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Evidence of meteorite impact-induced thermal shock in quartz

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Unknown spectacular microfracturing of quartz grains in a polymictic impact breccia from the recently established impact strewn field in the Czech Republic is interpreted as caused by thermal shock during impact and is seen in connection with recently postulated thermal shock during the formation of silica ballen structures.
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361
X. Минералогия астроблем и метеоритов
Evidence of meteorite impact-induced thermal shock in quartz
K. Ernstson
University of Würzburg, 97074 Würzburg, Germany; kernstson@ernstson.de
Introduction
Thermal shock is a mechanical load caused by a
rapid change of temperature (heating or cooling) in
a material that may fracture if the stress exceeds its
tensile strength. While thermal shock plays an im
portant role in material sciences and in industrial
production, it has rarely been considered in impact
cratering. A priori, this is easily understood, because
hypervelocity impact shock producing the well
known rock and mineral deformations (shock meta
morphism) precedes a thermal shock on pressure
release. However, if the shock pressures are enough
to melt or vaporize target rocks (beginning at a few
tens of GPa), then on rapid contact of melt or vapor
with less or no shocked rocks in the immediately fol
lowing excavation stage, extreme thermal shock can
be expected to exert mechanical load and a kind of
secondary shock fracturing. Here, I report on ob
servations in quartz of shocked impactites with evi
dence of strong thermal shock fracturing and of bal
len quartz formation by thermal shock, the latter
originally suggested in a recent paper [1].
Quartz microfracturing
Very unusual microfracture patterns in quartz
have been observed to occur in meteorite impact
rocks in a manner so far not described in the lite
rature to our knowledge (Fig. 1). The polymictic
impact breccia in Fig. 1A comes from the recent
ly proposed impact event in the Czech Republic [2,
3]. Individual small quartz grains are distri buted
throughout the matrix in most cases not contacting
each other (Fig. 1, B) but revealing extreme mosa
iclike microfracturing Fig. 1, C—F). In many of the
grains this microfracturing is more or less sharply
limited to a concentric rim zone encasing a large
ly untouched core with a few subplanar fractures
in some cases. Multiple sets of true planar features
(PFs) known as an impact shock effect also oc
cur (Fig. 1, C). Similarly unusual quartz fracturing
has been reported also from the NalbachSaarland
impact event [4] where abundant isolated quartz
grains in an impact melt glass are observed, as is the
case for the recently described granitic impact melt
rock sheet in SE Bavaria [5]. In all three cases a tec
Fig. 1. A: Polymictic impact breccia from the Czech impact site [2, 3]. B: Closeup of the breccia showing intensely fractured
quartz grains that are floating in the breccia matrix without contacts. C: Selected quartz grains from the breccia matrix
with multiple sets of planar fractures (PFs, densely developed cleavage). DF: Quartz grains from the breccia matrix
exhibiting the spectacularly fractured rim regions around a widely untouched core with some planar and subplanar
fractures. BF: Thin section photomicrographs, crossed polarizers
Modern Problems of Theoretical, Experimental, and Applied Mineralogy (Yushkin Readings — 2020) -
Proceedings of Russian conference with international participation - Syktyvkar, Komi Republic, Russia
362
Юшкинские чтения — 2020
tonic origin and a direct shockwave microfractur
ing can reasonably be excluded.
Ballen structures
Ballen structures in silica form a characteristic
texture in quartz that in general is considered a re
sult from various stages of phase transformation
and recrystallization of amorphous silica like e. g.,
diaplectic glass and hence are regarded as shock
indicator (e. g., [6]). A different model has recent
ly been suggested [1] that proposes a formation of
ballen in quartz in an extreme thermal shock event.
This idea has been taken up and is here shown to be
a probable indicator of impact thermal shock in oth
er impact events (Fig. 2).
Discussion and conclusion
The presented here results of thin section
ana lyses of quartz shocked in three young impact
events in the Czech Republic and in Germany reveal
observations strongly supporting processes of ther
mal shock that in the past have in general been dis
regarded in impact cratering research. Noticeably,
in all three impact events both effects the extremely
strange microfracturing and the ballen quartz for
mation occur together with impact melt glass and
diaplectic silica and, hence, point to affinity. In par
ticular the microfractured quartz grains «floating»
isolated in the matrix imply high confining pressure
for maintaining coherence, and the microfractur
ing limited to a rim all around the grains can best,
and probably only, be explained by a sudden com
plete immersion in rock melt and/or vapor short
ly after shock wave passage, experiencing extreme
thermal shock heating and rapid cooling. The bal
len quartz formation fits well into this process and
emphatically supports the model presented in [1].
Hence, thermal shock should become a more con
sidered feature in future impact cratering research.
References
1. Chanou, A., Grieve, R. A. F., Osinski, G. R. Formation
of ballen in silica by thermal shock // Bridging the Gap III:
Impact Cratering In Nature, Experiments, and Modeling.
2015. 21—26 September. University of Freiburg,
Germany. LPI Contribution 1861. P. 1112.
2. Molnár, M., Ventura, K., Švanda, P., Štaffen, Z.,
Rappenglück, M.A., Ernstson, K. Chrudim — Pardubice:
Evidence for a Young Meteorite Impact Strewn Field in
the Czech Republic // 48th Lunar and Planetary Science
Conference. 2017. 1920.
3. Molnár, M., Švanda, P., Bene, L., Ventura, K.,
Ernstson, K. Asphaltic (bituminous) breccias with carbo
lite (carbon allotrope) and ballen structures in silica as
indicative of thermal shock: more evidence of a Holocene
meteorite impact event in the Czech Republic // 49th
Lunar and Planetary Science Conference. 2017. 1423.
4. Ernstson, K., Müller, W., GawlikWagner, A. The
Saarlouis semi crater structure: notable insight into the
Saarland (Germany) meteorite impact event achieved
// 49th Lunar and Planetary Science Conference. 2018.
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5. Ernstson, K., Poßekel, J., Rappenglück, M.A. Near
ground airburst cratering: petrographic and ground pen
etrating radar (GPR) evidence for a possibly enlarged
Chiemgau impact event (Bavaria, SE Germany) // 51st
Lunar and Planetary Science Conference. 2020. 1231.
6. Ferrière, L., Koeberl, C., Reimold, W. U. Char
acterisation of ballen quartz and cristobalite in impact
breccias: new observations and constraints on ballen for
mation // Eur. J. Mineralogy. 2009. V. 21. P. 203—217.
Fig. 2. Silica ballen structures as a shock indicator. A: Czech impact; ballen as diaplectic silica glass [3]. B: Shocked granitic
melt rock sheet near Bach/Regensburg (Bavaria, Germany) [5]. C: Saarland impact (Germany) [4]. Photomicrographs,
A crossed polarizers, B, C plane light
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Formation of ballen in silica by thermal shock // Bridging the Gap III: Impact Cratering In Nature, Experiments, and Modeling
  • A Chanou
  • R A F Grieve
  • G R Osinski
Chanou, A., Grieve, R. A. F., Osinski, G. R. Formation of ballen in silica by thermal shock // Bridging the Gap III: Impact Cratering In Nature, Experiments, and Modeling. 2015. 21-26 September. University of Freiburg, Germany. LPI Contribution 1861. P. 1112.