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New Clues from Earth's Most Elusive Impact Crater: Evidence of Reidite in Australasian Tektites from Thailand

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New Clues from Earth's Most Elusive Impact Crater: Evidence of Reidite in Australasian Tektites from Thailand See discussions, stats, and author profiles for this publication at: https://www.researchgate.net/publication/321956231 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 CITATIONS READS 0 64 4 authors, including: Aaron J. Cavosie Timmons Erickson Curtin University Curtin University 100 PUBLICATIONS 2,285 CITATIONS 27 PUBLICATIONS 159 CITATIONS SEE PROFILE SEE PROFILE All content following this page was uploaded by Aaron J. Cavosie on 16 March 2018. 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.
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