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

Australasian 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 Australasian 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 FIELD Tektites 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 tektites 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 modeled 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 diffraction (EBSD) mapping of granular zircon in tektites 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 TEKTITES Muong Nong–type tektites (MN-type, or layered 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. MNtype 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 Australasian 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 eological Society of A erica. For permission to copy, contact editing@geosociety.org. 10° 0° 10°