Multi-scale three-dimensional characterization of iron particles in dusty olivine: Implications for paleomagnetism of chondritic meteorites

Einsle, J. F. , Harrison, R. J., Kasama, T., Conbhuí, P. Ó., Fabian, K., Williams, W., Woodland, L., Fu, R. R., Weiss, B. P. and Midgley, P. A. (2016) Multi-scale three-dimensional characterization of iron particles in dusty olivine: Implications for paleomagnetism of chondritic meteorites. American Mineralogist, 101(9), pp. 2070-2084. (doi: 10.2138/am-2016-5738CCBY)

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Abstract

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.

Item Type:Articles
Additional Information:The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme ( FP/2007–2013)/ERC grant agreements 291522-3DIMAGE (P.A.M.) and 320750-Nanopaleomagnetism (J.F.E., R.J.H., and P.A.M.). B.P.W. and R.R.F. were supported by NASA Emerging Worlds program grant NNX15AH72G, the NASA Solar System Exploration and Research Virtual Institute grant NNA14AB01A, and a generous gift from Thomas F. Peterson Jr. The research leading to these results has received funding from the European Research Council under the European Union’s Seventh Framework Programme ( FP/2007–2013)/ERC Grant Agreement No. 320832-Imagine. (W.W. and P.O.C.). Additionally, funding for this work was also made possible through NERC grant NE/J020966/1 Predicting the reliability with which the geomagnetic field can be recorded in igneous rocks (W.W).
Status:Published
Refereed:Yes
Glasgow Author(s) Enlighten ID:Einsle, Dr Joshua Franz
Authors: Einsle, J. F., Harrison, R. J., Kasama, T., Conbhuí, P. Ó., Fabian, K., Williams, W., Woodland, L., Fu, R. R., Weiss, B. P., and Midgley, P. A.
College/School:College of Science and Engineering > School of Geographical and Earth Sciences > Earth Sciences
Journal Name:American Mineralogist
Publisher:Mineralogical Society of America
ISSN:0003-004X
ISSN (Online):1945-3027
Copyright Holders:Copyright © 2016 Mineralogical Society of America
First Published:First published in American Mineralogist 101(9):2070-2084
Publisher Policy:Reproduced in accordance with the copyright policy of the publisher

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