Observations From Magnetic Force Microscopy on Crystallographically Oriented Magnetite Inclusions in Clinopyroxenes

Crystallographically oriented magnetite inclusions in pyroxenes are a product of subsolidus exsolution during slow cooling and are common features in gabbroic and granulite facies rocks. These elongate inclusions often record a stable component of the direction and intensity of the Earth's magnetic field and consequently, are of significant interest to paleomagnetism. However, the high coercivities sometimes found in these inclusions, which average > 80 mT, cannot be wholly explained by their extreme shape anisotropy. In this study, we visually evaluate the microstructure within crystallographically oriented magnetite inclusions in clinopyroxene crystals from the Messum Volcanic Complex using atomic force microscopy (AFM) and magnetic force microscopy (MFM). Both techniques measure the deflection of a flexible silicon cantilever probe as it is scanned over a sample surface. The probe can be scanned at varying heights above the sample allowing for the measurement of either near-surface forces related to sample topography (a proxy for mineralogy) or stray electromagnetic forces dominant farther from the sample surface. The topography and magnetic domain state within individual magnetite inclusions varied significantly between samples, but all inclusions fall within two broadly defined categories: segmented and non-segmented inclusions. The more common segmented inclusions contain a second, nonmagnetic phase forming walls parallel to \{100\} of magnetite which partition the remaining magnetite into orthogonal parallelepipeds with sides 50-200 nm long, creating a boxwork texture. This nonmagnetic phase was independently observed using an analytical transmission electron microscope and is rich in Fe, Ti, and Cr and has a spinel structure. The segmented inclusions are the product of a second stage of exsolution occurring during continued cooling of the host rock. The temperature and duration over which this oxide exsolution event occurred are poorly constrained and dependent on factors such as clinopyroxene composition, fO2, and cooling rate. A unique kind of highly stable magnetic domain state is produced in the segmented inclusions, whereby individual boxes of magnetite act as single domains whose directions are influenced by neighboring boxes and fixed in the surrounding nonmagnetic phase. The vertical components of magnetization of adjacent magnetite boxes are often oriented antiparallel to one another. The stability of this distribution of magnetic single domains was tested by gathering MFM images of segmented inclusions before, during, and after the application of a 72.5 mT DC field. Only 12% of the magnetite boxes changed polarity in the presence of this applied field, indicating that the boxwork texture is highly stable. Less common in occurrence, non-segmented inclusions show topographically (i.e., mineralogically) homogenous interiors with long magnetic domains running lengthwise down the inclusions. The lengthwise domain boundaries are diffuse, possibly indicating a vortex arrangement of direction. Crystals containing non-segmented inclusions were observed to change their magnetic directions when exposed to fields of <10 mT. Thus, crystallographically oriented magnetite inclusions in clinopyroxenes can have a wide range of coercivities. A full understanding of the inclusions' paragenesis and their mutual interactions is necessary before they can be viewed as faithful recorders of the Earth's magnetic field.