| Popis: |
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. The IODP-ICDP expedition 364 drilled into the peak ring of the Chicxulub impact crater, which is characterized by a well-developed hydrothermal system. This system was active for up to 2 Ma, reaching temperatures of 350-450°C. The main goal of our study is the investigation and characterization of heat treatment on shocked magnetite, the most important magnetic mineral in the shocked granitoid basement, and impact lithologies from drill core M0077A. In this study, we used a combination of microscopic, rock-magnetic, and paleomagnetic methods to investigate the potential post-shock temperature effects in magnetite. Our preliminary results suggest the presence of three types of magnetite. The first type found in the crystalline basement shows large fractured grains of pure magnetite, with scattered paleomagnetic directions. The second type consists of newly formed Al- and Mg- rich spinel, appearing in skeletal crystals at the uppermost impact melt layer, with stable 29r chron directions. A third type of magnetite is found throughout all lithologies in assemblage with sulphides, both interpreted of hydrothermal origin. We observe a general irreversibility in the temperature-dependent magnetic susceptibility (k-T curves) of the basement magnetite, and reversible k-T curves at close proximity with melt layers. We interpret this to indicate the hydrothermal system to not have reached annealing temperatures, in contrast with the slow-cooling, high-temperature deeper melt layers. |
