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The Vredefort impact structure, South Africa, is a 2.02 Ga deeply eroded meteorite scar that provides an opportunity to study large impact craters at their lower stratigraphic levels. A series of anomalous granophyre dikes in the core of the structure are believed to be composed of an impact melt, which intruded downwards from the crater floor, exploiting fractures in basement rocks. However, the melt emplacement mechanisms and timing are not constrained. The granophyre dikes contain supracrustal xenoliths captured at higher levels, presently eroded. By studying these clasts and shocked minerals within, we can better understand the nature of dikes, magnitude of impact melt movement, conditions that affected target rocks near the impacted surface, and erosional rates. We report “former reidite in granular neoblastic” (FRIGN) zircon within a granite clast enclosed in the granophyre. High-pressure zircon transformation to reidite (ZrSiO4) and reversion to zircon resulted in zircon grains composed of fine neoblasts (∼0.5–3 µm) with two or three orthogonal orientations. Our finding provides new independent constraints on the emplacement history of Vredefort granophyre dikes. Based on the environment, where other FRIGN zircons are found (impact glasses and melts), the clast was possibly captured near the top of the impact melt sheet and transported to the lowermost levels of the structure, traveling some 8–10 km. Our finding not only provides the highest-pressure shock estimates thus far discovered in the Vredefort structure (≥30 GPa), but also shows that microscopic evidence of high shock pressures can be found within large eroded craters at their lowest stratigraphic levels.

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Context. Shock-induced changes in ordinary chondrite meteorites related to impacts or planetary collisions are known to be capable of altering their optical properties. Thus, one can hypothesize that a significant portion of the ordinary chondrite material may be hidden within the observed dark C/X asteroid population. Aims. The exact pressure-temperature conditions of the shock-induced darkening are not well constrained. Thus, we experimentally investigate the gradual changes in the chondrite material optical properties as a function of the shock pressure. Methods. A spherical shock experiment with Chelyabinsk LL5 was performed in order to study the changes in its optical properties. The spherical shock experiment geometry allows for a gradual increase of shock pressure from ∼15 GPa at a rim toward hundreds of gigapascals in the center. Results. Four distinct zones were observed with an increasing shock load. The optical changes are minimal up to ∼50 GPa. In the region of ∼50-60 GPa, shock darkening occurs due to the troilite melt infusion into silicates. This process abruptly ceases at pressures of ∼60 GPa due to an onset of silicate melting. At pressures higher than ∼150 GPa, recrystallization occurs and is associated with a second-stage shock darkening due to fine troilite-metal eutectic grains. The shock darkening affects the ultraviolet, visible, and near-infrared (UV, VIS, and NIR) region while changes to the MIR spectrum are minimal. Conclusions. Shock darkening is caused by two distinct mechanisms with characteristic pressure regions, which are separated by an interval where the darkening ceases. This implies a reduced amount of shock-darkened material produced during the asteroid collisions.

We report the results of the complex study of the bulk interior of Bursa L6 ordinary chondrite using optical microscopy, scanning electron microscopy with energy dispersive spectroscopy, electron microprobe analysis (EMPA), X-ray diffraction (XRD), magnetization measurements, and M€ ossbauer spectroscopy. The main and minor ironbearing phases and their chemical compositions were determined by these techniques. The detected iron-bearing phases in the bulk interior of Bursa L6 are the following: olivine; orthopyroxene; Ca-rich clinopyroxene; troilite; chromite; hercynite; ilmenite; the a 2-Fe(Ni, Co), a-Fe(Ni, Co), and c-Fe(Ni, Co) phases; and ferrihydrite resulting from meteorite terrestrial weathering. Using the EMPA, the values of fayalite and ferrosilite were obtained as~25.2% and~21.4%, respectively. The unit cell parameters for silicate crystals were determined from XRD, then the Fe 2+ and Mg 2+ occupations of the M1 and M2 sites in these crystals were estimated. Further calculations of the ratios of the Fe 2+ occupancies in the M1 and M2 sites in olivine and orthopyroxene based on XRD and M€ ossbauer spectroscopy appeared to be in a good agreement. The temperatures of equilibrium cation distributions for olivine and orthopyroxene obtained from these techniques are consistent: 623 K (XRD) and 625 K (M€ ossbauer spectroscopy) for olivine and 1138 K (XRD) and 1122 K (M€ ossbauer spectroscopy) for orthopyroxene.

A series of precise nondestructive analytical methods (Raman spectroscopy, cathodoluminescence, and EBSD—electron backscatter diffraction) has been employed to investigate the internal textures of kyanite porphyroblasts from diamondiferous and diamond‐free ultrahigh‐pressure metamorphic rocks (Kokchetav massif, Northern Kazakhstan). Such internal kyanite characteristics as twinning, radial fibrous pattern, and spotty zoning were identified by means of Raman and cathodoluminescence imaging, whereas an intergrowth of two kyanite crystals was distinguished only by Raman imaging. The EBSD analysis recorded an ~10–25° changing of orientations along the elongation in the investigated kyanite porphyroblasts. The absence of a radial fibrous pattern and a spotty zoning on the EBSD maps indicates that these textures are not related to variations in crystallographic orientation. The absence of clear zoning patterns (cores, mantles, and rims) on the Raman, cathodoluminescence, or EBSD maps of the kyanite porphyroblasts indicates the rapid single‐stage formation of these porphyroblasts near the peak metamorphic conditions and the lack of recrystallization processes. The obtained results provide important implications for deciphering of mineral internal textures, showing that the data obtained by cathodoluminescence mapping can be clearly reproduced by Raman imaging, with the latter method occasionally being even more informative. This observation is of significant importance for the study of minerals that are unexposed on a thin section surface or Fe‐ and Ni‐rich minerals that do not show luminescence emission. The combination of the Raman spectroscopic, cathodoluminescence, and EBSD techniques may provide better spatial resolution for distinguishing different domains and textural peculiarities of mineral than the selective application of individual approaches.

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