Transformations to granular zircon revealed: Twinning, reidite, and ZrO2 in shocked zircon from Meteor Crater (Arizona, USA) Aaron J. Cavosie1,2,3, Nicholas E. Timms1, Timmons M. Erickson1, Justin J. Hagerty4, and Friedrich Hörz5 1TIGeR (The Institute for Geoscience Research), Department of Applied Geology, Curtin University, Perth, WA 6102, Australia 2NASA Astrobiology Institute, Department of Geoscience, University of Wisconsin–Madison, Madison, Wisconsin 53706, USA 3Department of Geology, University of Puerto Rico–Mayagüez, Mayagüez, Puerto Rico 00681, USA 4U.S. Geological Survey (USGS) Astrogeology Science Center, Flagstaff, Arizona 86001, USA 5NASA Johnson Space Center, Science Department/Jets/HX5/ARES, Houston, Texas 77058, USA ABSTRACT Granular zircon in impact environments has long been recognized but remains poorly A N35° 2.0’ W111° 0.9’ understood due to lack of experimental data to identify mechanisms involved in its genesis. Meteor Crater in Arizona (USA) contains abundant evidence of shock metamorphism, includ- N ing shocked quartz, the high-pressure polymorphs coesite and stishovite, diaplectic SiO2 glass, and lechatelierite (fused SiO2). Here we report the presence of granular zircon, a new shocked-mineral discovery at Meteor Crater, that preserve critical orientation evidence of specific transformations that occurred during formation at extreme impact conditions. The zircon grains occur as aggregates of sub-micrometer neoblasts in highly shocked Coconino Sandstone (CS) comprised of lechatelierite. Electron backscatter diffraction shows that each MCC2 site grain consists of multiple domains, some with boundaries disoriented by 65° around <110>, (main shaft) a known {112} shock-twin orientation. Other domains have {001} in alignment with {110} of neighboring domains, consistent with the former presence of the high-pressure ZrSiO4 polymorph reidite. Additionally, nearly all zircon preserve ZrO + SiO , providing evidence 2 2 250 m of partial dissociation. The genesis of CS granular zircon started with detrital zircon that W111° 1.8’ N35° 1.3’ experienced shock twinning and reidite formation at pressures from 20 to 30 GPa, ultimately yielding a phase that retained crystallographic memory; this phase subsequently recrystal- B lized to systematically oriented zircon neoblasts, and in some areas partially dissociated to lechatelierite ZrO2. The lechatelierite matrix, experimentally constrained to form at >2000 °C, provided the ultrahigh-temperature environment for zircon dissociation (~1670 °C) and neoblast formation. The capacity of granular zircon to preserve a cumulative pressure-temperature granular record has not been recognized previously, and provides a new method for investigating his- tories of impact-related mineral transformations in the crust at conditions far beyond those zircon at which most rocks melt. GRANULAR ZIRCON structure (South Africa) (Moser, 1997; Cavosie Granular zircon is of interest to studies of et al., 2015a). shock metamorphism as it is considered to rep- vesicle resent the highest-shock-intensity morphotype SHOCK METAMORPHISM AT 100 µm of zircon, resulting from recrystallization of dia- METEOR CRATER plectic ZrSiO4 glass into newly grown domains Meteor Crater (also known as Barringer Cra- (neoblasts) at conditions in excess of 50 GPa ter) is a 1.2-km-diameter simple crater (Fig. 1A) Figure 1. A: Aerial photograph of Meteor Crater (Arizona, USA), showing main shaft (Wittmann et al., 2006). However, its formation that formed ~49 k.y. ago in northern Arizona where sample MCC2 was collected (image conditions remain poorly constrained as it has (Shoemaker, 1963; Kring, 2007). The impact source: NASA Earth Observatory, earthobser- not been produced by experiment. The Meteor punctured Permian and Triassic sedimentary vatory.nasa.gov). B: Backscattered electron Crater (Arizona, USA) granular zircon described rocks, however the CS has received the most image of sample MCC2, showing granular zircon in highly vesicular lechatelierite matrix. here consist of aggregates of sub-micrometer study for effects of shock metamorphism, as neoblasts and are similar to those found in most other lithologies at Meteor Crater are car- other impact environments, including distal bonates. The CS is a Permian eolianite that is ejecta (Bohor et al., 1993), impact melt (Kamo widespread across the Colorado Plateau; it is a CS have been recognized based on increasing et al., 1996), tektites (Deloule et al., 2001), lunar mature, white, fine-grained quartz arenite (~97 shock intensity of SiO2 phases (Kieffer, 1971; breccia (Grange et al., 2013), and meteorites wt% SiO2) that contains few minerals besides Kieffer et al., 1976). Class 1 includes fractured (Zhang et al., 2011). The examples cited above detrital quartz (Kring, 2007). Reports of shock detrital quartz grains (<1 GPa), classes 2 and are readily distinguished from grains consist- metamorphism of CS include the first discover- 3 feature collapsed pores and microcrystalline ing of neoblasts up to 100 µm across that have ies of both coesite (Chao et al., 1960) and stisho- coesite (5–13 GPa) as well as stishovite and only been reported from the Vredefort impact vite (Chao et al., 1962) in nature. Five classes of coesite (25–30 GPa), class 4 includes diaplectic GEOLOGY, September 2016; v. 44; no. 9; p. 703–706 | Data Repository item 2016228 | doi:10.1130/G38043.1 | Published online 19 July 2016 GEOLOGY© 2016 The Authors.| Volume Gold 44 |Open Number Access: 9 | www.gsapubs.orgThis paper is published under the terms of the CC-BY license. 703 Downloaded from http://pubs.geoscienceworld.org/gsa/geology/article-pdf/44/9/703/3549653/703.pdf by guest on 25 September 2021 ABgrain 13 C n=14,577 pts I III II lechatelierite 1 µm 1 µm 1 µm DETwin orientations: domains I & II n=9,610 pts Reidite orientations: domains II & III n=12,027 pts (001) {110} {112} (001) {110} {112}red {100}dark blue aligned aligned aligned 67° aligned 65°<110> aligned {112} (001) 90° apart {110} // to (001) {112}red // to {100}blue Figure 2. Granular zircon (grain 13) with twin and reidite orientations. A: Backscattered electron image showing granu- lar zircon in lechatelierite matrix. B: Band contrast image showing neoblasts. C: Orientation map color coded for Euler coordinate projection, showing three domains with distinct orientations (labeled I–III). D: Pole figures of domains I and II, showing twin relationships. E: Pole figures of domains II and III, showing reidite relationships. glass and vesicular lechatelierite, and class 5 contact with vesicular lechatelierite (Fig. 1B; 2013a, 2013b; Thomson et al., 2014; Cavosie consists entirely of lechatelierite (>30 GPa and Item DR1), 12 of which were analyzed by EBSD et al., 2015b). In contrast, neoblasts in domains >1000 °C). In addition to SiO2 phases, impact (Table DR2). The grains range from 5 to 37 II and III (Fig. 2C) have a different orientation melts also occur at Meteor Crater (e.g., Hörz et µm in length, and many appear to retain their relationship; poles to (001) for each domain al., 2015), however no other shocked minerals original shape; their small size and generally are aligned with a <110> direction in the other have been reported. round shape are consistent with detrital zircon. domain (Fig. 2E). This results in further align- On the polished surface, single grains expose ment of the second <110> for each domain, as SAMPLES AND METHODS between 111 and 2329 neoblasts each (Figs. 2A well as alignment of a {100} with {112} of the The sample of CS analyzed was collected and 2B), which range from 0.26 to 0.42 µm in other domain (Fig. 2E). The crystallographic from near the central mining shaft at Meteor mean diameter. relations described above for domains II and Crater (Fig. 1A). The rock originated from below III are the same orientation relations previously the current crater floor, and was excavated dur- Systematic Orientation Relationships described between zircon and the high-pressure ing early mining operations. Sample MCC2 is Neoblasts in all of the granular zircon grains ZrSiO4 polymorph reidite, where (001)zircon is white and powdery with no visible crystals and are crystallographically contiguous and may parallel to {110}reidite, there is alignment of a is highly porous and vesicular, similar to pumice have different orientations relative to each other, {110} between the two phases, and {112}zircon 1 (see the GSA Data Repository , Item DR1). In whereby domains comprise groups of adjacent is aligned with {100}reidite (Leroux et al., 1999; thin section, it is extremely vesicular (Fig. 1B), neoblasts in similar orientation (Item DR3). Cavosie et al., 2015b; Reddy et al., 2015). Given contains very few crystals, and is almost com- Grain 13 (Figs. 2A and 2B) is an example of that domains I and II in grain 13 define a {112} pletely isotropic silica glass, typical for shock a granular zircon composed of three distinctly twin relation, it follows that the orientation class 5 (Kieffer, 1971). Zircon grains were iden- different orientation domains that each contain relation between domains II and III was estab- tified in a thin section using backscattered elec- from 48 to 134 similarly oriented neoblasts on lished by transformation of domain III to reidite tron (BSE) imaging, and analyzed by electron the polished surface (Fig. 2C, labeled I–III). The (Figs. 2C–2E). While reidite was not identified backscatter diffraction (EBSD) using a 50 nm orientation of individual neoblasts within each (indexed) during analysis of Meteor Crater zir- step size (Item DR2; Table DR1). domain show minor, non-systematic disper- con, orientation relationships consistent with the sion relative to each other that do not overlap former presence of reidite were found in eight RESULTS in orientation with adjacent domains [compare of 12 grains analyzed by EBSD, and {112} twin locations of poles to (001) in Figs. 2D and 2E]. orientations were found in seven of the 12 grains Granular Zircon Domain boundaries are irregular at the neoblast (Item DR3; Table DR2). A total of 14 zircon grains, all granular, scale but are otherwise approximately straight.