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Introduction: Planar deformation features (PDFs)
in quartz are considered unambiguous microscopic
evidence of shock metamorphism (e.g., [1,2]). However,
the occurrence of tectonic features that can closely
resemble PDFs [3-6], and the effect of post-impact
deformation that can completely obliterate former
PDFs, make a clear identification of PDFs sometimes
difficult, especially if the investigation is limited to the
optical microscope. While generally other evidence
permit to unambiguously identify impact crater, the
problem arises when only unclear PDFs can be observed.
This issue has recently triggered an animated
debate about the possible misinterpretation as PDFs of
some unusual features in old quartz grains [7,8]. Consequently,
a statistical criterion for the identification of
truly shock-related PDFs has been proposed, based on
a maximum number of unindexed planes after U-stage
measurements [9]. However, this rigid criterion might
not fit some natural situations. For example, the number
of unindexed planes in measured PDFs in quartz
from El'gygytgyn, Arctic Russia [10], is slightly higher
than the proposed limit.
Electron microscopy on shocked quartz from
El'gygytgyn has revealed crystal deformation that locally
affects PDFs and hampers their correct indexation
by means of optical microscope and U-stage. As
the area has not been affected by significant regional
tectonics after the impact, which is quite recent [11],
and as the target rock does not exhibit similar features,
the observed deformation must have occurred during
the impact event. Crystal plastic deformation, related
to the impact event, has been previously described in
quartz grains that contain PDFs from the Charlevoix
structure [12] and ascribed to the elastic rebound of the
central uplift while the rock was still at ~300°C. A
shear component in the shock deformation has been
invoked for the formation of PDFs with specific orientation
and of Brazil twins in quartz from the
Rochechouart impact structure [13].
Petrographic and microstructural analysis of the deformation
observed in shocked quartz from El'gygytgyn
is here presented and interpreted in the framework
of the impact event.
Methods: A polished, 35 µm thick thin section
from sample 101Q6_W11-13, 326.65 m depth below
the lake bottom in the drill core [10], has been selected
for this study. Scanning Electron Microscopy (SEM)
was performed at the Department of Lithospheric Research
of the University of Vienna (Austria), with a
FEI Inspect S50, and at SURF (Belgium), with a JEOL
JSM-7100F field emission SEM. Backscattered electron
(BSE) images were processed with the free software
ImageJ (http://rsbweb.nih.gov/ij/) for microstructural
analysis. Electron backscattered diffraction
(EBSD) analysis was done at the RBINS (Belgium),
with a FEI Quanta 200 ESEM equipped with an
EDAX-AMETEK Hikari camera. EBSD processing
software is OIM Data Collection and Analysis v.5.31.
Results: El'gygytgyn suevite contains individual
quartz grains, which are variously shocked, from unshocked
to completely molten [10]. The grains selected
for this study contain up to five sets of intersecting
PDFs. In SEM images, PDFs appear as thin (< 1µm)
straight lines crosscutting the quartz grain (Fig. 1a).
Locally, PDFs are bent, form pull-apart micro-basins
(Fig. 1b), drag folds, and similar features, which clearly
indicate that some degree of deformation occurred
after PDF formation.
This type of deformation seems to be localized
along specific PDF orientations, with characteristic
angles between them, generally of 60° or 45°. EBSD
analysis shows that crystals involved in this deformation
process do not contain deformation bands or
any evidence of dynamic recrystallization, supporting
the localization of deformation exclusively along PDF.
The only pervasive deformation effect that is observed
is the possible occurrence of Dauphiné twinning,
which was interpreted as resulting from the retrogression
from β-quartz into α-quartz during cooling of
impact-heated grains in quartz from the Charlevoix
impact structure [14].
Discussion: Microstructural analysis of these deformation
features in El'gygytgyn shocked quartz suggests
that crystal plastic deformation occurred in quartz
when embedded in the suevite. The observed deformation
that affects quartz grains containing PDFs is
very likely related to the impact event because (i)
completely undeformed quartz grains that preserve
original magmatic features are embedded in the same
impact breccia, (ii) this deformation does not affect
quartz grains in the unshocked target rock outside the
crater, (iii) there are no indications of foreign origin for
the investigated grains, and (iv), no significant regional
tectonic deformation was noted in the area after the
impact event, or is exhibited in the suevite. Therefore,
the observed deformation must have followed the
PDFs formation, and is likely coeval with the suevite
deposition. This would imply that the crystals were
still hot due to the impact, as suggested in [14;15].
A possible explanation is that the shear stress component,
necessary for activating slip systems, was likely
produced by a combination of simple axial compression,
due to gravitational compaction of the suevite,
and crystal orientation with respect to the flattening
direction. The deformation was mostly localized along
PDFs, filled with amorphous material that, therefore,
represented a weak heterogeneity that acted as shear
plane. A similar localization process has been modeled
in [16]. Only PDFs with a favorable orientation with
respect to the principal stress direction were exploited,
localizing most of the deformation and locally producing
a little offset. A quantitative analysis of the process
is in progress.
Assuming the described deformation mechanism,
we might expect to observe similar features in other
impact structures. Apart from the observations by C.
Trepmann [12-14], none have yet been reported, either
because the occurrence of other unambiguous impact
evidence made detailed research on apparently badly
preserved shocked quartz grains unnecessary, or because
at El'gygytgyn the combination of local factors
has allowed this type of localized deformation.
Conclusion: Crystal deformation has overprinted
PDFs in quartz from El'gygytgyn without any possible
exogenous causes than the impact itself, making the
proper identification and indexation of PDFs challenging.
The most probable scenario is strain localization
along PDFs that were oriented favorably with respect
to the stress directions and that, therefore, were exploited
as shear planes. Local shear stress was probably
induced by gravitational compaction of the suevite.
This impact-related deformation further confirms
that identification of PDFs by only mean of optical
microscopy might be challenging in some cases. Electron
microscopy, chemical etching, micro-Raman, and
similar analytical techniques might be necessary for an
unambiguous characterization of true PDFs in absence
of any other evidence for an impact event. The strict
application of a statistical evaluation, as proposed in
[9], might lead to ambiguous interpretations if synimpact
deformation, like that described in El'gygytgyn
samples, or post-impact deformation, in old structures,
has strongly affected the preservation of PDFs.
Originele taal-2 | English |
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Titel | Bridging the gap III. Abstracts Volume |
Status | Published - 2015 |
Evenement | Bridging the gap III: impact cratering in nature, experiments, and modeling - Freiburg, Germany Duur: 21 sep 2015 → 26 sep 2015 |
Conference
Conference | Bridging the gap III: impact cratering in nature, experiments, and modeling |
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Land/Regio | Germany |
Stad | Freiburg |
Periode | 21/09/15 → 26/09/15 |
Vingerafdruk
Duik in de onderzoeksthema's van 'IMPACT-RELATED DEFORMATION OF PDFs IN QUARTZ: WHEN IDENTIFICATION OF TRUE PDFs BECOMES CHALLENGING'. Samen vormen ze een unieke vingerafdruk.Projecten
- 1 Actief
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OZR2725: ULB-VUB Joint Research Group: Brussels Institute for Geochemical Techniques in Earth Sciences - BIGE
Claeys, P. & Mattielli, N.
5/12/14 → 2/10/28
Project: Fundamenteel