The impact pseudotachylitic breccia controversy: Insights from first isotope analysis of Vredefort impact-generated melt rocks

Reimold, W. U.; Hauser, Natalia; Hansen, Bent; Thirlwall, M. F.; Hoffmann, Marie C.

doi:10.1016/j.gca.2017.07.040

Cited by 25 publications

(51 citation statements)

References 50 publications

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“…Most of the ordinary chondrites have δ 18 O values between 3‰ and 6‰ (e.g., [42]) and an addition of <1% of such material would make no measurable difference to a magma with a δ 18 O value of 8‰ to 9‰. Given these constraints, the pseudotachylite veins were most consistent with local, frictional melting of the nearby OGG and ILG granite gneiss, as has been previously suggested [43].…”

Section: Oxygen Isotopessupporting

confidence: 65%

Post-Impact Faulting of the Holfontein Granophyre Dike of the Vredefort Impact Structure, South Africa, Inferred from Remote Sensing, Geophysics, and Geochemistry

et al. 2021

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Better characterization features borne from long-term crustal modification processes is essential for understanding the dynamics of large basin-forming impact structures on Earth. Within the deeply eroded 2.02 Ga Vredefort Impact Structure in South Africa, impact melt dikes are exposed at the surface. In this study, we utilized a combination of field, remote sensing, electrical resistivity, magnetic, petrographical, and geochemical techniques to characterize one such impact melt dike, namely, the Holfontein Granophyre Dike (HGD), along with the host granites. The HGD is split into two seemingly disconnected segments. Geophysical modeling of both segments suggests that the melt rock does not penetrate below the modern surface deeper than 5 m, which was confirmed by a later transecting construction trench. Even though the textures and clast content are different in two segments, the major element, trace element, and O isotope compositions of each segment are indistinguishable. Structural measurements of the tectonic foliations in the granites, as well as the spatial expression of the dike, suggest that the dike was segmented by an ENE–WSW trending sinistral strike-slip fault zone. Such an offset must have occurred after the dike solidified. However, the Vredefort structure has not been affected by any major tectonic events after the impact occurred. Therefore, the inferred segmentation of the HGD is consistent with long-term crustal processes occurring in the post-impact environment. These crustal processes may have involved progressive uplift of the crater floor, which is consistent with post-impact long-term crustal adjustment that has been inferred for craters on the Moon.

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Section: Oxygen Isotopessupporting

confidence: 65%

Post-Impact Faulting of the Holfontein Granophyre Dike of the Vredefort Impact Structure, South Africa, Inferred from Remote Sensing, Geophysics, and Geochemistry

et al. 2021

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“…Koeberl et al (1996) provided Re-Os isotopic analyses of the granophyre, suggesting~0.2 % of the meteoritic component within this melt rock. The chemical composition of granophyre dikes has been described as uniform (Reimold et al 1990(Reimold et al , 2017Therriault et al 1997), but variations in chemical composition have been later documented (Lieger and Riller 2012). Recent geophysical analysis has shown that one of the Vredefort granophyre dikes in the core of the structure terminates at a depth of 3-5 m below the current erosional surface (Fourie et al 2019).…”

Section: Background Geology Vredefort Impact Structure and Granophyrementioning

confidence: 99%

“…The chemical composition of granophyre dikes has been described as uniform (Reimold et al. 1990, 2017; Therriault et al. 1997), but variations in chemical composition have been later documented (Lieger and Riller 2012).…”

Section: Background Geologymentioning

confidence: 99%

Modeling the geochemical evolution of impact melts in terrestrial impact basins: Vredefort granophyre dikes and Sudbury offset dikes

Huber

Kovaleva

Riller

2020

Meteorit & Planetary Scien

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The Vredefort impact structure, South Africa, is comparable to the Sudbury impact structure, Canada, in size, age, and target rock composition. Both impact structures feature impact melt dikes. The melt sheet of the Sudbury impact (Sudbury Igneous Complex; SIC) is genetically linked to the Sudbury offset dikes in the underlying target rock. At Vredefort, the melt sheet was eroded so that only the granophyre dikes retain compositional melt sheet characteristics. XRF analyses of 43 samples from four granophyre dikes are similar to previous studies, but identify an anomalous mafic phase within one of the dikes. The results from the Vredefort granophyre dikes are compared to the Sudbury offset dikes and shown to follow similar geochemical trends, controlled by crystallization of feldspar and pyroxene. The mafic granophyre phase is compositionally remarkably similar to the offset dike compositions. The program Rhyolite-MELTS was used to test possible melt sheet compositions. Modeling results are broadly consistent with the overall chemical and mineral composition of the dikes. Modeling is consistent with offset dikes being derived from the basal mafic layer of the SIC, and the granophyre dikes being derived from alkalidepleted bulk continental crust. For all modeled compositions, crystallization primarily occurred at temperatures between 1150°C and 1000°C. The emplacement of the felsic granophyre dikes from a homogenized crustal melt suggests emplacement within tens of years after the impact event. The presence of the mafic phase in one of the granophyre dikes is explained by its emplacement following some differentiation of the Vredefort melt sheet.

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“…Pronounced differences in composition between granite and granophyre are expected, as granophyre represents a fraction of the impact melt, formed as a result of the mixing of different target rocks, such as shale, quartzite, granite, lamprophyre, mafic lava, carbonate, etc., in various possible proportions [3,46]. On the other hand, the pseudotachylitic melt is advocated to form strictly in situ and correspond geochemically to its host rock [4,11]. Some pseudotachylites in granites were found to be slightly more mafic (i.e., enriched in Mg and Fe) than their host rocks [7,8] due to the initial melting of mafic and hydrous phases with lower fracture toughness in non-equilibrium conditions [47,48].…”

Section: Geochemical Properties Of Pseudotachylitementioning

confidence: 99%

“…Having been deeply eroded during 2020 My of its history, it allows investigation of the impactites at the lowest levels of this large impact structure [1][2][3][4][5]. For example, impact-related pseudotachylites, formed at 8-10 km below the original level of the Vredefort structure, have been investigated in detail; their geochemical, isotopic, and (micro)structural properties have been studied continuously for over a century [4,[6][7][8][9][10][11][12]. However, the properties of the impactites formed in Vredefort at the higher structural levels, closer to the original surface, are practically unknown, with the upper portions of the structure being eroded [13,14].…”

Section: Introductionmentioning

confidence: 99%

Properties of Impact-Related Pseudotachylite and Associated Shocked Zircon and Monazite in the Upper Levels of a Large Impact Basin: a Case Study From the Vredefort Impact Structure

Kovaleva

Dixon

2020

Minerals

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The Vredefort impact structure in South Africa is deeply eroded to its lowermost levels. However, granophyre (impact melt) dykes in such structures preserve clasts of supracrustal rocks, transported down from the uppermost levels of the initial structure. Studying these clasts is the only way to understand the properties of already eroded impactites. One such lithic clast from the Vredefort impact structure contains a thin pseudotachylite vein and is shown to be derived from the near-surface environment of the impact crater. Traditionally, impact pseudotachylites are referred to as in situ melt rocks with the same chemical and isotopic composition as their host rocks. The composition of the sampled pseudotachylite vein is not identical to its host rock, as shown by the micro-X-ray fluorescence (µXRF) and energy-dispersive X-ray (EDX) spectrometry mapping. Mapping shows that the melt transfer and material mixing within pseudotachylites may have commonly occurred at the upper levels of the structure. The vein is spatially related to shocked zircon and monazite crystals in the sample. Granular zircons with small granules are concentrated within and around the vein (not farther than 6–7 mm from the vein). Zircons with planar fractures and shock microtwins occur farther from the vein (6–12 mm). Zircons with microtwins (65°/{112}) are also found inside the vein, and twinned monazite (180°/[101]) is found very close to the vein. These spatial relationships point to elevated shock pressure and shear stress, concentrated along the vein’s plane during impact.

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The impact pseudotachylitic breccia controversy: Insights from first isotope analysis of Vredefort impact-generated melt rocks

Cited by 25 publications

References 50 publications

Post-Impact Faulting of the Holfontein Granophyre Dike of the Vredefort Impact Structure, South Africa, Inferred from Remote Sensing, Geophysics, and Geochemistry

Post-Impact Faulting of the Holfontein Granophyre Dike of the Vredefort Impact Structure, South Africa, Inferred from Remote Sensing, Geophysics, and Geochemistry

Modeling the geochemical evolution of impact melts in terrestrial impact basins: Vredefort granophyre dikes and Sudbury offset dikes

Properties of Impact-Related Pseudotachylite and Associated Shocked Zircon and Monazite in the Upper Levels of a Large Impact Basin: a Case Study From the Vredefort Impact Structure

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