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New clues from Earth's most elusive impact crater: Evidence of reidite in Australasian tektites from Thailand

Article in Geology · December 2017 DOI: 10.1130/G39711.1

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The user has requested enhancement of the downloaded file. New clues from Earth’s most elusive impact crater: Evidence of reidite in Australasian tektites from Thailand

Aaron 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, Australia 2Lunar and Planetary Institute, Universities Space Research Association, Houston, Texas 77058, USA 3Natural History Museum, 1010 Vienna, Austria 4Department of Lithospheric Research, University of Vienna, 1090 Vienna, Austria

ABSTRACT in Australasian tektites from Thailand supports a Australasian tektites are enigmatic drops of siliceous impact melt found in an ~8000 × location for the source crater in Southeast Asia. ~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 MUONG NONG–TYPE TEKTITES to be 40–100 km in diameter; yet remarkably, the young and presumably large structure Muong Nong–type tektites (MN-type, or lay- remains undiscovered. Here we report new evidence of a rare high-pressure phase in Austral- ered tektites) are one of three types of tektites asian tektites that further constrains the location of the source crater. The former presence of (see review by Koeberl, 1986). They occur in reidite, a high-pressure polymorph of zircon, was detected in granular zircon grains within multiple strewn fields; however, most MN-type Muong Nong–type tektites from Thailand. The zircon grains are surrounded by tektite glass tektites are from the Australasian field. MN- and are composed of micrometer-sized neoblasts that contain inclusions of ZrO2. Each grain type tektites are high-silica glass (~80 wt% consists of neoblasts in three distinct crystallographic orientations as measured by electron SiO2), and distinguished from other tektites by backscatter diffraction, where all [001] directions are orthogonal and aligned with one <110> having a layered structure, a high abundance direction from the other two orientations. The systematic orientation relationships among of vesicles, higher volatile content, including zircon neoblasts are a hallmark of the high-pressure polymorphic transformation to reidite H2O, and a variety of relict grains and clasts and subsequent reversion to zircon. The preserved microstructures and dissociation of zircon (Koeberl, 1992). Phases reported in Austral- to ZrO2 and SiO2 require a pressure >30 GPa and a temperature >1673 °C, which represent asian MN-type tektites and microtektites include the most extreme conditions thus far reported for Australasian Muong Nong–type tektites. lechatelierite (shock-melted, vesicular SiO2) and

The data presented here place further constraints on the distribution of high-pressure phases the high-pressure SiO2 polymorphs coesite and in Australasian tektites, including coesite and now reidite, to an area centered over Southeast stishovite (Walter, 1965; Glass and Wu, 1993). Asia, which appears to be the most likely location of the source crater. Quartz, zircon, chromite, rutile, and monazite,

THE AUSTRALASIAN STREWN FIELD 30° Tektites are glassy impact ejecta that are important for understanding impact processes, as 20° they occur over widely dispersed “strewn fields”, and can preserve geochemical information on source craters (e.g., Taylor, 1973; Koeberl, 1986). 10° The Australasian field is the largest of at least four known fields (Fig. 1), yet it remains the only 0° one without an identified source crater (Koeberl, 1994). The impact that formed Australasian tek- 10° tites is conspicuously young relative to those that formed other strewn fields, occurring only ~790 20° k.y. ago (Schwarz et al., 2016), and it is estimated to have formed a large crater, ranging in mod- t eled diameter from 40 km (Glass and Koeberl, 30° o

2006) to >100 km (Lee and Wei, 2000; Prasad A n et al., 2007). However, the crater has thus far t 40° a r eluded discovery; proposed areas include sites granular zircon/former reidite ct in Antarctica (Schmidt, 1962), Siberia (Glass, i c shocked quartz and coesite a 1979), China (Mizera et al., 2016), and Southeast Asia (e.g., Koeberl, 1992; Glass and Simonson, 50° tektite/microtektite locality 2013). Here we use electron backscatter diffrac- 60° 90° 120° 150° tion (EBSD) mapping of granular zircon in tek- tites from Thailand to reveal the former presence 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 of reidite. Reidite is a high-pressure polymorph and Simonson, 2013). Circle (heavy line) encloses localities of tektites and microtektites with of ZrSiO4 that has not previously been reported shocked quartz and coesite (Glass and Wu, 1993). Star indicates approximate location of in tektites, and documenting its former presence analyzed tektites.

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 ©GEOLOGY 2017 Geological | Volume Society 46 | ofNumber America. 3 For| www.gsapubs.org permission to copy, contact [email protected]. 203 all with suspected shock damage as identified A MN-d MN-i MN-j via asterism in X-ray diffraction patterns, are 20 µm glass also present in Australasian MN-type tektites glass and microtektites (Glass, 1970; Glass and Bar- 50 µm low, 1979; Glass and Wu, 1993). The mineral- glass ogy and geochemistry of tektites are consistent 20 µm with their derivation from supracrustal material, rather than basement rocks (e.g., Taylor, 1973; B SIMS pit SIMS pit SIMS pit 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. 10 µm 10 µm 10 µm parting SAMPLES AND METHODS C Granular zircon grains from three MN-type parting tektites from Thailand were analyzed. The sam- parting ples consist of crushed tektite chips mounted in ZrO epoxy. The grains were previously analyzed by 2 ZrO2 secondary ion mass spectrometry (SIMS) for U-Pb age; none yielded reliable ages because the volumes analyzed are dominated by com- 1 µm 5 µm 1 µm mon Pb (Deloule et al., 2001) (see Item DR1 in ZrO2 GSA Data Repository1). In this study, the grains were characterized using backscattered electron D (BSE) and cathodoluminescence (CL) imaging and EBSD mapping (Item DR1; Table DR1 in bright dark the Data Repository). Due to the rarity of the core rim 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 1 µm 5 µm ZrO 1 µm to 200 nm, and generally yielded high-quality 2 EBSD patterns (Table DR1). Each EBSD map Figure 2. Scanning electron microscopy images of granular zircon grains in Muong Nong– has an elliptical “hole” in the image due to the type (MN-type) tektites. A: Backscattered electron (BSE) images of polished chips containing absence of diffraction patterns from SIMS pits. 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 RESULTS 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. Electron Imaging Results: BSE and CL The zircon grains are equant, range from 35 to (Fig. 2D; Item DR2). In contrast, tektite glass of similarly oriented neoblasts range in size 55 µm in diameter, and are surrounded by glass away from zircon neoblasts is not cathodolumi- from a single ~1 µm neoblast up to irregularly (Fig. 2A). Each grain is polycrystalline, with a nescent (Fig. 2D), suggesting that differences in shaped aggregates of neoblasts ~10 µm across granular texture. Between 250 and 418 zircon glass composition are spatially associated with (Fig. 3A). Pole figures reveal that each grain crystallites, or neoblasts, with a mean diameter the zircon grains. Neoblasts in grain MN-j appear comprises three distinct and non-overlapping of 1.1 µm are exposed on the polished surface of concentrically zoned, comprising bright cores orientation clusters that are mutually perpendic- each grain, and surrounded by intergranular glass and dark rims in CL images (Fig. 2D). Inclu- ular, with coincidence among (001) and {110}

(Figs. 2B and 2C). Crack-like partings filled with sions of ZrO2 in neoblasts, conspicuous in BSE poles (Fig. 3B). Within each orientation cluster, glass are present in all grains (Figs. 2B and 2C). images (Fig. 2C) and identified by energy disper- crystallographic poles are dispersed by up to The neoblasts are dark in CL images, whereas sive spectroscopy, range from tens of nanometers ~40° (Fig. 3B). These relationships are illus- intergranular glass proximal to neoblasts is up to ~1 µm across. Within zircon neoblasts, the trated on misorientation axis plots, which show brighter than zircon, creating an “inverted” effect distribution of ZrO2 inclusions is not uniform, the position of misorientation axes for neighbor- with most located near neoblast edges rather than pair pixels in the EBSD maps (Fig. 3C; Item 1 GSA Data Repository item 2018049, Item DR1 in cores. No other phases were observed in the DR3). Misorientation axes in the 85°–95° range (additional information on samples and methods), zircon neoblasts or the tektite glass. are systematically clustered, and align with Item DR2 (additional SEM images), Item DR3 (addi- poles to (001) and {110} of the zircon neoblasts; tional EBSD images and data), Table DR1 (EBSD ana- EBSD Mapping misorientation axes in other angular ranges are lytical conditions), and Table DR2 (neoblast charac- teristics), is available online at http://www.geosociety​ Zircon neoblasts in each composite grain are not systematically oriented (Item DR3). .org/datarepository​ ​/2018/ or on request from editing@ systematically aligned in three crystallographic The orientation relationships described here geosociety.org. orientations (Fig. 3). Local orientation domains for granular zircon are only known to result

204 www.gsapubs.org | Volume 46 | Number 3 | GEOLOGY A MN-d MN-i MN-j underlying zircon, and may indicate that the ZrO2 inclusions either are poorly crystalline or are otherwise poorly ordered, and thus appear electron transparent.

DISCUSSION

step: step: step: Implications of Reidite and ZrO in 150 nm 200 nm Y 150 nm 2 10 µm 10 µm 10 µm Australasian Tektites X Coesite is the highest-pressure phase pre- Euler orientations: E1 E2 E3 0° 180° 0° 360° 0° 180° viously reported in Australasian tektites and B. microtektites (Glass and Wu, 1993), and forms Pole figures Y (001) (001) (001) during shock unloading in crystalline target X 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 n=14,553 n=21,264 n=21,058 higher, as has been demonstrated both in shock experiments (Kusaba et al., 1985; Leroux et al., {110} {110} {110} 1999) and from conditions recorded by coexist- 90˚ 90˚ ing phases in natural samples (e.g., Wittmann 90˚ 90˚ et al., 2006). Detecting the former presence of 90˚ reidite thus substantially increases the pressure 90˚ estimates derived from unmelted phases in Aus- 90˚ tralasian tektites. 90˚ Most occurrences of reidite are in non-granu- 90˚ lar shocked zircon grains within rocks that have not experienced bulk fusion (e.g., Wittmann et C. Misorientation axis plots (85°-95°) al., 2006; Cavosie et al., 2015a; Erickson et Y al., 2017). In contrast, granular zircon is most n=203 n=1764 X n=1656 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, Figure 3. Electron backscatter diffraction (EBSD) data for granular zircon grains in Austral- Arizona, USA (Cavosie et al., 2016); the Acra- asian Muong Nong–type (MN-type) tektites. A: Maps showing crystallographic orientations in man impact structure, Australia (Timms et al., Euler coordinate space. Elliptical areas without data are secondary ion mass spectrometry 2017); and the Ries impact structure, Germany (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. (Erickson et al., 2017). Misorientation axes coincide with poles for (001) and {110}. Stereonets are equal area, lower Our results also provide the first direct evi- hemisphere projections in sample x-y-z reference frame. dence for extreme high-temperature conditions recorded by unmelted phases. The ubiquitous from transformation to, and reversion from, orthogonal orientations of zircon. The reversion presence of lechatelierite in Australasian MN- the high-pressure polymorph reidite (Cavosie from reidite produces additional systematic dis- type tektites has been cited as evidence of high et al., 2016; Erickson et al., 2017; Timms et persion of ~10° about each axis, which mani- temperature, ranging from 1700 to 2000 °C al., 2017). Transformation of zircon to reidite fests on pole figures as highly dispersed orienta- (Walter, 1965; Glass and Barlow, 1979; Macris results in alignment of [001]zircon with <110> reidite tion domains (Fig. 3B) that are systematically et al., 2014). The presence of ZrO2 resulting (Leroux et al., 1999; Cavosie et al., 2015a; misoriented (Fig. 3C). from zircon dissociation reported here requires Erickson et al., 2017). Up to eight orientation The surrounding and intergranular glass did that the tektite samples experienced tempera- variants of reidite in two approximately orthogo- not produce EBSD patterns. Inclusions of ZrO2 tures in excess of 1673 °C (e.g., Timms et al., nal orientation groups can form from a single within zircon neoblasts yielded EBSD patterns 2017). The distribution of ZrO2 inclusions along zircon crystal (Erickson et al., 2017; Timms et but they did not index as zirconia polymorphs neoblast margins is consistent with formation of al., 2017). Reversion of reidite to zircon fol- (monoclinic, tetragonal, or cubic); instead, they zircon neoblasts followed by partial dissocia- lows the reverse transformation relationship, but index as zircon in the same orientation as the tion of the neoblasts to ZrO2 + SiO2. If correct, can occur via the same or symmetrically equiva- surrounding neoblast (Item DR3). The patterns subsequent zircon growth likely occurred, as lent axes, resulting in up to three approximately from ZrO2 inclusions likely originated from ZrO2 was not observed in contact with glass.

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