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| Titel |
Influence of H2O Rich Fluid Inclusions on Quartz Deformation |
| VerfasserIn |
Anja Thust, Renée Heilbronner, Holger Stünitz, Alexandre Tarantola, Harald Behrens |
| 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 |
250044133
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| Zusammenfassung |
| The effect of H2O on the strength of quartz is well known and has been discussed many times
in the literature (e.g. Griggs & Blacic 1965, Kronenberg 1994). In this project we study the
H2O interactions between natural dry quartz and H2O rich fluid inclusions during
deformation in the solid medium Griggs apparatus.
High pressure and temperature experiments were carried out using a quartz single crystal
containing a large number of H2O-rich fluid inclusions. Adjacent to the fluid inclusions the
crystal is essentially dry (< 100 H/106Si, as determined by FTIR). Two sample orientations
where used: (1) -¥{m} orientation: normal to one of the prism planes, (2) O+ orientation: 45Ë
to and 45Ë to [c]. Confining pressures were 700 MPa, 1000 MPa and 1500 MPa, with a
constant displacement rate of ~ 10-6 s-1 and a constant temperature of 900Ë C.
Additionally, experiments where carried out at lower temperatures (800Ë C, 700Ë C) and
faster strain rate (~ 10-5 s-1). During increasing pressure and temperature we remained
close to the fluid inclusion isochore and exceeded the α - β transition as late as
possible.
The strengths of the majority of the samples are between 150 and 250 MPa (the weakest
is 84 MPa, the strongest 414 MPa). Low strength can be explained by dynamic
recrystallization and deformation by dislocation creep, higher strength correlates with a lower
H2O content and absence of dislocation creep.
In the undeformed material, the H2O rich fluid inclusions contain different chlorides like
antarticite (CaCl2-
6H2O) and hydrohalite (NaCl-
2H2O), as measured with micro
thermometry. They show a large range in size from 50 μm to 700 μm and their spatial
distribution is extremely heterogeneous. After deformation the inclusions are more
homogeneously distributed throughout the sample and dramatically reduced in size (<
0.1μm). Regions with a high density of very small fluid inclusions are the regions with the
highest concentration of deformation and yield an H2O content up to 3000 H/106Si. There is
also a difference in fluid homogenization for the two different orientations. In crystals
with orientation -¥{m}, the fluid distribution is more homogeneous and to a finer
scale than in crystals of O+ orientation. After deformation the salinity of the fluid
inclusions is increased, from ~10.6 wt.% equivalent NaCl up to ~12 wt.% equivalent
NaCl.
FTIR spectra (point measurements, 100*100 μm) also show a change in H2O content.
Before deformation, the absorption spectra show no evidence for H2O in the quartz, H2O is
only detected in the fluid inclusions (broad absorption band indicating molecular water).
After deformation, the absorption spectra display a discrete peak, indicating OH- bonding in
the quartz lattice.
The release of H2O from fluid inclusions is an important process for crystal plastic
deformation. Fluid inclusion rupture, micro cracking and the fast crack healing at these
temperatures promote the distribution of H2O through the quartz and influences the strength
of the material.
Reference:
Griggs, D.T. & Blacic, J.D., 1965: Quartz: Anomalous weakness of synthetic crystals.
Science 174, 293-295.
Kronenberg A.K., (1994): Hydrogen specifications and chemical weakening of quartz,
Rev. Mineral. Ser. 29 (1994), pp. 123–176. |
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