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| Titel |
The generation and evolution of anisotropic gas-permeability during viscous deformation in conduit-filling ignimbrites |
| VerfasserIn |
Stephan Kolzenburg, Kelly Russell |
| Konferenz |
EGU General Assembly 2015
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 17 (2015) |
| Datensatznummer |
250107402
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| Publikation (Nr.) |
 EGU/EGU2015-7102.pdf |
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| Zusammenfassung |
| Gas-permeability plays a governing role in the pre-explosive pressurization of volcanic
edifices. Pressurization may only occur once the total volume flux of gases emitted by an
underlying magmatic or hydrothermal source exceeds the flow capacity of the permeable
pathways present in the edifice.
We have measured the physical properties (strain, porosity, permeability and ultrasonic
wave velocities) of breadcrust bombs recovered from the deposits of the 2350 B.P. eruption
of Mt Meager, BC, Canada. These rocks represent a conduit-infilling pyroclastic
breccia that underwent various degrees of welding and deformation and present a
remarkable opportunity to constrain the nature and timescale of mechanical processes
operating within explosive volcanic conduits during repose periods between eruptive
cycles.
Here we present data from permeability measurements along the directions of maximum
and minimum shortening which help quantifying the effect of vesicle microstructure on
permeability. Permeability is measured by applying a range of confining pressures (between
3.4 and 17.2 MPa) to each sample and imposing a constant head (of 0.2 to 3.5 MPa) across
the sample. The permeability is then determined using a modified version of Darcy’s law
applicable to compressible fluids.
These rocks display a profound directionality in the measured physical properties
resulting from the deformation-induced fabric. For all samples the permeability across the
elongation fabric is highly correlated to the sample porosity whereas along the elongation
fabric there is little effect of porosity on permeability. At porosity values of about 20% the
permeability seems to reach a minimum at 10-16 m2 and does not change significantly with
further reduction of porosity. Further, the effect of confining pressure on the permeability of
these samples appears to be more pronounced across the elongation fabric than along the
elongation fabric.
The deformation fabric has a significant effect on the gas-permeability of the
deposit. Porosity, on the other hand, appears to play a secondary role. This, fabric
dependent, anisotropic permeability evolution of fragmental deposits during welding
directly affects the gas escape from, and transport through the deposit and, therewith,
plays a key role in the gas-pressure distribution and evolution within the volcano. |
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