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

美国切萨皮克湾冲击构造钻探的母岩和冲击岩性的岩石磁特性及其各向异性

基本信息

批准号：
65021913
负责人：
Privatdozentin Dr. Agnes Kontny
金额：
--
依托单位：
Abteilung Strukturgeologie und Tektonophysik
依托单位国家：
德国
项目类别：
Infrastructure Priority Programmes
财政年份：
2008
资助国家：
德国
起止时间：
2007-12-31 至 2012-12-31
项目状态：
已结题

来源：
https://gepris.dfg.de/gepris/projekt/65021913?language=en
关键词：
Rock magnetic properties their anisotropy

项目摘要

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.

切萨皮克湾的 ICDP-USGS Eyreville 钻探从 35 Ma 旧撞击构造中心区域 1766 m 深的钻探中获得了最完整的撞击构造地质剖面之一，可以研究不同的目标和撞击岩性。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。这项研究的主要目标是寻找有关重磁化机制的不同假设的新线索。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，这种模式被撞击所破坏，这符合位移基底的解释。在后一个单元中观察到嵌入岩屑和钙镁质角砾岩单元中的巨型块体，而基底衍生的块体没有显示出任何冲击引起的剩余磁化强度。化学剩磁是唯一的再磁化机制，与文献中的大多数假设相反，没有任何 TRM 的迹象可以得到证实（4C 改性具有丰富的机械孪晶和缺陷结构，显着的金属缺陷和 360 的 TC）。 °C）和硫铁矿基体中的次生磁铁矿（解释为由含铁流体形成，主要源自熔体蚀变）是稳定剩磁的磁性载体由于在铁钙石单元中仅观察到一个 NRM 方向，因此可能两种矿物都在接近撞击事件的时间处获得了磁化强度。这些数据可能会增强我们对冲击岩石中冲击引起的采集过程的理解，并有助于了解磁黄铁矿的微观结构与冲击压力之间的关系，而这之间的关系尚不清楚。此外，还将研究仅含有磁铁矿的样品，以便了解这些样品是否确实表现出我们从观察到的纳米晶体中怀疑的超顺磁行为的迹象，并找到大范围居里温度的线索。 500 至 640°C。