Tai Tektite

"It present evidence of reidite and ZrO2 in granular zircon within Muong Nong-type tektites from Thailand, providing new insights.
Tektites are drops of glassy "impact" melt found in areas called strewn fields. The Australasian field spans 1000s of km across the Indian, Southern, and western equatorial Pacific oceans, with onland finds from Thailand to Antarctica.
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. 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. 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.

Samples: Granular zircon grains in MN-type tektites from Thailand, previously analyzed for U-Pb age by secondary ion mass spectrometry (SIMS), were characterized using backscattered electron (BSE), cathodoluminescence (CL) imaging, and elec-tron backscatter diffraction (EBSD).
Results: Each zircon is polycrystalline with a ‘granular’ texture, with mean neoblast diameters of 1.1 μm. Some neoblasts are concentrically zoned in CL. Inclusions of ZrO2 range up to ~1 μm across; most are located near neoblasts edges, rather than in cores. No other phases were observed. Neoblasts in each grain are systematically aligned in three crystallographic orientations. Each grain comprises three distinct orientation clusters that are mutually perpendicular, with coincidence among (001) and {110} poles. High-angle misorientation axes (85-95°) show that neighbor-pair pixels in EBSD maps are systematically clustered, and align with poles to (001) and {110} of neoblasts. ZrO2 inclu-sions index as zircon in the same orientation as the surrounding neoblast. The inclusions are likely poorly ordered, and appear electron transparent."

My Conclusion:
 The evidence of former zircon-reidites in Tai-tektites is further evidence of the origin of the melts with contact to the supracrustal basement, which was influenced by high pressure. Since all glassy tektite melts worldwide have a proven composition of volcanic rocks, an impact origin is excluded. So far it has never been proven that tektites have a composition of a target rock. Also the general consensus that tektites represent completely melted supracrustal material is not prove.


"Electron backscatter diffraction (EBSD) data for granular zircon grains in Australasian
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 separations
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."

Source: Aaron J. Cavosie et al.



Reidite and ZrO2 in MUONG NONG-TYPE Australian Tektites and the significance of granular zircon in siliceous melt
A. J. Cavosie -- Lunar and Planetary Science XLVIII (2017) 1806.pdf

New clues from Earth’s most elusive impact crater: Evidence of Reidite in Australasian Tektites from Thailand
A. J. Cavosie et al. -- The Geological Society of America

New clues from Earth’s most elusive impact crater: Evidence of Reidite in Australasian Tektites from Thailand
A. J. Cavosie et al. -- GSA Data Repository item 2018049