Lunar and Planetary Science XLVIII (2017) 1806.pdf
REIDITE AND ZrO2 IN MUONG NONG-TYPE AUSTRALASIAN TEKTITES AND THE SIGNIFICANCE OF GRANULAR ZIRCON IN SILICEOUS IMPACT MELT A. J. Cavosie1,2 , N.E. Timms 1, T.M. Erickson1, and C. Koeberl3,4 1TIGeR (The Institute of Geological Research), Department of Applied Geology, Curtin University, Perth, Western Australia, Australia, 2NASA Astrobiology Institute, Department of Geoscience, University of Wis- consin-Madison, Madison WI, USA, 3Natural History Museum, Burgring 7, 1010 Vienna, Austria, 4Department of Lithospheric Research, University of Vienna, Althanstrasse 14, 1090 Vienna, Austria
Introduction: The origin of Australasian tektites, Results: Each zircon is polycrystalline with a enigmatic drops of siliceous impact melt found over ‘granular’ texture, with mean neoblast diameters of an area of ~8,000 x ~13,000 km centered on South- 1.1 μm. Some neoblasts are concentrically zoned in
east Asia, has long been debated. The impact event CL. Inclusions of ZrO2 range up to ~1 μm across; occurred only 0.78 m.y. ago, yet the source crater most are located near neoblasts edges, rather than remains undiscovered. While there is general con- in cores. No other phases were observed. Neoblasts sensus that Australasian tektites represent melted in each grain are systematically aligned in three supracrustal material, their formation conditions and crystallographic orientations (Fig. 1). Each grain provenance remain poorly constrained. Here we comprises three distinct orientation clusters that are
present evidence of reidite and ZrO2 in granular zir- mutually perpendicular, with coincidence among con within Muong Nong-type tektites from Thai- (001) and {110} poles. High-angle misorientation land, providing new insights into their genesis. axes (85-95°) show that neighbor-pair pixels in EBSD Background: Tektites are drops of glassy im- maps are systematically clustered, and align with
pact melt found in areas called strewn fields. The poles to (001) and {110} of neoblasts. ZrO2 inclu- Australasian field spans 1000s of km across the In- sions index as zircon in the same orientation as the dian, Southern, and western equatorial Pacific surrounding neoblast. The inclusions are likely oceans, with on-land finds from Thailand to Antarc- poorly ordered, and appear electron transparent. tica. The mineralogy and geochemistry of tektites are consistent with derivation from a supracrustal source [2]; the presence of 10Be requires the source to be near-surface material [3,4]. Muong Nong-type (MN-type) tektites: MN-type
tektites are high-silica glass (~80 wt. % SiO2), have a layered structure, a high abundance of vesicles,
high volatile and H2O contents, and a variety of relict minerals [2]. Phases in Australasian MN-type tektites include lechatelierite, coesite, and stishovite; quartz, zircon, and other minerals with suspected shock damage have been identified via X-ray asterism [5]. The absence of baddeleyite, combined with geochemical and microstructural data, has been cited to infer that Australasian MN- type tektites represent lower temperature melts compared to other tektites [2]. Samples: Granular zircon grains in MN-type tek- tites from Thailand, previously analyzed for U-Pb age by secondary ion mass spectrometry (SIMS) Figure 1. Map of granular zircon (top) showing [6], were characterized using backscattered electron variations in orientation using an inverse pole figure (BSE), cathodoluminescence (CL) imaging, and elec- color scheme. Pole figures (bottom) show the zircon tron backscatter diffraction (EBSD). consists of three orthogonal domains with coinci- dence between (001) and {110} poles. Lunar and Planetary Science XLVIII (2017) 1806.pdf
Pressure constraints for MN-type tektites: The ~40 km) [8,15]. Target rocks at both sites include orientation relationships described here for granular siliceous zircon-bearing supracrustal rocks, such as zircon are only known to result from transformation Coconino sandstone at Meteor Crater [7] and to, and reversion from, the high-pressure polymorph Yardea dacite at Acraman [15], which locally experi- reidite [7,8,9]. Transformation of zircon to reidite enced total fusion during impact. Physical condi-
results in alignment of [001]zircon with <110>reidite, with tions of impact melt as recorded in granular zircon at a systematic dispersion of 10° about the axes [7-12]. both sites, i.e., P> 30 GPa, T>1673 °C [7-8], are es- Reversion of reidite to zircon follows the reverse sentially identical to those derived from zircon in transformation relationship, resulting in up to three MN-type tektites here. At the deeply-eroded Acra- orthogonal orientations of zircon [8,9]. The rever- man impact structure, the crater environment of the sion from reidite produces additional systematic impact melt is uncertain; however its derivation from dispersion of ~10° about each axis, which manifests dacite [15] indicates an origin in siliceous supra- on pole figures as highly dispersed orientation do- crustal rock. Likewise, at Meteor Crater, shock- mains with systematic misorientations. The former melted Coconino sandstone was excavated from presence of reidite requires these tektites to have only ~80 m [17], and thus is also a siliceous supra- originated from within the 30 GPa isobar near crustal rock. Our results establish unambiguously ‘ground zero’ [10,13]. that unique microstructural features observed in Temperature constraints for MN-type tektites: MN-type tektites occur in impact melt derived from The presence of lechatelierite in MN-type tektites high-silica source rocks, and, therefore, support requires high temperatures [5], and our results pro- previous hypotheses for a siliceous supracrustal vide additional evidence of high-temperature condi- source for MN-type tektites [1-5]. Given that sili- tions. Most reported occurrences of reidite in other ceous igneous rocks and sediments are rare on the impact environments consist of lamellae in shocked Moon and Mars [18-19], tektites may be an impact zircon within rocks that have not melted [9,11,12,14]. product unique to Earth, as it is the only body in the In contrast, tektites are quenched impact melt, and Solar System with an evolved, silica-rich crust. contain shocked zircon grains that recrystallized to References: [1] Glass, B. P. and Simonson, B. neoblasts after reidite. Granular zircon after reidite (2013) Distal Impact Ejecta Layers, Springer- appears to be a product of super-heated impact melt Verlag, Berlin, 716p. [2] Koeberl, C. (1992) [7-8,15], and is readily distinguished by orientation Geochim. Cosmochim. Acta, 56, 1033-1064. [3] Ma, data from granular zircon in non-melted shocked P. et al. (2004) Geochim. Cosmochim. Acta, 68, 3883-
bedrock [16]. The presence of ZrO2 is additional 3896. [4] Koeberl, C. et al. (2015) Meteoritics & evidence that the tektite samples experienced tem- Planet. Sci., 50, A169. [5] Glass, B. P. and Wu, J. peratures in excess of 1673 °C, at which zircon dis- (1993) Geology, 21, 435-438. [6] Deloule E. et al.
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