Browsing by Author "Kovaleva, Elizaveta"

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    Evidence from the Vredefort Granophyre Dikes points to crustal relaxation following basin-size impact cratering
    (Academic Press Inc., 2021-11-24) Huber, Matthew D.; Kovaleva, Elizaveta; Clark, Martin D.; Riller, Ulrich; Fourie, Francois D.
    The timescale of the modification stage of basin-sized impact structures is not well understood. Owing to ca. 10 km of erosion since its formation, the Vredefort impact structure, South Africa, is an ideal testing ground for deciphering post-impact modification. Here, we present geophysical and geochemical evidence from the Vre defort Granophyre Dikes, which were derived from the - now eroded - Vredefort impact melt sheet. The dikes have been studied mostly in terms of their composition, while the timing and duration of their emplacement remain controversial. We examined the modern depth extent of five dikes, with three from the inner crystalline core of the central uplift, and two from the boundary between the core and the supracrustal collar of the central uplift, using two-dimensional electrical resistivity tomography. We found that the core dikes terminate near the present erosion surface (i.e., <5 m depth). In contrast, the dikes at the core-collar boundary extend to a depth ≥ 9 m. These observations suggest that the core dikes are exposed near their lowermost terminus. In addition, we obtained bulk geochemical composition of the dikes, finding that the andesitic composition phase is present in the core-collar dikes that is not found in the core dikes. The presence of this phase indicates the episodic emplacement of impact melt into subvertical crater floor fractures. We conclude that the dike formation was protracted and occurred over a time span of at least 104 years. The sequential formation of the Vredefort Granophyre Dikes points to horizontal extension of crust below the impact melt sheet above a kinematic velocity discontinuity, a crustal instability resulting from the dynamic collapse o
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    Nanoscale mapping of ZrSiO4phases in naturally shocked zircon using electron energy loss spectroscopy
    (Walter de Gruyter GmbH, 2025) Kovaleva, Elizaveta; Roddatis, Vladimir; Syczewski, Marcin
    Coexistence in natural samples of zircon (ZrSiO4) and reidite (a high-pressure polymorph of ZrSiO4) is attributed to the effects of hypervelocity impact events. The grains and intergrowths in those minerals can be merely a few nanometers in size, which makes phase identification by standard methods of structure analysis difficult. However, analytical scanning transmission electron microscopy (STEM) utilizing electron energy loss spectroscopy (EELS) can provide important information on phase transition mechanisms and pressure-temperature conditions associated with the shock event at the nanoscale. Here we demonstrate that the valence as well as oxygen core-loss EELS can be employed for nanoscale mapping of zirconreidite distributions in zircon-reidite aggregates. Moreover, other accompanying phases, e. g., baddeleyite, could also be identified and mapped by this method. We further compare the EELS maps with a 4D-STEM nanobeam precession electron diffraction data, and demonstrate the advantages of the EELS mapping, which provides spatial resolution down to the nanometer scale and is independent of crystal orientation.