Mineral magnetism of shocked ferrimagnetic minerals

亚铁磁性矿物的磁性取决于其晶格、各向异性、化学成分和粒度。后一个参数受微结构的强烈控制，这对解释受冲击磁性矿物的磁性具有重要意义。断裂和晶格缺陷是磁畴尺寸减小、矫顽力增大和磁相变(如磁黄铁矿34K相变、磁铁矿Verwey相变)被抑制的主要原因。特别是对于充分研究铁磁性矿物的冲击诱导变质，显微结构和磁测量相结合是必不可少的。本项目主要关注极端条件下的两个重要方面--冲击波对地球上天然物质的影响和铁陨石中外来磁性矿物的磁性矿物学。为了获得变形结构和磁性之间的一般关联，将来自海洋目标(洛克内和切萨皮克湾)中两个撞击结构的样品的特定磁性、载流子和显微结构与陨石坑外(洛克恩)和陨石坑内未变形的巨型块体(切萨皮克湾)的冲击磁铁矿和含磁铁矿靶岩性进行比较。我们将验证这一假设，即震后热液蚀变是否可以显著地叠加与震荡相关的微结构和相关的磁性。我们尤其想要关注Verwey转换(TV)，因为较低的TV被描述为冲击冲击岩性。因此，本研究的重点是将低温磁化率、高温磁化率和饱和等温剩磁与矿物学和微结构研究相结合的磁矿物学研究。然后，同样的方法将被用于铁陨石的研究，其磁性除了金属相外，通常还受到柯石英、施赖伯利石和硅铅矿等外来磁性矿物的控制。这些相的磁相转变温度与它们的化学成分以及它们的结晶学和微结构配置有关的记录很少。然而，为了对地外物质和地球物质中与冲击有关的磁化过程有一个普遍的了解，获得初始的“未受冲击的”状态与随后的与冲击和蚀变有关的叠加之间的一般关联是至关重要的。

Magnetic properties of ferrimagnetic minerals depend on their crystal lattice, anisotropy, chemical composition and grain size. The latter parameter is strongly controlled by microstructures, which are significant for the interpretation of the magnetic properties of shocked magnetic minerals. Fracturing and lattice defects are the main causes for magnetic domain size reduction and generate an increase in coercivity and the suppression of magnetic transitions (e.g. 34 K transition in pyrrhotite, Verwey transition in magnetite).Especially for an adequate investigation of shock-induced modifications in ferromagnetic minerals, a combination of microstructural and magnetic measurements is therefore essential.This project focusses on two significant aspects of extreme conditions – the consequence of shock waves on natural material on Earth and on the magnetic mineralogy of exotic magnetic minerals in iron meteorites. In order to obtain general correlations between deformation structures and magnetic properties, the specific magnetic properties and carriers as well as microstructures of samples from two impact structures in marine targets (Lockne and Chesapeake Bay) will be compared with shocked magnetite ore and magnetite-bearing target lithologies from outside the crater (Lockne) as well as from undeformed megablocks within the crater (Chesapeake Bay). We will test the hypothesis if shock-related microstructures and associated magnetic properties can significantly be overprinted by postshock hydrothermal alteration. We especially want to focus on the Verwey transition (TV) as lower TVs are described for shocked impact lithologies. Hence, the main focus of this study lies on magneto-mineralogical investigations which combine low- and high-temperature magnetic susceptibility and saturation isothermal remanent magnetization with mineralogical and microstructural investigations. The same methods will then be used for the investigation of iron meteorites, whose magnetic properties are often controled by exotic magnetic minerals like cohenite, schreibersite and daubreelite in addition to the metal phases. Magnetic transition temperatures of those phases are poorly documented in relation to their chemical composition as well as to their crystallographic and microstructural configuration. For a general understanding of shock-related magnetization processes in extraterrestrial and terrestrial material, however, it is crucial to obtain a general correlation between the initial “unshocked” state and the subsequent shock- and alteration-related overprints.