Nano-scale investigation of granular neoblastic zircon, Vredefort impact structure, South Africa: Evidence for complete shock melting

Kovaleva, Elizaveta; Kusiak, Monika A.; Kenny, Gavin G.; Whitehouse, Martin J.; Habler, Gerlinde; Schreiber, Anja; Wirth, Richard

doi:10.1016/j.epsl.2021.116948

Cited by 19 publications

(25 citation statements)

References 43 publications

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“…3D ) that are indicative of so-called “former reidite in granular neoblastic” (FRIGN) zircon ( 16 – 17 ). The interpretation that these recrystallization textures indicate the former presence of reidite is disputed ( 18 ), but they have only been documented at impact structures; thus, we consider them related unequivocally to an impact event. U-Pb analyses by secondary ion mass spectrometry (SIMS) targeted the three grain types described above (data S2 and S3).…”

Section: Resultsmentioning

confidence: 99%

A Late Paleocene age for Greenland’s Hiawatha impact structure

et al. 2022

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The ~31-km-wide Hiawatha structure, located beneath Hiawatha Glacier in northwestern Greenland, has been proposed as an impact structure that may have formed after the Pleistocene inception of the Greenland Ice Sheet. To date the structure, we conducted 40 Ar/ 39 Ar analyses on glaciofluvial sand and U-Pb analyses on zircon separated from glaciofluvial pebbles of impact melt rock, all sampled immediately downstream of Hiawatha Glacier. Unshocked zircon in the impact melt rocks dates to ~1915 million years (Ma), consistent with felsic intrusions found in local bedrock. The 40 Ar/ 39 Ar data indicate Late Paleocene resetting and shocked zircon dates to 57.99 ± 0.54 Ma, which we interpret as the impact age. Consequently, the Hiawatha impact structure far predates Pleistocene glaciation and is unrelated to either the Paleocene-Eocene Thermal Maximum or flood basalt volcanism in east Greenland. However, it was contemporaneous with the Paleocene Carbon Isotope Maximum, although the impact’s exact paleoenvironmental and climatic significance awaits further investigation.

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Section: Resultsmentioning

confidence: 99%

A Late Paleocene age for Greenland’s Hiawatha impact structure

et al. 2022

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“…This might be a similar process to the formation of calcite in water where precursors assemble to larger structures and, finally, mosaic crystals (Raz et al, 2000). The formation of mosaic crystals of zircon from nanometre-sized subunits of zircon has been recently observed and described with zircon from the Vredefort impact structure (Kovaleva et al, 2021). The important point here is that it is not classical nucleation and growth but nanometre or sub-nanometre-sized precursors that assemble to crystals due to surface charging.…”

Section: Discussionmentioning

confidence: 86%

New insights in the mechanisms of the reaction 3.65 Å phase = clinoenstatite + water down to nanoscales

et al. 2021

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Abstract. The reaction of 3.65 Å phase <=> clinoenstatite + water was investigated in five experiments at 10 GPa, 470–600 ∘C, using a rotating multi-anvil press. Under these P/T conditions, clinoenstatite exists in its high-pressure modification, which, however, is not quenchable to ambient conditions but transforms back to low-pressure clinoenstatite. The quenched run products were characterized by electron microprobe analyses (EMPA), powder X-ray diffraction (XRD), Raman spectroscopy and by high-resolution transmission electron microscopy (HRTEM) on focused ion beam (FIB)-cut foils. We bracketed the reaction in the T range 470 to 510 ∘C (at 10 GPa). The hydration of clinoenstatite to the 3.65 Å phase at 470 ∘C was very sluggish and incomplete even after 96 h. Clinoenstatites range in size from less than 1 to up to 50 µm. Usually clinoenstatite has a very small grain size and shows many cracks. In sub-micron-sized broken clinoenstatite, an amorphous phase (0.91Mg:1.04Si, with about 20 wt % H2O) was observed, which further transformed with increasing reaction time into the 3.65 Å phase (1Mg:1Si, with 34 wt % H2O). Thus, the sub-micron-sized fractured clinoenstatite transformed via an amorphous water-bearing precursor phase to the 3.65 Å phase. The dehydration to clinoenstatite was faster but still incomplete after 72 h at 600 ∘C. From the backscattered electron images of the recovered sample of the dehydration experiment, it is obvious that there is a high porosity due to dehydration of the 3.65 Å phase. Again, the grain size of clinoenstatite ranges from less than 1 up to 50 µm. There are still some clinoenstatite crystals from the starting material present, which can clearly be distinguished from newly formed sub-micron-sized clinoenstatite. Additionally, we observe a water-rich crystalline phase, which does not represent the 3.65 Å phase. Its Raman spectra show the double peaks around 700 and 1000 cm−1 characteristic for enstatite and strong water bands at 3700 and 3680 cm−1. The Mg:Si ratio of 0.90:1.04 was determined by EMPA, totalling to 81 wt %, in accordance with its high water content. Diffraction patterns from high-resolution images (fast Fourier transform – FFT) are in agreement with an orthoenstatite crystal structure (Pbca). The surprising observation of this study is that, in both directions of the investigated simple reaction, additional metastable phases occur which are amorphous in the hydration and crystalline in the dehydration reaction. Both additional phases are water rich and slightly deviate in composition from the stable products 3.65 Å phase and clinoenstatite, respectively. Thus, as a general remark, conventional investigations on reaction progress should be complemented by nanoscale investigations of the experimental products because these might reveal unpredicted findings relevant for the understanding of mantle processes. The extreme reduction in grain size observed in the dehydration experiments due to the formation of nanocrystalline clinoenstatite rather than the slowly released fluids might cause mechanical instabilities in the Earth's mantle and, finally, induce earthquakes.

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“…The SiO 2 reacted with MgO to form MgSiO 3 . Additionally, with the increase in ZrSiO 4 concentration, the peak strength of ZrO 2 was also raised gradually (Kovaleva et al, 2021).…”

Section: Microstructure and Phase Analysismentioning

confidence: 93%

“…The phase composition was formed by the different reactions during the MAO process (Baghdadabad et al, 2020;Kovaleva et al, 2021).…”

Section: Formation Mechanism Of the Coatingmentioning

confidence: 99%

Effect of ZrSiO4 Concentration on the Microstructure and Corrosion Resistance of MAO Coatings Formed on AZ91 Magnesium Alloy

Chen

Xiang

et al. 2022

Front. Mater.

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As a metallic material with lightweight and high specific strength, magnesium alloy has excellent application prospects. However, the rapid corrosion rate and localized corrosion behavior of magnesium alloys limit the practical application in the automobile industry. In this study, to improve the corrosion resistance of AZ91 alloy, the film of different concentrations containing zirconium (AZR0, AZR5, AZR10, AZR15) was prepared on the surface of AZ91 alloy by micro-arc oxidation technology in the Na2SiO3-Na3PO4 system. Furthermore, the influence of electrolyte composition on the corrosion resistance of the MAO film was systematically investigated. The experimental results revealed that the ZrSiO4 particles added in the electrolyte could enter into the MAO film and ZrSiO4 particles were also decomposed into ZrO2 and Mg2Zr5O11 in the process of micro-arc oxidation. More importantly, the formation of micro-cracks and other defects in the film could be reduced after this process. The addition of 15 g l−1 ZrSiO4 in the electrolyte was contributed to the best comprehensive properties of MAO-processed AZ91 specimens, including improved Vickers hardness of 167.16 Hv, Young’s modulus of 652 MPa, and enhanced corrosion resistance (RP = 9.82 × 105 Ω cm2). This approach could provide the approach for developing Mg-based materials with high anticorrosion in industrial fields.

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Nano-scale investigation of granular neoblastic zircon, Vredefort impact structure, South Africa: Evidence for complete shock melting

Cited by 19 publications

References 43 publications

A Late Paleocene age for Greenland’s Hiawatha impact structure

A Late Paleocene age for Greenland’s Hiawatha impact structure

New insights in the mechanisms of the reaction 3.65 Å phase = clinoenstatite + water down to nanoscales

Effect of ZrSiO4 Concentration on the Microstructure and Corrosion Resistance of MAO Coatings Formed on AZ91 Magnesium Alloy

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