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

ICDP-USGS在切萨皮克湾的Eyreville钻探获得了最完整的撞击结构地质剖面之一。从1766米深的岩心样本钻到35 Ma的冲击构造的中心区域，可以研究不同的目标和冲击岩性及其岩石磁性。造成地壳磁异常的主要磁性矿物是磁黄铁矿和磁铁矿。本研究的主要目的是为冲击相关过程中再磁化机制的不同假设寻找新的线索。我们的初步调查表明，目标岩石含有磁铁矿和磁黄铁矿，它们是造成区域磁异常模式的原因。这一模式被撞击所破坏，这与嵌在岩屑和滑石角砾岩单元中的位移基底衍生的巨型岩块的解释相一致。在后一个单元中观察到磁性矿物的唯一改变，而基底源块体没有表现出任何冲击变形。我们发现冲击诱导的剩余磁化和化学剩余磁化是唯一的再磁化机制，并且没有TRM的迹象可以证实，这与文献中的大多数假设相反。激波磁黄铁矿（4C变质，具有丰富的机械孪晶和缺陷结构，明显的金属缺陷和360°C的TC）和苏云石基体中的次生磁铁矿（解释为主要由熔融蚀变产生的含铁流体形成）是苏云石单元中稳定残余磁化（Jr > Ji）的磁性载体。由于在软石中只观察到一个NRM方向，可能这两种矿物都是在接近撞击事件发生的时间获得磁化的。我们计划用钠铁矿样品和实验激波磁黄铁矿进行低温磁化实验。这些数据可以增强我们对冲击岩石中冲击诱导采集过程的理解，并有助于理解磁黄铁矿中微观结构与冲击压力之间的关系，这一关系目前还没有得到很好的约束。此外，还将研究仅含磁铁矿的样品，以了解这些样品是否显示出超顺磁行为的迹象，正如我们从观察到的纳米晶体中怀疑的那样，并在居里温度从500°C到640°C的大范围内找到线索。

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.