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| Titel |
Outgassing in the lab: Permeability development in two-phase magmas during simple shear |
| VerfasserIn |
Alexandra Kushnir, Caroline Martel, Rémi Champallier, Laurent Arbaret |
| 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 |
250125453
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| Publikation (Nr.) |
 EGU/EGU2016-5035.pdf |
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| Zusammenfassung |
| The mechanisms governing permeability development in rising magmas influence the nature
of volcanic outgassing and, consequently, eruption style. Of particular interest for any given
system are: 1) the conditions under which permeability develops; 2) when permeability
develops; and 3) the structure of the permeable network. To explore this, we performed a
series of in-situ permeability measurements made during non-coaxial deformation of
two-phase magmas at various shear strain rates.
Samples were synthesized prior to each experiment by sintering a haplogranitic (HPG8)
powder at a temperature of 1150˚ C. During synthesis, both the confining (PC) and pore fluid
(Pf) pressures were equal to 300 MPa; the confining medium and pore fluid were argon. An
effective pressure of 0 MPa (PC=Pf) ensured that bubbles trapped between the sintering
grains were pressurized while the sample retained its cylindrical geometry. The magmas were
then isothermally decompressed to 60 MPa to allow bubble expansion. Synthesized samples
were impermeable and had bubble fractions between 0.11 and 0.14. Prior to deformation,
the temperature was lowered to 880˚ C and a differential pore fluid pressure was
applied across the sample. The magmas were deformed in torsion until the pore fluid
pressures above and below the samples equilibrated, providing an in-situ measure of
permeability.
At low strain rates (<∼2×10−4 s−1) permeability was not established, even at very large
strains (γ >7). In these samples, bubbles acted as passive strain markers and recorded the
total strain on the sample. At shear strain rates between ∼2×10−4 and 4.5×10−4 s−1,
samples experienced strain hardening until they became permeable at high strain (γ >3). The
permeable network in these samples was constructed of en echelon, Mode I fractures
distributed around the sample periphery. The bubble density adjacent to these features was
reduced with respect to the rest of the sample, suggesting aspiration of surrounding bubbles
into the fractures. Above shear strain rates of 4.5×10−4 s−1, permeability developed
shortly after the onset of inelastic deformation. Again, permeability was established
through a series of en echelon fractures, but there was no reduction in bubble density
surrounding these features, suggesting that there was insufficient time to drain the abutting
bubbles.
These experiments highlight a transition in deformation mechanism that has a profound
effect on sample outgassing. Critically, the Mode I fracture geometry may expedite volatile
movement upwards from the interior of the magma column and outward to the conduit
periphery. |
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