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| Titel |
Quantitative Neutron and X-ray texture analysis of Quartz mylonites - a comparative study |
| VerfasserIn |
Jens Walter, Bernd Leiss, Giorgio Pennacchioni, Christian Randau |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250057546
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| Zusammenfassung |
| We investigated five mylonitic quartz veins from the Adamello pluton (southern Alps, Italy)
which accumulated different amounts of shear deformation in the range between 4.5 to over
15. All quartz mylonites consist of a fine grained (35-40 μm grainsize) dynamically
recrystallized aggregate and show a strong crystallographic preferred orientation
(CPO or texture), dominated by a strong Y-maximum of c-axis, whose intensity
is expected to strengthen with increasing strain. The CPO has been measured by
different texture diffraction methods at the Neutron Material Science Diffractometer
STRESS-SPEC at the research reactor FRM II in Garching near Munich and with an x-ray
diffractometer optimized for geological sample material located at the University of
Göttingen.
The high penetration capabilities of neutrons allowed the texture determination of the
complete cubic quartz sample volume of about 4 cm3. In contrast, 90% of the x-ray CuKα
radiation detected from the sample in reflection geometry has penetration depths of up
to 45 μm in quartz (Wenk 1998). Applying a beam diameter of about 7 mm by
means of a glass fibre polycapillary results in a measured sample volume of 1.73
mm3. Combining the measurements of three orthogonal sample directions, the
measured sample volume sums up to 5.19 mm3. Due to the defocussing effect in
X-ray diffraction, a correction function derived from randomly oriented powder
samples has to be applied and only incomplete pole figures (tilt angle 75Ë ) could be
obtained. To obtain complete pole figures, we (1) combined the measurements of three
orthogonal sample sections and (2) applied the WIMV-algorithm (e.g. Wenk et al
1998) as an orientation distribution function (ODF) to recalculate complete pole
figures. For a quantitative comparison with the X-ray data, the WIMV-algorithm was
also applied on the neutron diffraction data to also obtain a quantitative texture
analysis.
The experimental and recalculated pole figures of the three orthogonal sample directions
measured by X-ray are very similar between the individual directions. This proves a high
texture homogeneity and a reliable defocussing correction. Consequently the added
experimental pole figures from the three sample directions show a very good agreement as
well. The comparison between the X-ray and neutron pole figures shows in general a
good accordance. Minor differences are related to grain statistics, small sample
heterogeneities and minor effects from the defocusing correction. From the geological view
these are negligible. Furthermore, this study shows that the STRESS-SPEC neutron
diffractometer, although optimized for material science applications is well suitable for the
measurement of geological samples, when critical grain-size/volume ratios are not
exceeded.
References:
Wenk, H.-R. (1998): Pole figure measurements with diffraction techniques. In: Texture
and Anisotropy (edited by Kocks, U. F., Tomé, C. N., Wenk, H.-R.), Cambridge University
Press 1998, pp. 126-177
Wenk, H. R., S. Matthies, J. Donovan, and D. Chateigner (1998): BEARTEX: A
Windows-based program system for quantitative texture analysis, J. Appl. Crystallogr., 31(2),
262–269, doi: 10.1107/ S002188989700811X. |
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