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NewcluesfromEarth'smostelusiveimpactcrater:EvidenceofreiditeinAustralasiantektitesfromThailand

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DOI:10.1130/G39711.1

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GEOLOGY | Volume 46 | Number 3 | www.gsapubs.org 203

New clues from Earth’s most elusive impact crater: Evidence of reidite in Australasian tektites from ThailandAaron J. Cavosie1, Nicholas E. Timms1, Timmons M. Erickson2, and Christian Koeberl3,4

1The Institute for Geoscience Research (TIGeR), Department of Applied Geology, Curtin University, Perth, WA 6102, Australia2Lunar and Planetary Institute, Universities Space Research Association, Houston, Texas 77058, USA3Natural History Museum, 1010 Vienna, Austria4Department of Lithospheric Research, University of Vienna, 1090 Vienna, Austria

ABSTRACTAustralasian tektites are enigmatic drops of siliceous impact melt found in an ~8000 ×

~13,000 km strewn field over Southeast Asia and Australia, including sites in both the Indian and Pacific oceans. These tektites formed only 790,000 yr ago from an impact crater estimated to be 40–100 km in diameter; yet remarkably, the young and presumably large structure remains undiscovered. Here we report new evidence of a rare high-pressure phase in Austral-asian tektites that further constrains the location of the source crater. The former presence of reidite, a high-pressure polymorph of zircon, was detected in granular zircon grains within Muong Nong–type tektites from Thailand. The zircon grains are surrounded by tektite glass and are composed of micrometer-sized neoblasts that contain inclusions of ZrO2. Each grain consists of neoblasts in three distinct crystallographic orientations as measured by electron backscatter diffraction, where all [001] directions are orthogonal and aligned with one <110> direction from the other two orientations. The systematic orientation relationships among zircon neoblasts are a hallmark of the high-pressure polymorphic transformation to reidite and subsequent reversion to zircon. The preserved microstructures and dissociation of zircon to ZrO2 and SiO2 require a pressure >30 GPa and a temperature >1673 °C, which represent the most extreme conditions thus far reported for Australasian Muong Nong–type tektites. The data presented here place further constraints on the distribution of high-pressure phases in Australasian tektites, including coesite and now reidite, to an area centered over Southeast Asia, which appears to be the most likely location of the source crater.

THE AUSTRALASIAN STREWN FIELDTektites are glassy impact ejecta that are

important for understanding impact processes, as they occur over widely dispersed “strewn fields”, and can preserve geochemical information on source craters (e.g., Taylor, 1973; Koeberl, 1986). The Australasian field is the largest of at least four known fields (Fig. 1), yet it remains the only one without an identified source crater (Koeberl, 1994). The impact that formed Australasian tek-tites is conspicuously young relative to those that formed other strewn fields, occurring only ~790 k.y. ago (Schwarz et al., 2016), and it is estimated to have formed a large crater, ranging in mod-eled diameter from 40 km (Glass and Koeberl, 2006) to >100 km (Lee and Wei, 2000; Prasad et al., 2007). However, the crater has thus far eluded discovery; proposed areas include sites in Antarctica (Schmidt, 1962), Siberia (Glass, 1979), China (Mizera et al., 2016), and Southeast Asia (e.g., Koeberl, 1992; Glass and Simonson, 2013). Here we use electron backscatter diffrac-tion (EBSD) mapping of granular zircon in tek-tites from Thailand to reveal the former presence of reidite. Reidite is a high-pressure polymorph of ZrSiO4 that has not previously been reported in tektites, and documenting its former presence

in Australasian tektites from Thailand supports a location for the source crater in Southeast Asia.

MUONG NONG–TYPE TEKTITESMuong Nong–type tektites (MN-type, or lay-

ered tektites) are one of three types of tektites (see review by Koeberl, 1986). They occur in multiple strewn fields; however, most MN-type tektites are from the Australasian field. MN-type tektites are high-silica glass (~80 wt% SiO2), and distinguished from other tektites by having a layered structure, a high abundance of vesicles, higher volatile content, including H2O, and a variety of relict grains and clasts (Koeberl, 1992). Phases reported in Austral-asian MN-type tektites and microtektites include lechatelierite (shock-melted, vesicular SiO2) and the high-pressure SiO2 polymorphs coesite and stishovite (Walter, 1965; Glass and Wu, 1993). Quartz, zircon, chromite, rutile, and monazite,

GEOLOGY, March 2018; v. 46; no. 3; p. 203–206 | GSA Data Repository item 2018049 | https://doi.org/10.1130/G39711.1 | Published online 20 December 2017

© 2017 Geological Society of America. For permission to copy, contact editing@geosociety.org.

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granular zircon/former reiditeshocked quartz and coesitetektite/microtektite locality

to Antarctica

Figure 1. Map showing Australasian strewn field (after Glass and Wu, 1993). Long-dashed line is historical extent of field; short-dashed line indicates extension to Antarctica (e.g., Glass and Simonson, 2013). Circle (heavy line) encloses localities of tektites and microtektites with shocked quartz and coesite (Glass and Wu, 1993). Star indicates approximate location of analyzed tektites.

204 www.gsapubs.org | Volume 46 | Number 3 | GEOLOGY

all with suspected shock damage as identified via asterism in X-ray diffraction patterns, are also present in Australasian MN-type tektites and microtektites (Glass, 1970; Glass and Bar-low, 1979; Glass and Wu, 1993). The mineral-ogy and geochemistry of tektites are consistent with their derivation from supracrustal material, rather than basement rocks (e.g., Taylor, 1973; Koeberl, 1986, 1994). The abundance of 10Be also requires Australasian tektites to be sourced from near-surface material, indicating that they were ejected early during crater formation (Ma et al., 2004; Koeberl et al., 2015). The large size of some MN-type tektites, up to 24 kg (Koeberl, 1992), further suggests that they are proximal to the source crater.

SAMPLES AND METHODSGranular zircon grains from three MN-type

tektites from Thailand were analyzed. The sam-ples consist of crushed tektite chips mounted in epoxy. The grains were previously analyzed by secondary ion mass spectrometry (SIMS) for U-Pb age; none yielded reliable ages because the volumes analyzed are dominated by com-mon Pb (Deloule et al., 2001) (see Item DR1 in GSA Data Repository1). In this study, the grains were characterized using backscattered electron (BSE) and cathodoluminescence (CL) imaging and EBSD mapping (Item DR1; Table DR1 in the Data Repository). Due to the rarity of the zircon grains, the mount was not re-ground to remove SIMS pits but was polished with col-loidal silica to remove surface damage. The grains were mapped using step sizes from 150 to 200 nm, and generally yielded high-quality EBSD patterns (Table DR1). Each EBSD map has an elliptical “hole” in the image due to the absence of diffraction patterns from SIMS pits.

RESULTS

Electron Imaging Results: BSE and CLThe zircon grains are equant, range from 35 to

55 µm in diameter, and are surrounded by glass (Fig. 2A). Each grain is polycrystalline, with a granular texture. Between 250 and 418 zircon crystallites, or neoblasts, with a mean diameter of 1.1 µm are exposed on the polished surface of each grain, and surrounded by intergranular glass (Figs. 2B and 2C). Crack-like partings filled with glass are present in all grains (Figs. 2B and 2C). The neoblasts are dark in CL images, whereas intergranular glass proximal to neoblasts is brighter than zircon, creating an “inverted” effect

1 GSA Data Repository item 2018049, Item DR1 (additional information on samples and methods), Item DR2 (additional SEM images), Item DR3 (addi-tional EBSD images and data), Table DR1 (EBSD ana-lytical conditions), and Table DR2 (neoblast charac-teristics), is available online at http://www.geosociety .org /datarepository /2018/ or on request from editing@geosociety.org.

(Fig. 2D; Item DR2). In contrast, tektite glass away from zircon neoblasts is not cathodolumi-nescent (Fig. 2D), suggesting that differences in glass composition are spatially associated with the zircon grains. Neoblasts in grain MN-j appear concentrically zoned, comprising bright cores and dark rims in CL images (Fig. 2D). Inclu-sions of ZrO2 in neoblasts, conspicuous in BSE images (Fig. 2C) and identified by energy disper-sive spectroscopy, range from tens of nanometers up to ~1 µm across. Within zircon neoblasts, the distribution of ZrO2 inclusions is not uniform, with most located near neoblast edges rather than in cores. No other phases were observed in the zircon neoblasts or the tektite glass.

EBSD MappingZircon neoblasts in each composite grain are

systematically aligned in three crystallographic orientations (Fig. 3). Local orientation domains

of similarly oriented neoblasts range in size from a single ~1 µm neoblast up to irregularly shaped aggregates of neoblasts ~10 µm across (Fig. 3A). Pole figures reveal that each grain comprises three distinct and non-overlapping orientation clusters that are mutually perpendic-ular, with coincidence among (001) and {110} poles (Fig. 3B). Within each orientation cluster, crystallographic poles are dispersed by up to

~40° (Fig. 3B). These relationships are illus-trated on misorientation axis plots, which show the position of misorientation axes for neighbor-pair pixels in the EBSD maps (Fig. 3C; Item DR3). Misorientation axes in the 85°–95° range are systematically clustered, and align with poles to (001) and {110} of the zircon neoblasts; misorientation axes in other angular ranges are not systematically oriented (Item DR3).

The orientation relationships described here for granular zircon are only known to result

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parting

Figure 2. Scanning electron microscopy images of granular zircon grains in Muong Nong–type (MN-type) tektites. A: Backscattered electron (BSE) images of polished chips containing zircon grains. Inset boxes show areas featured in B. B: BSE images of granular zircon grains. Dashed ellipses are secondary ion mass spectrometry (SIMS) pits. Inset boxes show areas featured in C and D. C: BSE images of zircon neoblasts with ZrO2 inclusions. White arrows in C and D point to the same locations in grains MN-d and MN-i. D: Cathodoluminescence images of same areas shown in C.

GEOLOGY | Volume 46 | Number 3 | www.gsapubs.org 205

from transformation to, and reversion from, the high-pressure polymorph reidite (Cavosie et al., 2016; Erickson et al., 2017; Timms et al., 2017). Transformation of zircon to reidite results in alignment of [001]zircon with <110> reidite (Leroux et al., 1999; Cavosie et al., 2015a; Erickson et al., 2017). Up to eight orientation variants of reidite in two approximately orthogo-nal orientation groups can form from a single zircon crystal (Erickson et al., 2017; Timms et al., 2017). Reversion of reidite to zircon fol-lows the reverse transformation relationship, but can occur via the same or symmetrically equiva-lent axes, resulting in up to three approximately

orthogonal orientations of zircon. The reversion from reidite produces additional systematic dis-persion of ~10° about each axis, which mani-fests on pole figures as highly dispersed orienta-tion domains (Fig. 3B) that are systematically misoriented (Fig. 3C).

The surrounding and intergranular glass did not produce EBSD patterns. Inclusions of ZrO2 within zircon neoblasts yielded EBSD patterns but they did not index as zirconia polymorphs (monoclinic, tetragonal, or cubic); instead, they index as zircon in the same orientation as the surrounding neoblast (Item DR3). The patterns from ZrO2 inclusions likely originated from

underlying zircon, and may indicate that the ZrO2 inclusions either are poorly crystalline or are otherwise poorly ordered, and thus appear electron transparent.

DISCUSSION

Implications of Reidite and ZrO2 in Australasian Tektites

Coesite is the highest-pressure phase pre-viously reported in Australasian tektites and microtektites (Glass and Wu, 1993), and forms during shock unloading in crystalline target rocks shocked to 30 and 60 GPa (Stöffler and Langenhorst, 1994). However, coesite can form at pressures <10 GPa in porous targets (Ferrière and Osinski, 2012), which is likely to be the case for the Australasian tektite source mate-rial given its near-surface origin. In contrast, reidite requires shock pressures of 30 GPa or higher, as has been demonstrated both in shock experiments (Kusaba et al., 1985; Leroux et al., 1999) and from conditions recorded by coexist-ing phases in natural samples (e.g., Wittmann et al., 2006). Detecting the former presence of reidite thus substantially increases the pressure estimates derived from unmelted phases in Aus-tralasian tektites.

Most occurrences of reidite are in non-granu-lar shocked zircon grains within rocks that have not experienced bulk fusion (e.g., Wittmann et al., 2006; Cavosie et al., 2015a; Erickson et al., 2017). In contrast, granular zircon is most often found in or associated with impact melt (e.g., El Goresy, 1965; Schmieder et al., 2015); however, it can also form in non-impact settings (e.g., Cavosie et al., 2015b). As shown here for tektites, orientation data can be used to distin-guish granular zircon formed after reidite, from granular zircon that did not pass through rei-dite stability, a result that has previously been demonstrated in impactites from Meteor Crater, Arizona, USA (Cavosie et al., 2016); the Acra-man impact structure, Australia (Timms et al., 2017); and the Ries impact structure, Germany (Erickson et al., 2017).

Our results also provide the first direct evi-dence for extreme high-temperature conditions recorded by unmelted phases. The ubiquitous presence of lechatelierite in Australasian MN-type tektites has been cited as evidence of high temperature, ranging from 1700 to 2000 °C (Walter, 1965; Glass and Barlow, 1979; Macris et al., 2014). The presence of ZrO2 resulting from zircon dissociation reported here requires that the tektite samples experienced tempera-tures in excess of 1673 °C (e.g., Timms et al., 2017). The distribution of ZrO2 inclusions along neoblast margins is consistent with formation of zircon neoblasts followed by partial dissocia-tion of the neoblasts to ZrO2 + SiO2. If correct, subsequent zircon growth likely occurred, as ZrO2 was not observed in contact with glass.

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Figure 3. Electron backscatter diffraction (EBSD) data for granular zircon grains in Austral-asian Muong Nong–type (MN-type) tektites. A: Maps showing crystallographic orientations in Euler coordinate space. Elliptical areas without data are secondary ion mass spectrometry (SIMS) pits. B: Pole figures showing data from maps in A for (001) and {110}. Angular separa-tions of 90° are shown for {110}. C: Plots showing high-angle (85° to 95°) misorientation axes. Misorientation axes coincide with poles for (001) and {110}. Stereonets are equal area, lower hemisphere projections in sample x-y-z reference frame.

206 www.gsapubs.org | Volume 46 | Number 3 | GEOLOGY

The absence of SiO2 phases indicates that dis-sociated silica dissolved into the surrounding high-Si glass, which may in part be responsible for the variable CL response (Fig. 2D). The sus-pected disordered character of the ZrO2 inclu-sions, as well as their small volume, may explain why baddeleyite was not previously detected in X-ray diffraction studies of Australasian tektites (Glass, 1970; Glass and Barlow, 1979; Deloule et al., 2001).

Constraints on the Source of Australasian Tektites

The question of how such a young (ca. 790 ka) and potentially large (40–100 km) impact crater can evade discovery remains a matter of debate. As described above, published petrologi-cal and geochemical data constrain the source of MN-type tektites to near-surface, high-silica, zircon-bearing, supracrustal material. Evidence presented here for the former presence of reidite firmly establishes that these tektites originated within the 30 GPa isobar near the site of impact. The observation that all known occurrences of shocked quartz and coesite in Australasian microtektites are found in a circular area centered on Southeast Asia (Glass and Wu, 1993), com-bined with our discovery that tektites from Thai-land contain the highest-pressure phase thus far reported, as well as the recognition that MN-type tektites have large masses indicative of proximity to source, strongly suggests that the Australasian tektite source crater is located in Southeast Asia, rather than in other previously proposed sites.

ACKNOWLEDGMENTSSupport was provided by the NASA Astrobiology program (grant NNAI3AA94A), a Curtin Research Fellowship, and the Microscopy and Microanalysis Facility at Curtin University, Australia. C. Macris, M. Wielicki, and an anonymous person provided helpful reviews. M. Schmieder and B. Glass commented on an earlier version. D. Brown is thanked for editorial handling.

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Manuscript received 13 September 2017 Revised manuscript received 22 November 2017 Manuscript accepted 27 November 2017

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