Rock magnetic properties and their anisotropy from host rock and impact lithologies of drillings at the Chesapeake Bay impact structure, USA
Final Report Abstract
Meteorite impact structures are an important feature of most planetary surfaces. The effects of shock waves on the intrinsic magnetic properties of minerals and rocks are therefore essential for the understanding of magnetization processes and magnetic field anomalies related to impact structures on earth and other planetary bodies. This project combined rock magnetic measurements with microstructural studies in drill cores from the 35 Ma old Chesapeake Bay impact structure (CBIS), USA, in order to better understand the relationship between shock pressure, microstructures and magnetic properties, which are not well constrained up to now. Our 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 units. The only modification of magnetic minerals has been observed in the suevite, whereas 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 hypotheses found in the literature. Shocked pyrrhotite and secondary magnetite in the suevite matrix are the magnetic carriers of a stable remanent magnetization in the suevite units. We conducted low-temperature magnetization experiments and high-resolution microscopy with the suevite samples as well as with experimentally shocked pyrrhotite. Magnetite and pyrrhotite, the main magnetic carriers of the CBIS can be characterized as follows: MAGNETITE can be subdivided into three population types, named primary, shocked and secondary magnetite. Whereas primary magnetite is mainly characterized by grain sizes in the multidomain (MD) range, shocked magnetite has been largely affected by shockinduced fragmentation that resulted in grain sizes in the pseudo-single-domain (PSD) range. Secondary magnetite formed after the impact in the suevite matrix and occurs as clusters consisting of nm-sized crystals. Many samples with shocked and secondary magnetite show a decreased or suppressed Verwey transition temperature (TV), and this observation can be linked with a non-stoichiometric composition of these magnetites. Nonstoichiometry is mainly a product of oxidation and the ratio between the oxidized and stoichiometric volume fraction determines if and how strong TV is modified (see Mang and Kontny submitted to JGR). PYRRHOTITE can be subdivided into a shocked and unshocked population. A large fraction of shocked pyrrhotite grains are strongly depleted in iron resulting in a smythite -like composition (Fe9S11). This pyrrhotite type, referred to as iron-deficient pyrrhotite, shows an absent or largely suppressed 34 K transition (Besnus transition) and the TC is higher (340 - 365°C) than for 4C pyrrhotite (Fe7S8: 320°C). These features are a product of post-impact alteration and are not directly related to shock. During the third year of this project we conducted a series of shock experiments on natural pyrrhotite in order to examine what deformation structures develop in shocked pyrrhotite and how they modify its magnetic properties. The main reason for these experiments was that naturally shocked samples are usually altered and make it often impossible to distinguish alteration from shock-induced magneto-mineralogical properties. Our experiments showed that pyrrhotite, shocked up to 8 GPa, is characterized by mechanical deformation, including grain size reduction and the formation of planar fractures (PFs), planar deformation features (PDFs), and abundant internal defects. Associated with these features is an increase in coercivity and saturation isothermal remanent magnetization (SIRM), indicating an increase in SD behavior. Shock pressures between 20 and 30 GPa produce large amorphous domains in the crystal lattice and at 30 GPa shock-melting occurs, which is associated with the formation of native iron. Especially the occurrence of native iron has a significant effect on the magnetic properties of shocked pyrrhotite.
Publications
- 2009. Megablocks and melt pockets in the Chesapeake Bay impact structure constrained using magnetic field measurements and properties of the Eyreville and Cape Charles cores. GSA Special Paper, 458, 195-208
Shah, A.K., Daniels, D., Kontny, A. Brozena, J.
- 2009. Rock magnetic properties of the ICDP-USGS Eyreville core, Chesapeake Bay impact stucture. GSA Special Paper, 458, 119-136
Elbra T., Kontny, A., Pesonen L.J.
- 2012. Iron-deficiency in naturally shocked pyrrhotite-evidence of shock metamporphism?Meteoritics and Planetary Science Letters (47), 2, 277–295
Mang, C., Harries, D., Kontny, A., Hecht, L., Langenhorst, F.
(See online at https://doi.org/10.1111/j.1945-5100.2012.01329.x) - 2012: Impact-related modifications in magnetite and pyrrhotite and their consequences on the rock magnetic properties of rocks from the Chesapeake Bay impact structure, Virginia, USA. KIT Karlsruhe, 114 pp.
Mang, C.
- 2013. Shock experiments up to 30 GPa and their consequences on microstructures and magnetic properties in pyrrhotite. Geochem. Geophys. Geosyst., 14, 1-22
Mang, C., Kontny, A., Fritz, J., Schneider, R.
(See online at https://doi.org/10.1029/2012GC004242)
