^1,2Joshua F. Einsle, ¹Richard J. Harrison, ³Takeshi Kasama, ⁴Pádraig Ó Conbhuí, ⁵Karl Fabian, ⁴Wyn Williams, ⁷Leonie Woodland, ⁸Roger R. Fu, ⁹Benjamin P. Weiss,²Paul A. Midgley
American Mineralogist 101,  2070-2084 Link to Article [http://dx.doi.org/10.2138/am-2016-5738CCBY]
¹Department of Earth Sciences, University of Cambridge, Downing Street, Cambridge CB2 3EQ, U.K.
²Department of Materials Science & Metallurgy, University of Cambridge, Charles Babbage Road, Cambridge CB3 0FS, U.K.
³Center for Electron Nanoscopy, Technical University of Denmark, Kongens Lyngby, Denmark
⁴Grant Institute of Earth Science, University of Edinburgh, Kings Buildings, West Mains Road, Edinburgh EH9 3JW, U.K.
⁵Geological Survey of Norway, Leiv Eirikssons vei 39, 7491 Trondheim, Norway
⁶Centre for Arctic Gas Hydrate, Environment and Climate; Department of Geology, University of Tromsø, NO-9037 Tromsø, Norway
⁷The Stephen Perse Foundation, Union Road, Cambridge CB2 1HF, U.K.
⁸Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, U.S.A.
⁹Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts
Copyright: The Mineralogical Society of America

Dusty olivine (olivine containing multiple sub-micrometer inclusions of metallic iron) in chondritic meteorites is considered an ideal carrier of paleomagnetic remanence, capable of maintaining a faithful record of pre-accretionary magnetization acquired during chondrule formation. Here we show how the magnetic architecture of a single dusty olivine grain from the Semarkona LL3.0 ordinary chondrite meteorite can be fully characterized in three dimensions, using a combination of focused ion beam nanotomography (FIB-nT), electron tomography, and finite-element micromagnetic modeling. We present a three-dimensional (3D) volume reconstruction of a dusty olivine grain, obtained by selective milling through a region of interest in a series of sequential 20 nm slices, which are then imaged using scanning electron microscopy. The data provide a quantitative description of the iron particle ensemble, including the distribution of particle sizes, shapes, interparticle spacings and orientations. Iron particles are predominantly oblate ellipsoids with average radii 242 ± 94 × 199 ± 80 × 123 ± 58 nm. Using analytical TEM we observe that the particles nucleate on sub-grain boundaries and are loosely arranged in a series of sheets parallel to (001) of the olivine host. This is in agreement with the orientation data collected using the FIB-nT and highlights how the underlying texture of the dusty olivine is crystallographically constrained by the olivine host. The shortest dimension of the particles is oriented normal to the sheets and their longest dimension is preferentially aligned within the sheets. Individual particle geometries are converted to a finite-element mesh and used to perform micromagnetic simulations. The majority of particles adopt a single vortex state, with “bulk” spins that rotate around a central vortex core. We observed no particles that are in a true single domain state. The results of the micromagnetic simulations challenge some preconceived ideas about the remanence-carrying properties of vortex states. There is often not a simple predictive relationship between the major, intermediate, and minor axes of the particles and the remanence vector imparted in different fields. Although the orientation of the vortex core is determined largely by the ellipsoidal geometry (i.e., parallel to the major axis for prolate ellipsoids and parallel to the minor axis for oblate ellipsoids), the core and remanence vectors can sometimes lie at very large (tens of degrees) angles to the principal axes. The subtle details of the morphology can control the overall remanence state, leading in some cases to a dominant contribution from the bulk spins to the net remanence, with profound implications for predicting the anisotropy of the sample. The particles have very high switching fields (several hundred millitesla), demonstrating their high stability and suitability for paleointensity studies.