Coesite, a high-pressure silica polymorph, is a diagnostic indicator of impact cratering in quartz-bearing target rocks. The formation mechanism of coesite during hypervelocity impacts has been debated since its discovery in the 1960s. In impactites, coesite is preserved as a metastable phase in crystalline rocks that experienced peak shock pressures above ~30-40 GPa (Stöffler & Langenhorst, 1994), and in porous sedimentary rocks shocked at pressures as low as ~10 GPa (Kowitz et al., 2016). There is a general consensus that coesite within impactites originates by crystallization from a dense amorphous phase during shock unloading, when the pressure release path passes through the coesite stability field. Here, we present a combination of TEMbased electron diffraction analyses in order to obtain the necessary high-resolution images and crystallographic data to unravel the spatial and temporal relations of quartz and coesite. Such nanoscale approach revealed evidence for direct solid-state quartz-to-coesite transformation in shocked coesite-bearing quartz ejecta from the Australasian tektite/microtektite strewn field, which is the largest and youngest (~0.8 Myr old) on Earth. These ejecta consist of a mixture of coesite and quartz in variable proportions, the latter showing planar deformation features (PDFs) with typical {10-11} and {10-12} orientations. Coesite crystals range in size from ~500 nm to few nanometres, with rounded or elongated habit. They show evident twinning and planar disorder along (010) planes. Where quartz and coesite are in contact, no appreciable amorphous or ‘glassy’ volume was detected. Instead, quartz boundaries are always lobate or sawtooth-like, with euhedral coesite crystals penetrating through the quartz boundaries. Moreover, PDFs in quartz clearly extend in the coesite domains, suggesting that the latter forms directly at the expense of shocked quartz crystals. Our observations indicate that quartz transforms directly to coesite after PDF formation and through a solidstate process without entering the silica liquid stability field. The recurrent pseudo iso-orientation between the (1-11) vector in quartz and the (010) vector of neighbouring coesite crystals point to a martensitic-like transformation as possible transition mechanism. Arguably, solid-state martensitic-like process could represent the dominant mechanism of coesite formation in a wide range of cratering events, at least for those with porous target rocks like at the Barringer (Kieffer et al., 1976) and Kamil craters (Folco et al., 2018). This implies lower peak impact pressure and temperature conditions for the formation of impact coesite than previously thought.
Campanale, F., Mugnaioli, E., Folco, L., Gemmi, M., Lee, M.R., Daly, L., et al. (2019). Evidence for direct solid-state quartz-to-coesite transformation in shocked ejecta from the Australasian tektite/microtektite strewn field. In Il tempo del pianeta Terra e dell’uomo: le geoscienze tra passato e futuro (pp.377-377). Roma : Società Geologica Italiana.
Evidence for direct solid-state quartz-to-coesite transformation in shocked ejecta from the Australasian tektite/microtektite strewn field
Mugnaioli E.;
2019-01-01
Abstract
Coesite, a high-pressure silica polymorph, is a diagnostic indicator of impact cratering in quartz-bearing target rocks. The formation mechanism of coesite during hypervelocity impacts has been debated since its discovery in the 1960s. In impactites, coesite is preserved as a metastable phase in crystalline rocks that experienced peak shock pressures above ~30-40 GPa (Stöffler & Langenhorst, 1994), and in porous sedimentary rocks shocked at pressures as low as ~10 GPa (Kowitz et al., 2016). There is a general consensus that coesite within impactites originates by crystallization from a dense amorphous phase during shock unloading, when the pressure release path passes through the coesite stability field. Here, we present a combination of TEMbased electron diffraction analyses in order to obtain the necessary high-resolution images and crystallographic data to unravel the spatial and temporal relations of quartz and coesite. Such nanoscale approach revealed evidence for direct solid-state quartz-to-coesite transformation in shocked coesite-bearing quartz ejecta from the Australasian tektite/microtektite strewn field, which is the largest and youngest (~0.8 Myr old) on Earth. These ejecta consist of a mixture of coesite and quartz in variable proportions, the latter showing planar deformation features (PDFs) with typical {10-11} and {10-12} orientations. Coesite crystals range in size from ~500 nm to few nanometres, with rounded or elongated habit. They show evident twinning and planar disorder along (010) planes. Where quartz and coesite are in contact, no appreciable amorphous or ‘glassy’ volume was detected. Instead, quartz boundaries are always lobate or sawtooth-like, with euhedral coesite crystals penetrating through the quartz boundaries. Moreover, PDFs in quartz clearly extend in the coesite domains, suggesting that the latter forms directly at the expense of shocked quartz crystals. Our observations indicate that quartz transforms directly to coesite after PDF formation and through a solidstate process without entering the silica liquid stability field. The recurrent pseudo iso-orientation between the (1-11) vector in quartz and the (010) vector of neighbouring coesite crystals point to a martensitic-like transformation as possible transition mechanism. Arguably, solid-state martensitic-like process could represent the dominant mechanism of coesite formation in a wide range of cratering events, at least for those with porous target rocks like at the Barringer (Kieffer et al., 1976) and Kamil craters (Folco et al., 2018). This implies lower peak impact pressure and temperature conditions for the formation of impact coesite than previously thought.File | Dimensione | Formato | |
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https://hdl.handle.net/11365/1118064