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| Titel |
Strain-dependent permeability of volcanic rocks. |
| VerfasserIn |
Jamie Farquharson, Michael Heap, Patrick Baud |
| 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 |
250121553
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| Publikation (Nr.) |
 EGU/EGU2016-326.pdf |
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| Zusammenfassung |
| We explore permeability evolution during deformation of volcanic materials using a suite
of rocks with varying compositions and physical properties (such as porosity ϕ).
40 mm × 20 mm cylindrical samples were made from a range of extrusive rocks,
including andesites from Colima, Mexico (ϕ∼0.08; 0.18; 0.21), Kumamoto, Japan
(ϕ∼0.13), and Ruapehu, New Zealand (ϕ∼0.15), and basalt from Mt Etna, Italy
(ϕ∼0.04).
Gas permeability of each sample was measured before and after triaxial deformation
using a steady-state benchtop permeameter. To study the strain-dependence of permeability in
volcanic rocks, we deformed samples to 2, 3, 4, 6, and 12 % axial strain at a constant strain
rate of 10−5 s−1. Further, the influence of failure mode—dilatant or compactant—on
permeability was assessed by repeating experiments at different confining pressures. During
triaxial deformation, porosity change of the samples was monitored by a servo-controlled
pore fluid pump.
Below an initial porosity of ∼0.18, and at low confining pressures (≤ 20 MPa), we
observe a dilatant failure mode (shear fracture formation). With increasing axial strain, stress
is accommodated by fault sliding and the generation of ash-sized gouge between the fracture
planes. In higher-porosity samples, or at relatively higher confining pressures (≥ 60 MPa), we
observe compactant deformation characterised by a monotonous decrease in porosity with
increasing axial strain. The relative permeability k′ is given by the change in permeability
divided by the initial reference state. When behaviour is dilatant, k′ tends to be
positive: permeability increases with progressive deformation. However, results
suggest that after a threshold amount of strain, k′ can decrease. k′ always is negative
(permeability decreases during deformation) when compaction is the dominant
behaviour.
Our results show that—in the absence of a sealing or healing process—the efficiency of a
fault to transmit fluids is correlated to the degree of strain to which is subjected. Volcanic
processes such as dome extrusion, which involve progressive strain on complex fault systems,
have been seen to cause fault sliding and the prolific generation of fault gouge. Our results
indicate that the permeability of these faults will tend to remain constant or increase during
continued extrusion, allowing magmatic gases to readily outgas through permeable fault
architectures despite the generation and accumulation of gouge. On the other hand, deeper
regions of the edifice that will typically be compacting due to the relatively higher
confining pressures, will exhibit a continuous decrease in permeability. The interplay
between permeability-increasing and permeability-decreasing processes within the
edifice is likely to influence outgassing and eruptive cycles at active volcanoes. |
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