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Identifying and dating large impact structures is challenging, as many of the traditional shock indicator phases can be modified by post-impact processes. Highly robust accessory phases, such as zircon, while faithful recorders of shock wave passage, commonly respond with partial U-Pb age resetting during impact events. Titanite is an accessory phase with lower Pb closure temperature than many other robust chronometers, but its potential as indicator and chronometer of impact-related processes remains poorly constrained. In this study, we examined titanite grains from the Sudbury (Ontario, Canada) and Vredefort (South Africa) impact structures, combining quantitative microstructural and U-Pb dating techniques. Titanite grains from both craters host planar microstructures and microtwins that show a common twin-host disorientation relationship of 74° about <102>.In the Vredefort impact structure, the microtwins deformed internally and developed high and low-angle grain boundaries that resulted in the growth of neoblastic crystallites. U-Pb isotopic dating of magmatic titanite grains with deformation microtwins from the Sudbury impact structure yielded a 207Pb/206Pb age of 1851 ± 12 Ma that records either the shock heating or the crater modification stage of the impact event. The titanite grains from the Vredefort impact structure yielded primarily pre-impact ages recording the cooling of the ultra-high temperature Ventersdorp event but domains with microtwins or planar microstructures show evidence of U-Pb isotopic disturbance. Despite that the identified microtwins are not diagnostic of shock-metamorphic processes, our contribution demonstrates that titanite has great potential to inform studies of the terrestrial impact crater record.
|Number of pages||15|
|Journal||Contributions to Mineralogy and Petrology|
|Early online date||14 Sep 2018|
|Publication status||Published - 1 Oct 2018|
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EMMU: Electron Microscopy and Microanalysis Unit
Darling, J., Dunlop, J., Storey, C., Coyne, J. & Koor, N.
1/01/18 → …