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.

Chesapeake湾的ICDP-USGS Eyreville钻探产生了影响结构中最完整的地质部分之一。从1766 m深入到35 MA旧冲击结构中心区域的核心样品可以研究不同的目标和撞击岩性及其岩石磁性。产生地壳磁异常的主要磁性矿物质是黄铁矿和磁铁矿。这项研究的主要目标是找到有关在与撞击相关过程中有关延迟机制的不同假设的新线索。我们的初步研究表明，目标岩石含有磁铁矿和黄铁矿，这些岩石是负责区域磁异常模式的原因。这种模式受到影响的破坏，这与对岩石和suevitic角砾岩单元中嵌入地下室衍生的巨型岩的解释一致。在以后的单元中观察到磁性矿物质的唯一修饰，而碱衍生的块没有显示任何冲击变形。我们发现，与文献中的大多数假设相比，我们发现冲击诱导的远期磁化和化学物质磁性磁化是唯一的重新磁性机制，并且无法确认TRM的迹象。冲击的黄铁矿（用丰富的机械双胞胎和缺陷结构修饰，在360°C时具有明显的金属缺乏和TC）和SUEVITE基质中的二次磁铁矿（解释为由还使用货物形成的稳定型号是由稳定的MAGNENENT MAGNETIAL的磁性携带者（JR）是稳定的。由于在SUEVITE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机附近获得了磁化强度。我们计划使用SUEVITE样品以及实验震惊的黄铁矿进行低温磁化实验。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，并有助于了解黄铁矿和冲击压力中微观结构之间的关系，到目前为止尚未得到很好的限制。此外，还将研究磁铁矿仅轴承样品，以了解这些样品是否确实显示出任何超顺磁行为的迹象，因为我们怀疑从观察到的纳米晶体中怀疑，并在500至640°C的大量居里温度下找到群集。