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| Titel |
Grain-scale alignment of melt in sheared partially molten rocks:
implications for viscous anisotropy |
| VerfasserIn |
Matěj Peč, Alejandra Quintanilla-Terminel, Benjamin Holtzman, Mark Zimmerman, David Kohlstedt |
| 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 |
250129445
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| Publikation (Nr.) |
 EGU/EGU2016-9561.pdf |
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| Zusammenfassung |
| Presence of melt significantly influences rheological properties of partially molten rocks by
providing fast diffusional pathways. Under stress, melt aligns at the grain scale and this
alignment induces viscous anisotropy in the deforming aggregate. One of the consequences
of viscous anisotropy is melt segregation into melt-rich sheets oriented at low angle to the
shear plane on much larger scales than the grain scale. The magnitude and orientation of
viscous anisotropy with respect to the applied stress are important parameters for
constitutive models (Takei and Holtzman 2009) that must be constrained by experimental
studies.
In this contribution, we analyze the shape preferred orientation (SPO) of individual
grain-scale melt pockets in deformed partially molten mantle rocks. The starting materials
were obtained by isostatically hot-pressing olivine + basalt and olivine + chromite + basalt
powders. These partially molten rocks were deformed in general shear or torsion at a
confining pressure, Pc = 300 MPa, temperature, T = 1200˚ – 1250˚ C, and strain rates of
10−3 – 10−5 s−1to finite shear strains, γ, of 0.5 - 5. After the experiment, high resolution
backscattered electron images were obtained using a SEM equipped with a field
emission gun. Individual melt pockets were segmented and their SPO analyzed
using the paror and surfor methods and Fourier transforms (Heilbronner and Barret
2014).
Melt segregation into melt-rich sheets inclined at 15˚ -20˚ antithetic with respect to the
shear plane occurs in three-phase system (olivine + chromite + basalt) and in two-phase
systems (olivine + basalt) twisted to high strain. The SPO of individual melt pockets within
the melt-rich bands is moderately strong (b/a ≈ 0.8) and is always steeper (20˚ -40˚ ) than
the average melt-rich band orientation. In the two-phase system (olivine + basalt) sheared to
lower strains, no distinct melt-rich sheets are observed. Individual grain-scale melt
pockets are oriented at 45˚ -55˚ antithetic with respect to the shear plane (i.e.,
sub-perpendicular to σ3) with a strong SPO (b/a ≈ 0.7) that decreases with increasing finite
strain.
Our observations of melt alignment at low strains are in agreement with observations
performed on analogue materials (borneol, Takei 2010) and provide further constraints for the
orientation of viscous anisotropy in the Earth’s mantle. The systematic difference in
grain-scale melt alignment between samples in which melt segregation did and did
not occur - irrespective of the deformation geometry and mineralogy - suggests
that melt segregation into bands leads to local stress rotation within the samples. |
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