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Microstructural Constraints on the Mechanisms of the Transformation to Reidite in Naturally Shocked Zircon

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Microstructural Constraints on the Mechanisms of the Transformation to Reidite in Naturally Shocked Zircon Contrib Mineral Petrol (2017) 172:6 DOI 10.1007/s00410-016-1322-0 ORIGINAL PAPER Microstructural constraints on the mechanisms of the transformation to reidite in naturally shocked zircon Timmons M. Erickson1 · Mark A. Pearce2 · Steven M. Reddy1 · Nicholas E. Timms1 · Aaron J. Cavosie1,3,4 · Julien Bourdet5 · William D. A. Rickard6 · Alexander A. Nemchin1,7 Received: 2 May 2016 / Accepted: 8 December 2016 / Published online: 12 January 2017 © Springer-Verlag Berlin Heidelberg 2017 Abstract Zircon (ZrSiO4) is used to study impact struc- using a combination of electron backscatter diffraction tures because it responds to shock loading and unload- (EBSD) and focused ion beam cross-sectional imaging ing in unique, crystallographically controlled manners. techniques. The zircon-bearing clasts were obtained from One such phenomenon is the transformation of zircon to within suevite breccia from the Nördlingen 1973 bore- the high-pressure polymorph, reidite. This study quan- hole, close to the center of the 14.4 Ma Ries impact crater, tifies the geometric and crystallographic orientation in Bavaria, Germany. We have determined that multiple relationships between these two phases using naturally sets (up to 4) of reidite lamellae can form in a variety of shocked zircon grains. Reidite has been characterized in non-rational habit planes within the parent zircon. How- 32 shocked zircon grains (shocked to stages II and III) ever, EBSD mapping demonstrates that all occurrences of lamellar reidite have a consistent interphase misori- entation relationship with the host zircon that is char- acterized by an approximate alignment of a {100}zircon with a {112}reidite and alignment of a {112}zircon with a Communicated by Gordon Moore. conjugate {112}reidite. Given the tetragonal symmetry of zircon and reidite, we predict that there are eight possible Electronic supplementary material The online version of this variants of this interphase relationship for reidite trans- article (doi:10.1007/s00410-016-1322-0) contains supplementary material, which is available to authorized users. formation within a single zircon grain. Furthermore, laser Raman mapping of one reidite-bearing grain shows that * Timmons M. Erickson moderate metamictization can inhibit reidite formation, [email protected] thereby highlighting that the transformation is controlled by zircon crystallinity. In addition to lamellar reidite, 1 Department of Applied Geology, Curtin University, GPO Box U1984, Perth, WA 6845, Australia submicrometer-scale granules of reidite were observed in one zircon. The majority of reidite granules have a topo- 2 CSIRO Mineral Resources, Australian Resources Research Centre, 26 Dick Perry Avenue, Kensington, WA 6151, taxial alignment that is similar to the lamellar reidite, Australia with some additional orientation dispersion. We confirm 3 NASA Astrobiology Institute, Department of Geoscience, that lamellar reidite likely forms via a deviatoric transfor- University of Wisconsin-Madison, Madison, WI 53706, USA mation mechanism in highly crystalline zircon, whereas 4 Department of Geology, University of Puerto Rico- granular reidite forms via a reconstructive transformation Mayagüez, Mayagüez, PR 00681, USA from low-crystallinity ZrSiO4 within the reidite stability 5 CSIRO Energy, Australian Resources Research Centre, 26 field. The results of this study further refine the formation Dick Perry Avenue, Kensington, WA 6151, Australia mechanisms and conditions of reidite transformation in 6 Department of Imaging and Applied Physics, Curtin naturally shocked zircon. University, GPO Box 1984, Perth, WA 6845, Australia 7 Department of Geosciences, Swedish Museum of Natural Keywords Shock metamorphism · EBSD, zircon · History, 104 05 Stockholm, Sweden Reidite · Ries impact crater 1 3 6 Page 2 of 26 Contrib Mineral Petrol (2017) 172:6 Table 1 Locations of natural reidite occurrences Impact crater Age (Ma) Shock stage Method of identification Number of grains reported References Ries impact structure, 14.7 II–IV Laser Raman spectroscopy II: 0, III: 1/12 (8%), IV: 0 Gucsik et al. (2004a) Germany II–IV Laser Raman spectroscopy II–III: 52/67 (78%), IV: 1/46 Wittmann et al. (2006) (2%) II–III EBSD, laser Raman 32/46 (70%) This study Eocene ejecta, New Jersey, 35.7 – X-ray diffraction 7/68 (10%) Glass and Liu (2001) Glass USAa et al. (2002) Eocene ejecta, Barbadosa 35.7 – – Glass et al. (2002) 4 Kamo et al. (2002) Chesapeake Bay impact 35.7 II–III Laser Raman spectroscopy – Wittmann et al. (2009) structure, USA II–III 1/27 (4%) Malone (2009) III–IV 1 Malone et al. (2010) Xiuyan Crater, China 0.1 II–III Laser Raman, XRD II: 7/30 (23%), III: 0/20 Chen et al. (2013) Rock elm impact structure, 460 II EBSD 2/135 (1%) Cavosie et al. (2015a) USA Proterozoic ejecta, Stac 1180 – EBSD 2/200 (1%) Reddy et al. (2015) Fada, Scotland Haughton Crater, Canada 39 IV Laser Raman 4/6 (67%) Singleton et al. (2015) a Ejecta horizons correlated to the Chesapeake Bay impact structure Introduction the scheelite-structured reidite facilitates an approximately 10% increase in density, and is inferred to be complete Zircon (ZrSiO4, space group I41/amd) is an important indi- at 900 °C and 12 GPa (Reid and Ringwood 1969; Ono cator and geochronometer of meteorite bombardment (Krogh et al. 2004). However, reidite forms well above 20 GPa in et al. 1984, 1993; Bohor et al. 1993). Shock microstructures shock experiments (Kusaba et al. 1985; Leroux et al. 1999; in zircon form at or above 20 GPa (Leroux et al. 1999) and Morozova 2015). The discrepancy in the pressure condi- provide diagnostic criteria to identify impact structures tions for reidite formation is likely the effect of an energy (French 1998; French and Koeberl 2010). Zircon is highly barrier associated with a transition state (Marqués et al. refractory, and shock microstructures can survive subse- 2008). quent metamorphism while similar shock microstructures Naturally occurring reidite was first identified using are obscured in phases such as quartz (Wittmann et al. 2006). X-ray diffraction (XRD) in zircon grains from Eocene Shock microstructures in zircon include {100}-parallel defor- ejecta associated with the ~90 km diameter, ca. 35.7 Ma mation bands (Erickson et al. 2013a; Timms et al. 2012), Chesapeake Bay impact structure (Glass and Liu 2001; mechanical twins along {112} (Moser et al. 2011; Timms Glass et al. 2002). Reidite has since been reported from et al. 2012; Erickson et al. 2013a, b; Thomson et al. 2014), the ca. 14.7 Ma, 24-km-wide Ries impact structure, south- decomposition of zircon to ZrO2 and SiO2 (El Goresy 1965; ern Germany, (Gucsik et al. 2004a; Wittmann et al. 2006), Wittmann et al. 2006; Timms et al. 2017), the development of bedrock at the Chesapeake bay structure (Wittmann et al. granular textured zircon (Bohor et al. 1993; Schmieder et al. 2009), the ca. 0.1 Ma, 1.8-km-diameter Xiuyan Crater, 2015; Cavosie et al. 2016; Timms et al. 2017), conversion northeastern China (Chen et al. 2013), the ca. 460 Ma, of zircon to reidite (Glass and Liu 2001; Glass et al. 2002), 6.5-km-wide Rock Elm impact structure, Wisconsin, USA and subsequent decomposition of reidite to ZrO2 and SiO2 (Cavosie et al. 2015a), the ca. 23 Ma, 23 km Haughton (Reddy et al. 2015). In the following study, we undertake a impact structure (Singleton et al. 2015), and within zircon microstructural study of zircon–reidite relationships to further grains from the ca. 1180 Ma Stac Fada ejecta deposit, Scot- refine the impact conditions that produce this transformation, land (Reddy et al. 2015) (Table 1). and establish the factors that control its development. Experimental shock deformation studies of zircon show Reidite (space group I41/a), the tetragonal high-pressure the conversion of zircon to reidite commences above 20 polymorph of ZrSiO4, was first recorded in experiments GPa and is complete by 52 GPa (Mashimo et al. 1983; by Reid and Ringwood (1969). Under static loading pres- Kusaba et al. 1985; Fiske et al. 1999; Leroux et al. 1999; sures at mantle conditions, the transformation of zircon to Morozova 2015). While conditions under which zircon 1 3 Contrib Mineral Petrol (2017) 172:6 Page 3 of 26 6 transforms to reidite during shock loading have been con- operate on the nanosecond to seconds timescale of shock strained experimentally, the mechanism by which the events because diffusion rates are too slow (Langenhorst conversion takes place is less constrained. The transfor- and Deutsch 2012); thus, a martensitic transformation mation mechanisms responsible for the conversion have mechanism is favored for impact-related reidite. been studied experimentally and investigated by transmis- While an experimental study by Leroux et al. (1999) sion electron microscopy (TEM) (Leroux et al. 1999). In used TEM to first establish the crystallographic relation- zircon grains experimentally shocked to 40 GPa, the par- ships between reidite and zircon lattices, studies of natu- tial transformation to reidite developed as 0.1 µm lamellae ral samples have primarily used XRD (e.g., Glass and Liu formed parallel to the {100} of the zircon, and contained 2001; Chen et al. 2013) or laser Raman (e.g., Gucsik et al. ~0.05-µm-wide {112} twins within its structure (Leroux 2004a; Wittmann et al. 2006, 2009; Chen et al. 2013) to et al. 1999). At 60 GPa, complete conversion of the zir- establish the presence of reidite. However, both XRD and con to reidite occurred and the reidite had many mechani- laser Raman analyses do not allow inter-crystalline rela- cal twins in {112}, along with localized aggregates of tionships to be quantified. In contrast, EBSD data provide nanocrystalline reidite. Based on TEM diffraction data, a rapid means of acquiring quantitative crystallographic the reidite lamellae were interpreted to have formed par- data with high spatial (~50 nm) and angular (~0.5°) resolu- allel to {100}zircon. The transformation was interpreted to tion (Cavosie et al.
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