Paleomagnetism of impact breccias from the Chicxulub crater - Implications for ejecta emplacement and hydrothermal processes

摘要

Results of a paleomagnetic study of 89 samples from impact breccias from the Chicxulub crater are presented and used to investigate on ejecta emplacement, hydrothermal and alteration processes. The impactite sequence sampled in cores from Yaxcopoil-1 borehole is ～100. m thick and formed by six subunits characterized by petrographic, scan images, geochemical and textural analyses, which point to different emplacement modes. Impact occurred within reverse polarity chron C29r; therefore, magnetization acquired at crater formation and shortly thereafter will present reverse polarity, which is supported by paleomagnetic measurements on melt samples from Yucatan-6 borehole. Results after step-wise thermal and alternating field demagnetization document a pattern of 23 upward and 29 downward inclinations at Yaxcopoil-1 around -33° and 38°, respectively. Measurements of magnetic hysteresis and variation of low-field susceptibility with temperature suggest low-Ti titanomagnetites and magnetite as main magnetic carriers. Petrographic observations indicate the presence of fine-grained magnetite, hematite and Fe-oxyhydroxides, related to hydrothermal alteration processes. Curie points are in the range of 520-580 °C. The wide range of unblocking temperature spectra points to variable grain sizes of a magnetic phase that unblocks around 580 °C. Hysteresis ratio plots indicate most samples fall in the pseudo-single domain field. In some samples, wasp-waist constrained hysteresis loops suggest magnetite and hematite. Analyses of vector plots and coercivity and unblocking temperature spectra do not show apparent differences within and between subunits. Subunits are characterized by distinct textural and compositional differences in size, type and relative abundance of clasts and melt-rich or carbonate-rich matrix types. Bulk properties vary with composition and clast contents, particularly within the Middle Suevite and Brecciated Melt Rock subunits that show higher values, while low values characterize the Lower Suevite carbonate-rich with rare basement clasts subunit. Magnetization acquisition mechanisms are discussed, with reference to impactite characteristics, and may relate to remagnetization from hydrothermal and post-impact alteration processes.