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| Titel |
The effect of crystal plasticity and mineral stability on the rheological
properties of magma during spine extrusion at Unzen, Japan |
| VerfasserIn |
Paul A. Wallace, Jackie E. Kendrick, Yan Lavallée, James D. Ashworth, Elisabetta Mariani, Felix W. von Aulock, Rebecca Coats, Takahiro Miwa |
| Konferenz |
EGU General Assembly 2016
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 18 (2016) |
| Datensatznummer |
250121722
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| Publikation (Nr.) |
 EGU/EGU2016-552.pdf |
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| Zusammenfassung |
| The presence of crystals in silicic magmas is known to have a significant effect on the
rheological properties inducing a non-Newtonian response. Plastic deformation of the
crystalline phase in magmatic suspensions is believed to be partially responsible for this
characteristic behaviour via accommodating strain, but little has been investigated on its role
in volcanic processes. The spine extrusion following the final stages of endogenous growth of
the 1991-95 lava dome eruption at Unzen volcano, Japan, has provided a unique opportunity
to investigate the contribution of the different deformation mechanisms and varying
petrological phenomena associated with magma ascent. The spine forms a shear zone
consisting of four structurally discrete units over a 6 m transect including: gouge (1), a
heavily sheared zone (2) to a moderately sheared zone (3), and an undeformed
magmatic core (4). Here we report the first systematic study of the microstructures,
mineralogy, crystal stability, geochemistry and crystal size distribution across this shear
zone.
The spine samples are porphyritic dacites with varying abundance of phenocrysts (20-30
vol.%), dominantly plagioclase, hornblende and biotite with minor quartz. The groundmass
contains the same mineralogy plus pyroxene, magnetite and ilmenite. The microlites (35
vol.%) show a strong trachytic texture in areas of high shear, providing evidence of strain
localisation. Brittle deformation is evident across the spine, with the higher sheared
samples showing more crystal size reduction of the phenocrysts. By performing
high-temperature (900˚ C) uniaxial compressive strength tests at constant strain rates (10−5
and 10−3 s−1), it can be inferred that crystals play a key role in the rheological
properties, by forming a rigid but weak network that serves to partition stress and thus
localise strain within the flowing melt. Electron backscatter diffraction (EBSD)
enables the identification of crystal plasticity in both phenocrysts and microlites, with
biotite displaying the greatest evidence of strain accommodation. This permanent
strain is induced when the shear stress exceeds a critical point on an orientated
lattice plane, resulting in a misorientation of the internal lattice. Crystal-plastic
behaviour may thus act as a strain marker for the viscous-brittle transition during ascent.
In the highly sheared zone, the rims of both hydrous minerals (hornblende and
biotite) and plagioclase show a reaction with the melt suggesting disequilibrium
conditions – a feature not as evident in the undeformed magmatic core of the spine.
The narrow localisation of the disequilibrium textures suggest that the increased
effects of gas flow in the permeable shear zone and/or thermal input due to strain
localisation may be contributing factors affecting mineral stability during magma
transport.
These deformation microstructures that occur in the shallow conduit, especially during
ascent of highly viscous magma, can lead to permeability anisotropy which can significantly
alter degassing efficiency and control the explosivity of an eruption. For this reason a
thorough petrological/rheological understanding of these deformation processes is vital in
constraining the complexities associated with on-going eruptions and shifts from effusive to
explosive activity. |
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