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| Titel |
Strain rate and shear stress at the grain scale generated during near equilibrium antigorite dehydration |
| VerfasserIn |
José Alberto Padrón-Navarta, Andrea Tommasi, Carlos J. Garrido, David Mainprice, Maxime Clément |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
en
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250134570
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| Publikation (Nr.) |
 EGU/EGU2016-15314.pdf |
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| Zusammenfassung |
| Dehydration reactions are an outstanding case of mineral replacement reactions because they
produce a significant transient fluid-filled porosity. Because fluids are present, these
reactions occur by interface-coupled dissolution–precipitation. Under poorly drained
conditions corresponding to foliated metamorphic rocks, they generate fluid pressure
gradients that evolve in time and space eventually controlling fluid migration [1].
Despite the general agreement on this fact, we still lack of a precise knowledge
of the complex coupling between the stresses generated during the reaction and
the timescales for mineral growth and how they ultimate control the rate of fluid
migration. Constraining these rates is challenge because the timescales of the feedback
between fluid flow and mineral growth rates at near equilibrium are beyond the
current experimental capabilities. For instance, numerical simulations suggest that the
draining times of a dehydration front by compaction are in the order of 10–100 ky [1]
difficult to translate into experimental strain rates. On the other hand, the natural
record of dehydration reaction might potentially provide unique constrains on this
feedback, but we need to identify microstructures related to compaction and quantify
them.
Features interpreted as due to compaction have been identified in a microstructural study
[2] of the first stages of the antigorite dehydration at high-pressure conditions in
Cerro del Almirez, Spain (ca. 1.6–1.9 GPa and 630–710 ˚ C). Compaction features
can be mostly observed in the metamorphic enstatite in the form of (1) gradual
crystallographic misorientation (up to 16˚ ) of prismatic crystals due to buckling, (3)
localized orthoenstatite(Pbca)/low clinoenstatite (P21/c) inversion (confirmed optically and
by means of Electron Backscattered Diffraction) and (4) brittle fracturing of prismatic
enstatite wrapped by plastically deformed chlorite. The coexistence of enstatite buckling and
clinoenstatite lamellae has not been previously reported and offers an unique opportunity to
estimate a lower bound for the strain rates and local shear stresses generated during
the grain growth and coeval compaction. Estimated values based on experimental
creep rates on pyroxene aggregates [3] result in strain rates in the order of 10−12 to
10−13 s−1 and shear stresses of 60–70 MPa. Lower shear stress values (20–40
MPa) are retrieved using the thermodynamic model clinoenstatite inversion of Coe
[4] in combination with the hydrostatic high-pressure experimental data on the
stability of low clinoenstatite (P21/c). These data suggest that, under low deviatoric
stress, fluid extraction and compaction near equilibrium in natural systems are only
marginally higher than the strain rate of the solid matrix. These observations support the
relatively long residence time of fluids in dehydration fronts and the necessity to
further explore and quantify the feedback between mineral grain growth and fluid
migration.
[1] Connolly (2010) Elements 6(3):165–172; [2] Padrón-Navarta et al. (2015). Contrib
Miner Petrol 169:35 [3] Raleigh et al. (1971). J Geophys Res 76(17): 4011–4022; [4] Coe
(1970). Contrib Miner Petrol 26(3):247–264 |
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