^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.

¹Queenie H.S. Chan, ¹Michael E. Zolensky, ²James E. Martinez, ³Akira Tsuchiyama, ³Akira Miyake
American Mineralogist 101, 2041-2050  Link to Article [http://dx.doi.org/10.2138/am-2016-5604]
¹ARES, NASA Johnson Space Center, Houston, Texas 77058, U.S.A.
²Jacobs Engineering, Houston, Texas 77058, U.S.A.
³Graduate School of Science, Kyoto University, Kitashirakawa Oiwake-cho, Sakyo-ku, Kyoto 606-8502, Japan
Copyright: The Mineralogical Society of America

Life on Earth shows preference toward the set of organics with particular spatial configurations. Enantiomeric excesses have been observed for α-methyl amino acids in meteorites, which suggests that chiral asymmetry might have an abiotic origin. A possible abiotic mechanism that could produce chiral asymmetry in meteoritic amino acids is their formation under the influence of asymmetric catalysts, as mineral crystallization can produce spatially asymmetric structures. Although magnetite plaquettes have been proposed to be a possible candidate for an asymmetric catalyst, based on the suggestion that they have a spiral structure, a comprehensive description of their morphology and interpretation of the mechanism associated with symmetry-breaking in biomolecules remain elusive. Here we report observations of magnetite plaquettes in carbonaceous chondrites (CC) that were made with scanning electron microscopy and synchrotron X-ray computed microtomography (SXRCT). We obtained the crystal orientation of the plaquettes using electron backscatter diffraction (EBSD) analysis. SXRCT permits visualization of the internal features of the plaquettes. It provides an unambiguous conclusion that the plaquettes are devoid of a spiral feature and, rather that they are stacks of individual magnetite disks that do not join to form a continuous spiral. Despite the lack of spiral features, our EBSD data show significant changes in crystal orientation between adjacent magnetite disks. The magnetite disks are displaced in a consistent relative direction that lead to an overall crystallographic rotational mechanism. This work offers an explicit understanding of the structures of magnetite plaquettes in CC, which provides a fundamental basis for future interpretation of the proposed symmetry-breaking mechanism.

^1,2A.P. Jordan, ^2,3T.J. Stubbs, ^1,2J.K. Wilson, ^1,2N.A. Schwadron, ^1,2H.E. Spence
Icarus (in Press) Link to Article [http://dx.doi.org/10.1016/j.icarus.2016.08.027]
¹Institute for the Study of Earth, Oceans, and Space, University of New Hampshire, Durham, New Hampshire, USA
²Solar System Exploration Research Virtual Institute, NASA Ames Research Center, Moffett Field, California, USA
³NASA Goddard Space Flight Center, Greenbelt, Maryland, USA
Copyright Elsevier

Large solar energetic particle events may cause dielectric breakdown in the upper 1 mm of regolith in permanently shadowed regions (PSRs). We estimate how the resulting breakdown weathering compares to meteoroid impact weathering. Although the SEP event rates measured by the Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO) are too low for breakdown to have significantly affected the regolith over the duration of the LRO mission, regolith gardened by meteoroid impacts has been exposed to SEPs for ∼106 yr. Therefore, we estimate that breakdown weathering’s production rate of vapor and melt in the coldest PSRs is up to 1.8−3.5×10−71.8−3.5×10−7 kg m−2−2 yr−1,−1, which is comparable to that produced by meteoroid impacts. Thus, in PSRs, up to 10–25% of the regolith may have been melted or vaporized by dielectric breakdown. Breakdown weathering could also be consistent with observations of the increased porosity (“fairy castles”) of PSR regolith. We also show that it is conceivable that breakdown-weathered material is present in Apollo soil samples. Consequently, breakdown weathering could be an important process within PSRs, and it warrants further investigation.