Impact structures: Effect of post-shock thermal conditions on mineral magnetism

地球上的大型撞击构造，如墨西哥的Chixulub，具有磁高的特征，但磁矿物学起源仍然缺乏约束，并讨论了撞击产生的熔体与热液活动的模型。冲击特征以及热激活过程是产生或破坏磁性矿物的最重要的驱动力，导致撞击构造上方的磁异常信号。本研究项目的主要目的是对地球地壳岩石中的两种主要磁性矿物--冲击磁铁矿和磁黄铁矿进行热处理研究和表征。为此，我们计划研究实验冲击和随后退火材料的岩石磁性和微结构特征，并将其与受冲击岩石的磁性矿物进行比较。为了进行比较，选择了希克苏勒陨石坑的IODP-ICDP钻芯M0077A和Nördlingen 1973的钻芯，因为这两个结构都显示了与撞击相关的磁异常的内部陨石坑。不同之处在于大小，对于直径200公里的希克苏鲁伯来说，假设是一个长期存在的热液系统是活跃的，而对于直径25公里的Nördlingen撞击结构则不是。用随温度变化的磁化率以及一系列的显微镜(反射光、扫描电子显微镜、磁力显微镜、透射电子显微镜)、X射线衍射技术和室温和低温磁化实验来表征冲击和退火态的样品。了解受冲击物质受热时的磁性和微结构特征，以及它们与新的磁性矿物形成的区别，可能是解释地球和其他行星上大型撞击坑上方磁异常模式的关键。因此，这项建议旨在区分导致大型撞击结构上磁信号增强或破坏的不同二次过程。

Large impact structures on Earth like the Chicxulub in Mexico are characterized by magnetic highs but the magneto-mineralogical origin is still poorly constrained and impact-generated melt versus hydrothermal activity models are discussed. Shock features along with thermally activated processes are the most important driving forces for the creation or destruction of magnetic minerals causing magnetic anomaly signals over impact structures. The main goal of this research project is the investigation and characterization of heat treatment on shocked magnetite and pyrrhotite, the two main magnetic minerals in crustal rocks of the Earth. For this purpose, we plan to study rock magnetic properties and microstructural features from experimentally shocked and afterwards annealed material and compare it to magnetic minerals from impacted rocks. For comparison, the IODP-ICDP drill core M0077A from the Chicxulub impact crater and from the Nördlingen 1973 drill core are selected, because both structures show inner craters with impact-related magnetic anomalies. The difference is the size and it is hypothesized for the 200 km diameter Chicxulub that a long-living hydrothermal system was active, but not for the 25 km diameter Nördlingen impact structure. Temperature dependent magnetic susceptibility along with a series of microscopic (reflected light, scanning electron microscopy, magnetic force microscopy, transmission electron microscopy), x-ray diffraction techniques and magnetization experiments at room temperature and low-temperature will be used to characterize shocked and annealed samples. An understanding of the magnetic and microstructural features due to heating of shocked material and their distinction from new magnetic mineral formations are likely a key to explain magnetic anomaly pattern over large impact craters on Earth and other planetary bodies. Therefore, this proposal aims on the discrimination of different secondary processes leading to enhancement or destruction of magnetic signals over large impact structures.