Multi-scale three-dimensional characterization of iron particles in dusty olivine: Implications for paleomagnetism of chondritic meteorites
Multi-scale three-dimensional characterization of iron particles in dusty olivine: Implications for paleomagnetism of chondritic meteorites
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DOI：
10.2138/am-2016-5738ccby
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发表时间：
2016-09-01
影响因子：
3.1
通讯作者：
Midgley, Paul A.
中科院分区：
文献类型：
--
作者：
Einsle, Joshua F.;Harrison, Richard J.;Midgley, Paul A.
球粒陨石中的灰橄榄石（含有多个亚微米金属铁包裹体的橄榄石）被认为是古磁剩余物的理想载体，能够保持球粒形成过程中获得的增生前磁化的忠实记录。在这里，我们展示了如何利用聚焦离子束纳米层析成像（FIB-nT）、电子层析成像和有限元微磁建模的组合，在三维空间中充分表征Semarkona LL3.0普通球粒陨石中单个橄榄石颗粒的磁性结构。我们提出了一个尘埃橄榄石颗粒的三维（3D）体积重建，通过选择性铣削通过一系列顺序的20 nm切片感兴趣的区域获得，然后使用扫描电子显微镜成像。这些数据提供了铁颗粒系综的定量描述，包括颗粒大小、形状、颗粒间距和方向的分布。铁颗粒主要是扁椭球体，平均半径为242 +/- 94 x 199 +/- 80 x 123 +/- 58瓮。通过透射电镜分析，我们观察到颗粒在亚晶界上成核，并松散地排列成一系列平行于橄榄石基体（001）的片状。这与使用FIB-nT收集的定向数据一致，并强调了尘埃橄榄石的底层结构如何受到橄榄石宿主的晶体学约束。粒子的最短维度垂直于薄片，它们的最长维度优先排列在薄片内。单个粒子的几何形状被转换成一个有限元网格，并用于执行微磁模拟。大多数粒子采用单一的涡旋状态，具有围绕中心涡旋核心旋转的“散装”自旋。我们没有观察到真正处于单畴态的粒子。微磁模拟的结果挑战了一些关于涡旋态剩磁携带特性的先入为主的观念。在不同的场中，粒子的主要轴、中间轴和次要轴与剩余矢量之间往往没有简单的预测关系。虽然涡旋核心的方向在很大程度上取决于椭球体的几何形状（即，长椭球平行于长轴，扁椭球平行于短轴），但涡旋核心和剩余物矢量有时会与主轴形成很大（几十度）的夹角。形貌的细微细节可以控制整体剩余物状态，在某些情况下导致体自旋对净剩余物的主要贡献，这对预测样品的各向异性具有深远的意义。这些粒子具有非常高的开关场（几百毫特斯拉），表明它们具有很高的稳定性和古强度研究的适用性。
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 x 199 +/- 80 x 123 +/- 58 urn. 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.