Planar deformation features in shocked quartz; a transmission electron microscopy investigation

Abstract

We present a detailed investigation by transmission electron microscopy (TEM) of the fine structure of planar deformation features (PDF) in quartz grains within which shock metamorphism has been detected optically and which originate from various sites: Slate Islands, La Malbaie and Manicouagan Lake, Canada; Ries Crater, Germany; the Toba caldera, Sumatra; the Vredefort complex in South Africa and the K/T boundary at Raton Basin, Colorado. PDFs appear on TEM micrographs as straight and very narrow (< 0.1 μm) bands, either glassy or crystalline. In the latter case some bands are microtwins while others contain a very high density of dislocations. We also noted a mixture of both configurations in the form of thin layers of partially disorganised crystal, some containing short dislocation segments while others appear to be very fine (10 nm) microcrystalline material embedded in a glassy matrix. Such contrasting microstructures may reflect different shock scenarios (different intensity or duration of the shock or possibly different target temperature). In at least one case (Vredefort) the post-shock thermal history (intense metamorphism) has strongly overprinted the shock-induced defects. TEM reveals many more PDFs than are optically detected, probably because only concentrations of such defects in narrow areas can induce an optically detectable contrast. Caution should therefore be exercised when applying criteria based on the optical observation of single or multiple sets of PDFs. PDFs in samples from Slate Islands and Raton Basin exhibit a peculiar morphology, a high density of very tiny bubbles precipitated on the dislocation lines. Comparison with the behavior of wet quartz strongly suggests that these materials experienced lengthy annealing at moderate pressure and temperature.