Mapping magnetic sources at the millimeter to micrometer scale in dunite and serpentinite by high-resolution magnetic microscopy

Abstract

Rock samples can have wide range of magnetic properties depending on composition, amount of ferromagnetic minerals, grain sizes and microstructures. Here, we used scanning magnetic microscopy, a highly sensitive and high-resolution magnetometric technique to map remanent magnetic fields over a planar surface of a rock sample.The technique allows for the investigation of discrete magnetic mineral grains, or magnetic textures and structures with submillimeter scale resolution. Here, we present a case-study of magnetic scans of pristine and serpentinized dunite thin sections from the Reinfjord Ultramafic Complex, in northern Norway. The magnetic mineralogy is characterized by electron microprobe, scanning electron-and optical-microscopy, and with rock magnetic methods. In serpentinized samples the magnetic carrier is end-member magnetite occurring as large discrete grains and small grains in micron scale veins. By contrast, the pristine dunite sample contains large Cr-spinel grains with very fine equant exsolutions ranging in composition from ferrichromite to end-member magnetite.Forward and inverse modeling of the magnetic anomalies is used to determine the remanent magnetization directions and intensities of discrete magnetic sources observed in the scanning magnetic microscopy. The finescale magnetization of the rock sample is used to investigate the magnetic carriers and the effect of serpentinization on the magnetic properties of the dunite. Modeling shows that the dipolar magnetic anomalies that are mapped by scanning magnetic microscopy are caused by grains with heterogeneous magnetic sources.The intensity of the magnetization and the amount of magnetic minerals are higher in the serpentinized sample than the pristine dunite sample, consistent with the measured bulk magnetic properties. Furthermore, the serpentinized samples show a larger variability in the direction of the magnetization and a stronger heterogeneity with respect to the pristine sample. The ability to rigorously associate components of the bulk magnetic properties to individual mineral phases creates new possibilities for rock magnetic, paleomagnetic, and exploration applications.

Magnetic Mineralogy and Petrophysical Properties of Ultramafic Rocks: Consequences for Crustal Magnetism

“…The gradual decrease in susceptibility is interpreted to be due to a range in composition of the Cr‐spinel host and Fe‐rich exsolution microstructures. This interpretation is in agreement with compositional ranges reported by Pastore, McEnroe, ter Maat, et al () on dunite samples from the CS, as well as reported progressive changes in T c in the Fe‐Cr spinel solid solution due to compositional variation in the exsolved phase by Francombe () and Robbins et al (). The T c estimates based on the thermomagnetic curves implies that some exsolution blebs have near‐end‐member magnetite compositions; however, the partially suppressed Verwey transition also indicates a degree of cation substitution or nonstoichiometry.…”

“…The chemical composition of the exsolution microstructures and host chrome‐spinel in CS rocks is reported by Pastore, McEnroe, ter Maat, et al (). The composition is obtained by electron microprobe analysis and is [(Fe 2+ 0.94 Mg 0.07 Ni 0.01 ) 1.02 (Fe 3+ 1.33 Cr 0.46 Al 0.15 Ti 0.03 ) 1.97 O 4 ] for the exsolution microstructures and [(Fe 2+ 0.76 Mg 0.25 ) 1.01 (Fe 3+ 0.46 Cr 0.74 Al 0.75 Ti 0.03 ) 1.97 O 4 ] for the host chrome‐spinel.…”

“…All CS samples contain chrome‐spinel, and those with high and intermediate bulk susceptibility contain abundant exsolution of a more iron‐rich phase, while those with low bulk susceptibility have little exsolution. The exsolved phase contains variable amounts of Al, Cr, and minor Mg, Ni, and Ti (Pastore, McEnroe, ter Maat, et al, ) leading to a range in T c consistent with the continuous loss of susceptibility from 200 °C to the sharpest decrease at 535–550 °C. The samples with the lowest T c (by inference, the highest degree of cation substitution) also contain sparser exsolution microstructures and have lower bulk susceptibility compared to those with higher T c .…”

“…Similar textures in chrome-spinel are reported by other authors (Colas et al, 2016;Eales et al, 1988;Loferski & Lipin, 1983;Ramdohr, 1980). The chemical composition of the exsolution microstructures and host chrome-spinel in CS rocks is reported by Pastore, McEnroe, ter Maat, et al (2018) ] for the host chrome-spinel. The host chrome-spinel contains more Cr and Al, and the exsolved phase is more Fe-rich.…”

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

“…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, ).…”