Rock magnetic properties and their anisotropy from host rock and impact lithologies of drillings at the Chesapeake Bay impact structure, USA

切萨皮克湾的ICDP-USGS Eyreville钻井产生了撞击结构最完整的地质剖面之一。钻进35 Ma老碰撞构造中心区的1766米深的岩心样品，可以研究不同的目标和碰撞岩性及其岩石磁性。造成地壳磁性异常的主要磁性矿物是磁黄铁矿和磁铁矿。这项研究的主要目的是寻找关于碰撞相关过程中再磁化机制的不同假说的新线索。我们的初步研究表明，目标岩中含有磁铁矿和磁黄铁矿，它们是区域磁异常模式的成因。这种模式被冲击所破坏，这与嵌入岩屑角砾岩单元中的移位的基底来源的巨型块体的解释是一致的。在后一单元中只观察到磁性矿物的变质作用，而来自基底的块体没有表现出任何冲击变形。我们发现激波诱导的剩磁和化学剩磁是唯一的剩磁机制，与文献中的大多数假设相反，没有TRM的迹象被证实。震荡型磁黄铁矿(4C变质，具有丰富的机械孪晶和缺陷结构，明显缺乏金属，360℃时为TC)和硫铁矿基质中的次生磁铁矿(解释为主要来自熔体蚀变的含铁流体形成)是硫铁矿单元中稳定剩余磁化(Jr&Gt；Ji)的磁性载体。由于在硫化物中只观察到一个NRM方向，可能这两种矿物都在接近撞击事件的时间获得了磁化。我们计划用硫锌矿样品以及实验上受到冲击的磁黄铁矿进行低温磁化实验。这些数据可能会加深我们对冲击岩石中冲击诱导捕获过程的理解，并有助于理解磁黄铁矿微观结构与冲击压力之间的关系，这一关系到目前为止还没有得到很好的约束。此外，还将研究只含有磁铁矿的样品，以了解这些样品是否像我们从观察到的纳米晶体中怀疑的那样显示出任何超顺磁性行为的迹象，并寻找居里温度从500°C到640°C的大范围的线索。

The ICDP-USGS Eyreville drilling at Chesapeake Bay produced one of the most complete geologic sections of an impact structure. Core samples from the 1766 m deep drilling into the central zone of the 35 Ma old impact structure allows the study of different target and impact lithologies and their rock magnetic properties. The dominant magnetic minerals creating crustal magnetic anomalies are pyrrhotite and magnetite. The main target of this study is to find new clues about the different hypotheses on remagnetization mechanisms during impact-related processes. Our initial investigations have shown that the target rocks contain magnetite and pyrrhotite, which are responsible for the regional magnetic anomaly pattern. This pattern is disrupted by the impact, which is in accordance with an interpretation of displaced basement-derived megablocks embedded in the lithic and suevitic breccia unit. The only modification of magnetic minerals has been observed in this latter unit, while the basement-derived blocks do not show any shock deformation. We found shock-induced remanent magnetization and chemical remanent magnetization to be the only remagnetization mechanisms, and no indication for a TRM can be confirmed, in contrast to most hypothesis in literature. Shocked pyrrhotite (4C modification with abundant mechanical twins and defect structures, a significant metal deficieny and a TC at 360 °C) and secondary magnetite in the suevite matrix (interpreted to be formed from Febearing fluids, which derived mainly from melt alteration) are the magnetic carriers of a stable remanent magnetization (Jr > Ji) in the suevite unit. Because only one NRM direction has been observed in the suevite, probably both minerals have acquired their magnetization close to the timing of the impact event. We plan to conduct low-temperature magnetization experiments with the suevite samples as well as with experimentally shocked pyrrhotite. These data might enhance our understanding of shock-induced acquisition processes in impact rocks and help to understand the relation between microstructures in pyrrhotite and shock pressure, which is not well constrained up to now. Furthermore, the magnetite only bearing samples will also be studied in order to learn if these samples do show any indications of superparamagnetic behaviour as we would suspect from the observed nanocrystals, and to find clues on the large range of Curie temperatures from 500 to 640 °C.