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| Titel |
Microstructures and Crystallographic Misorientation in Experimentally Deformed Natural Quartz Single Crystals |
| VerfasserIn |
Anja Thust, Renée Heilbronner, Holger Stünitz |
| Konferenz |
EGU General Assembly 2010
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 12 (2010) |
| Datensatznummer |
250043246
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| Zusammenfassung |
| Samples of natural milky quartz were deformed in a Griggs deformation apparatus at
different confining pressures (700 MPa, 1000 MPa, 1500Â MPa), with constant displacement
rates of 1 * 10-6s-1, axial strains of 3 - 19%, and at a temperature of 900Ë C. The
single crystal starting material contains a large number of H2O-rich fluid inclusions.
Directly adjacent to the fluid inclusions the crystal is essentially dry (50-150H/106Si,
determined by FTIR). The samples were cored from a narrow zone of constant “milkyness”
(i.e. same density of fluid inclusions)Â in a large single crystal in two different orientations (1)
normal to one of the prism planes (-¥{m} orientation) and (2) 45Ë to and to
(O+ orientation).During attaining of the experimental P and T conditions,
numerous fluid inclusions decrepitate by cracking. Rapid crack healing produces
regions of very small fluid inclusions (“wet” quartz domains). Only these regions are
subsequently deformed by dislocation glide, dry quartz domains without cracking and
decrepitation of fluid inclusions remain undeformed. Sample strain is not sufficient to
cause recrystallization, so that deformation is restricted to dislocation glide. In
experiments at lower temperatures (800, 700Ë C) or at lower strain rate (10-5s-1) there is
abundant cracking and semi-brittle deformation, indicating that 900Ë C, (10-6s-1)
represents the lower temperature end of crystal plastic deformation in these single
crystals.
Peak strengths (at 900Ë C) range between 150 and 250 MPa for most samples of both
orientations. There is a trend of decreasing strength with increasing confining pressure, as
described by Kronenberg and Tullis (1984) for quartzites, but the large variation in strength
due to inhomogeneous sample strain precludes a definite analysis of the strength/pressure
dependence in our single crystals.
In the deformed samples, we can distinguish a number of microstructures and inferred
different slip systems. In both orientations, deformation lamellae with a high optical relief
appear in the usual sub-basal orientation; often they are associated with “fluid inclusions
trails”, cracks or en echelon arrays.
In -¥{m} orientation, conjugate misorientation bands sub-parallel to the prism planes can
be observed. The barreled shape of the samples can be explained by prism glide.
Unfortunately, since prism glide does not affect the c-axis orientation it cannot be
recognized on a c-axis orientation image. Nevertheless, changes in the c-axis orientation are
observed locally, indicating either the activity of an additional slip system or a different
deformation process (not specified yet).
In O+ orientation, we observe the formation of internally kinked shear bands. They are up
to 100 μm wide and oriented at α ~ 90Ë w/r to the host c-axis, slightly oblique to the
sense of shear. The width of the kinked domains is ~20-40 μm and the average
misorientation (β) is ~ 5Ë . The dispersion of c-axis orientation with synthetic rotation of the
c-axis is evidence of basal glide.
References:
Kronenberg, A.K. & Tullis, J. (1984): Flow strength of quartz aggregates: grain size and
pressure effects due to hydrolyticÂ
weakening. JGR Vol. 89, 4281-4281. |
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