory (Church & McEnroe, ). The instrument measures the vertical component of the field and all imaging is carried out at room temperature (~20 °C).…”

“…These anomalies reflect the small‐scale variations of the magnetization, which commonly can record geologically meaningful information. Numerous studies have used the information derived from the fine scale magnetization for applications in the fields of paleomagnetism (e.g., Fu et al, ; Oda et al, ; Weiss et al, , ; Weiss et al, ), environmental magnetism (Noguchi et al, ), and rock magnetism (Church & McEnroe, ; de Groot et al, ; Egli & Heller, ; Hankard et al, ; Lima et al, ; Nakamura et al, ; Pastore et al, ; Schmidt et al, ; Tominaga et al, ; Weiss et al, ).…”

“…With the exception of early work by Thomas et al (), geology SMM studies have reported results acquired in near‐field‐free conditions. In the instrument used in this study, such conditions are achieved by active field cancelation which can reduce fields at the sample to <20 nT (Church & McEnroe, ). Measurements in near‐field‐free conditions can be used to investigate the remanent magnetization of the magnetic minerals.…”

“…The magnetic scanner uses a sensor with an active area of ~900 μm * 900 μm (Church & McEnroe, ). This area determines the spatial resolution of the instrument, together with the sample to sensor distance (~200 μm) and the sampling step (~30 μm).…”