Magnetic properties and microfabrics of shocked minerals and their influence on the magnetic anomalies in the Ries, Vredefort and Manicouagan impact structures

撞击陨石坑的目标岩石几乎是瞬间受到极高的应力、应变率和温度的影响，从而永久性地改变了岩石中的矿物质。这些冲击效应严重影响了冲击结构中铁磁性矿物的磁性，这反过来又导致它们具有明显的磁场异常。然而，对目标岩石中磁性矿物的撞击相关修饰及其剩余磁化率的了解仍然很少。因此，我们提出了一项综合岩石组构和磁矿物学研究，以了解三个复杂冲击构造的磁异常起源：Ries（德国），Vredefort（南非）和Manicouagan（加拿大）。所选择的冲击结构在直径上有一个数量级的不同，在压力和温度条件下提供了广泛的范围。它们含有受冲击结晶基底和/或冲击角砾岩的裸露露头，其中的结晶成分经历了广泛的冲击和后冲击条件。此外，这些冲击结构具有磁异常，其起源完全或部分与冲击过程有关。我们在这些结构中都有工作经验，并且熟悉它们的磁性特征。基于现场和实验室的岩石学、微观结构、古地磁和岩石磁学综合分析旨在更好地了解冲击变质作用如何影响受冲击目标岩石的磁残余，以及特定的P-T条件如何在冲击结构中产生磁场异常。以下关键问题将得到解决：(1)我们能否将不透明铁磁体中的冲击效应与非不透明相（例如，石英中的pdf）的既定冲击阶段指标等同起来？(2)在冲击加载和后冲击卸载过程中，快速变化的压力和温度条件对磁性矿物的影响和修饰是怎样的？(3)在特定的冲击压力和/或温度条件下产生的特定微观结构是否会转化为铁磁体的剩余磁化强度和磁性能的特定响应？(4)冲击诱发的剩余磁化如何影响包括冲击退磁在内的磁场异常？将冲击效应与特定的过程、P-T条件和岩石的磁性联系起来，对于解释撞击结构中的磁异常至关重要，这是我们太阳系中最常见的形态特征之一。

Impact-cratering subjects target rocks quasi-instantaneously to extremely high stresses, strain rates and temperatures that permanently modify the minerals within them. These shock effects bear heavily on the magnetic properties of ferrimagnetic minerals within impact structures, which in turn leads them to possess distinct magnetic field anomalies. However, the impact-related modifications of the magnetic minerals and their remanent magnetizations in the target rocks remain poorly understood. We therefore propose an integrated rock fabric and magneto-mineralogy study to understand the origin of the magnetic anomalies at three complex impact structures: Ries (Germany), Vredefort (South Africa) and Manicouagan (Canada). The chosen impact structures differ in diameter by an order of magnitude, offering a wide range in pressure and temperature conditions. They contain well-exposed outcrops of shocked crystalline basement and/or impact breccias with crystalline components that experienced a wide range of shock and post-shock conditions. Moreover, these impact structures possess magnetic anomalies whose origins are wholly or partially linked to shock processes. We have experience working in each of these structures and are familiar with their magnetic characteristics. The proposed integrated, field and laboratory-based, petrological, microstructural, paleomagnetic and rock magnetic analyses aim at a better understanding how shock metamorphism influences the magnetic remanence of shocked target rocks and how particular P-T conditions act to create magnetic field anomalies in impact structures. The following key questions will be addressed: (1) Can we equate shock effects in opaque ferrimagnets with established indicators of shock stages from non-opaque phases (e.g., PDFs in quartz)? (2) How are magnetic minerals affected/modified by the rapidly changing pressure and temperature conditions during shock loading and post-shock unloading? (3) Do specific microstructures arising from particular shock pressure and/or temperature conditions translate into specific responses in the remanent magnetization and magnetic properties of the ferrimagnets? (4) How does shock-induced remanent magnetization influence magnetic field anomalies including the case of shock demagnetization? Correlating shock effects to specific processes, P-T conditions and magnetic properties of the rocks is essential for the interpretation of magnetic anomalies in impact structures, which are among the most common morphologic features in our solar system.