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Paper Number: 111
Kovaleva,
E.1,2, Klötzli, U.2 and Habler, G.2
1Department
of Geology, University of the Free State, 205 Nelson Mandela Drive, 9300
Bloemfontein, South Africa, kovalevae@ufs.ac.za
2Department
of Lithospheric Research, University of Vienna, 14 Althanstrasse, 1090
Vienna, Austria
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Our discovery of planar microstructures in zircon from paleo-seismic
zones demonstrates that they are not exclusively attributed to
impactites, but can also form as a result of seismic activity in the
upper crust.
Deformed zircons were found in the high-grade metapelitic rocks of
the Ivrea-Verbano Zone, Southern Alps (Northern Italy). Sampled
granulitic mylonites contain pseudotachylytic veins resulting from
frictional melts and are associated with ultramylonites. Planar
microstructures formed in zircon at ~10 km depth due to unusually high
differential stresses and strain rates generated by earthquakes and/or
due to shearing at elevated temperatures in the vicinity of frictional
melts.
The interior of polished zircon grains ranging from 30 to 150 mµ in
length were investigated with optical microscope and scanning electron
microscope (SEM) techniques, including secondary electron (SE),
backscattered electron (BSE), forescatter electron (FSE),
cathodoluminescence (CL) imaging, crystallographic orientation mapping
by electron backscatter diffraction (EBSD) analysis, and secondary ion
mass spectroscopy (NanoSIMS). Among different cataclastic and
crystal-plastic deformation microstructures in zircon we identified
characteristic planar deformation bands (PDBs), planar fractures (PFs),
and curviplanar fractures (CFs). Planar features form in specifically
oriented grains with <c> axis parallel to the stretching
lineation. Planar deformation bands in zircon appear as contrast
lamellae in orientation contrast images and in EBSD maps, and in rare
cases can be observed with an optical microscope. They are
crystallographically controlled planar lattice volumes with
misorientation from the host grain, which can reach up to 3°. PDBs are
strictly parallel to {100} crystallographic planes, are 0.3-1 µm wide
with an average spacing of 5 µm. Discovered structures represent a
result of crystal-plastic deformation of zircon lattice with operating
dislocations having <100>{010} glide system and <001>
misorientation axis. PDBs are different from well-known impact-related
planar deformation features (PDFs), because they occupy other
crystallographic planes and are not amorphous [2,3]. Furthermore, we
have demonstrated that formation of PFs in zircon takes place not only
during impacts, but also in seismically active zones. We observe at
least two cases of PFs formation with {100} orientation: (1) as a result
of evolution of PDBs and (2) as micro-cleavage [1].
Portions of zircon grains that contain PDBs were investigated with
NanoSIMS, with the goal to study the effect of PDBs on trace elements
isotope distribution. We have found that PDBs cause re-distribution and
loss of radiogenic lead isotopes which result in systematic rejuvenation
of the affected domains. Moreover, PDBs may facilitate enhanced pipe
diffusion of REE, Hf, and possibly P and Ti. These effects can be very
important for zircon microchemistry and geochronology, and may be used
in the future as a tool for dating of seismic events.
According to the new findings, planar deformation bands and planar
fractures in zircon derived from the deep tectonic settings represent
newly recognized evidence of seismicity, and help to understand how
seismic energy is released at depth and interacts with metamorphic
processes [1].
References:
[1] Kovaleva E et al. (2015) Am Mineral 100:1834–1847
[2] Leroux
H et al. (1999) Earth Planet Sc Lett 169:291–301
[3] Timms NE et al. (2012) Meteorit Planet Sci 47:120–141
