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| Titel |
Fracture and healing in magmas: a dual role on permeability evolution |
| VerfasserIn |
Anthony Lamur, Yan Lavallée, Richard Wall, James Ashworth, Jackie Kendrick, Fabian Wadsworth |
| 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 |
250121709
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| Publikation (Nr.) |
 EGU/EGU2016-529.pdf |
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| Zusammenfassung |
| The development of a permeable network in silicic volcanic conduits controls outgassing and
plays a major role on the subsequent eruptive behaviour. Efficient outgassing, at higher
permeabilities, is achieved through the coalescence of pores and fractures. Whilst the
relationship between permeability and increasing connected porosity is now relatively well
constrained, the effects of fractures have, on the other hand, rarely been investigated. Here,
we present the results of an experimental study focusing on the impacts of tensile fracturing
and healing on permeability.
Permeability measurements have been performed on over 60 disk-shaped samples (26
mm diameter, 13 mm thickness) with connected porosities ranging from 2 to 45%.
Our results for unfractured samples display the same porosity-permeability trend
as previous studies and permeabilities span from 10−15 at low porosities to over
5x10−12 m2 at higher porosities. These samples were then broken via Brazilian tests
and the resultant permeability of the rocks were then measured across the fracture
zone. Whilst high porosity samples reached permeabilities of about 5x10−10 m2 (2
orders of magnitude higher than intact samples), low porosity samples, on the other
hand, reached permeabilities around 5x10−12 m2 (more than 3 orders of magnitude
above intact samples). Our results show that fracturing favours the development of a
permeable network that adheres to a different permeability-porosity relationship than
previously presented, and that this effect is emphasized in magmas with low connected
porosities.
The effect of fracture healing by diffusion on permeability has been investigated through
a series of experiments on borosilicate standard glass (NIST 717a). These experiments were
conducted at 560oC (viscosity of 1010.33 Pa.s) on pairs of columns pressed and held in
contact at constant load for times varying between 0.5s and 15000 s before being pulled apart
at a strain rate of 10−3s−1. Using Maxwell’s theory of viscoelasticity, we estimate the
relaxation timescale for this viscosity at ∼2s. Results show that healing starts after a
minimum contact time of 60 s, more than 1 order of magnitude slower than the time
predicted by the theory of viscoelasticity. Furthermore, healing and strength recovery
increase logarithmically from then. Strength recovery is estimated with respect to the
tensile strength of this glass; 25 MPa when measured under the same conditions
(560oC and strain rate of 10−3s−1). The time for full-healing can thus be estimated at
∼52000 s, or 2.5x104 times the relaxation timescale. We hereby propose that healing
efficiency depends on the destruction of the contact area through atomic diffusive
exchanges and bond creation during viscous sintering between the two parts of the
sample.
The combination of our two sets of experiments allows us to propose a model in
which fractures can promote the development of a permeable network which may
lead a volcano toward more effusive behaviour, or whereby healing can destroy
permeability and allow pressure build-up that could lead to explosive eruption. |
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