 |
| Titel |
Coesite as stress indicator in experimentally deformed quartz gouge |
| VerfasserIn |
Bettina Richter, Holger Stünitz, Renée Heilbronner |
| Konferenz |
EGU General Assembly 2015
|
| Medientyp |
Artikel
|
| Sprache |
Englisch
|
| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250104265
|
| Publikation (Nr.) |
 EGU/EGU2015-3683.pdf |
|
|
|
|
|
| Zusammenfassung |
| In shearing experiments conducted to study the behaviour of quartz gouge at the brittle -
viscous transition, coesite was found in samples that were deformed at confining pressures of
1.0 GPa or 1.5 GPa, at temperatures between 600Ë C and 800Ë C, and at constant
displacement rates of ~1.3 x 10-5 mms-1 or ~1.3 x 10-4 mms-1. The experiments were
performed in a Griggs type deformation apparatus and the starting material was obtained
from a hydrothermally grown single crystal. The crystal was crushed and sieved to a grain
size < 100 μm. 0.1 g of the powder, with 0.2 wt% water added, was introduced
in a 45°pre-cut between alumina forcing blocks forming a ~1 mm thick shear
zone.
In all experiments, the confining pressures (Ïă3)and the peak mean stresses (1/3
(Ïă1 + Ïă2 + Ïă3) for the general case or 1/2 (Ïă1 + Ïă3) for Ïă3 = Ïă2) remained below the quartz
- coesite transition. Only the highest principal stresses (Ïă1) reach the coesite stability field.
With the exception of low-temperature experiments, the occurrence of coesite coincides with
whether or not Ïă1 reached the coesite stability field. In samples deformed at 600oC coesite
did not form despite the fact that Ïă1 reached the coesite field, indicating some temperature
effect for the transformation kinetics.
In two samples, Ïă1 crosses the quartz-coesite phase transition and stays in the coesite
field at the beginning of the shearing deformation and - with ongoing weakening - crosses
back into the quartz stability field at higher strains. As expected, the reverse phase
transformation, from coesite to quartz, can be observed in these samples.
Coesite forms as soon as Ïă1 comes very close to or enters the coesite stability field.
Clusters of small idiomorphic tabular coesite crystals are distributed throughout the sample
and are commonly aligned with the [010] direction parallel to the Ïă1 direction. With
increasing deformation in the coesite stability field, coesite grains grow (forming up to 2 vol
%) and the [010] directions rotate into parallelism with the foliation (rigid particle
behaviour).
Once Ïă1 drops below the phase transition, the coesite grains are corroded, indicating a
back-transformation to quartz. A preferred growth direction of the new quartz grains with
respect to the old coesite grains is not obvious but the replacing quartz grains show a constant
crystallographic orientation (single crystal orientation).
In conclusion, in deformation experiments, the coesite formation seems to only depend on
the maximum compressive stress Ïă1 rather than on the confining pressure or the mean stress.
Ïă1 controls the quartz-to-coesite transformation as well as the reverse transformation except
where low temperatures slow down the transformation kinetics. Furthermore, the accuracy of
Ïă1 values measured with solid medium deformation apparatus lies within the same error
range as that of the quartz-coesite phase transitions determined with the piston cylinder
apparatus. |
|
|
|
|
|
|