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Pantasma melt glass
"Pantasma is a 14.5 km diameter circular structure
located in Oligocene felsic volcanic rocks in Nicaragua. Observations include
outward‐dipping volcanic flows, the presence of former melt‐bearing polymict
breccia, melt glass (with lechatelierite and coesite), and also a possible
ejecta layer containing Paleozoic rocks which originated from hundreds of
meters below the surface.
The Pantasma structure has been proposed to have an impact origin based on
the occurrence of decomposed zircon in melt glass found in the Pantasma
River within the structure.
Granular zircon grains in melt glass from the Pantasma structure consist of
~1 μm-sized neoblasts in multiple domains that are systematically oriented
such that all are systematically oriented such that all (001) are orthogonal,
and coincide with a {110} direction from another domain. The neoblast orientations
are uniquely produced by transformation of zircon to the high-pressure polymorph
reidite and its subsequent reversion to neoblastic zircon under high-temperature
conditions (>1673°C), as evidenced by their occurrence with ZrO2.
The Pantasma zircon is a 10 x 15 μm grain with a granular texture consisting
of ~1 μm-sized neoblasts embedded in glass. The zircon contains discrete irregular
partings filled by glass. Inclusions of ZrO2 in zircon neoblasts occur throughout
the grain, although none indexed as zirconia polymorphs during electron backscatter
diffraction (EBSD) analysis. Six distinct neoblast orientation clusters are
present."
| Left: This rare black glass
found inside the Pantasma structure, along the river banks. SEM observations
reveal the presence of schlieren and abundant relicts of partially to
completely molten grains of detrital minerals, including Fe-Ti oxydes
and quartz. Water content, determined on two samples, is 240-300 ppm,
major and trace elements similar to the Matagalpa volcanics formation
of Oligocene. |
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This melt glass is to compare with a composition
of a dacite volcanic rock
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Former reidite in granular neoblastic (FRIGN) zircon in Pantasma (Nicaragua)
glass. A: Backscattered electron (BSE) image showing a granular zircon
with ZrO 2 inclusions. B: Inverse pole figure map (IPFx) showing neoblast
orientations. Former reidite is indicated by 90°/<110> boundaries. C:
Pole figures showing data from B that reveal six domains (labeled) that
each shown minimal dispersion. The 90° misorientations for {110} pole
clusters among orientations 1,2 and 4,5 are shown; each contains a {110}
orientation that coincides with a (001) pole cluster. D: Plot showing
high-angle (85° to 95°) misorientation axes; clusters coincide with poles
for {110}. Stereonets are equal-area, lower-hemisphere projections in
sample x-y-z reference frame.
Source: Aaron J. Cavosie et al.
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My Conclusion:
Apart of whether the melt glass with the recrystallized zircon in it, found
in the Pantasma River, is actually related to the structure of the same name,
the proposed former Reidites in the glassy melt are not the result of an impact.
They come directly from basement-rocks and are volcanically re-crystallized
to zircon.
References:
First investigations on the PANTASMA structure (Nicaragua)
P. Rochette et al. -- 79th Annual Meeting of the Meteoritical Society (2016),
6128.pdf
Former REIDITE in granular neoblastitic ZIRCON (FRIGN zircon) from
the Luizi impact structure and proposed PANRASMA structure.
A. J. Cavosie et al. -- 49th Lunar and Planetary Science Conference 2018 (LPI
Contrib. No. 2083), 1816.pdf
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