Paleomagnetic analysis using SQUID microscopy

Abstract

[1] Superconducting quantum interference device (SQUID) microscopes are a new generation of instruments that map magnetic fields with unprecedented spatial resolution and moment sensitivity. Unlike standard rock magnetometers, SQUID microscopes map magnetic fields rather than measuring magnetic moments such that the sample magnetization pattern must be retrieved from source model fits to the measured field data. Here we present the first direct comparison between paleomagnetic analyses on natural samples using joint measurements from SQUID microscopy and moment magnetometry. We demonstrate that in combination with a priori geologic and petrographic data, SQUID microscopy can accurately characterize the magnetization of lunar glass spherules and Hawaiian basalt. The bulk moment magnitude and direction of these samples inferred from inversions of SQUID microscopy data match direct measurements on the same samples using moment magnetometry. In addition, these inversions provide unique constraints on the magnetization distribution within the sample. These measurements are among the most sensitive and highest resolution quantitative paleomagnetic studies of natural remanent magnetization to date. We expect that this technique will be able to extend many other standard paleomagnetic techniques to previously inaccessible microscale samples.

Fast inversion of magnetic field maps of unidirectional planar geological magnetization

“…We began by revisiting the NRM and sIRM inversions for the sample of Hawaiian basalt first presented in Weiss et al [Weiss et al, 2007a[Weiss et al, , 2007b]. This sample not only provides an excellent opportunity to test the technique on a well-studied specimen, but also allows the comparison with results previously obtained using spatial-domain inversion techniques.…”

“…The most interesting practical case consists of a unidirectional magnetization distribution with known fixed direction but variable strength [Baratchart et al, 2013;Weiss et al, 2007a]. Retrieving such a magnetization from scanning magnetic microscopy data measured on a plane above the sample is the goal of this paper.…”

“…[14] This is a realistic model for SMM, in which doubly polished 30 mm thin sections of geological samples are typically measured at distances greater than three times the sample thickness [Fong et al, 2005;Weiss et al, 2007a]. We assume that the magnetic field is measured on the plane z = h parallel to a sample located on the x-y horizontal plane, where h is the sensor-to-sample distance or liftoff (Figure 1).…”

“…[4] For the aforementioned spatial scales, the predominant imaging technique is scanning magnetic microscopy, in which a geological sample-typically a thin section-is displaced horizontally underneath a fixed magnetic sensor by a high-precision scanning stage while the magnetic field is recorded [Weiss et al, 2007a]. This is effectively equivalent to mapping the field on a plane above a fixed sample, with the advantage that the magnetic sensor experiences a constant background field that is easily subtracted from the measurements.…”

Advances in magneto‐optical imaging applied to rock magnetism and paleomagnetism

“…Recently, several scanning magnetic microscopes have been developed in particular to understand the magnetism of meteorites. Among these are Scanning Superconducting Quantum Interference Device (SQUID) microscopy [Gattacceca et al, 2006;Weiss et al, 2007], Scanning Giant MagnetoResistance (GMR) microscopy [Hankard et al, 2009], and Scanning Magneto-Impedance (MI) microscopy [Uehara and Nakamura, 2008]. [3] There are two fundamental problems specific to scanning magnetic microscopy.…”