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| Titel |
Experiments on the rheology of vesicle-bearing magmas |
| VerfasserIn |
Alessandro Vona, Amy G. Ryan, James K. Russell, Claudia Romano |
| 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 |
250129480
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| Publikation (Nr.) |
 EGU/EGU2016-9602.pdf |
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| Zusammenfassung |
| We present a series of high temperature uniaxial deformation experiments designed to
investigate the effect of bubbles on the magma bulk viscosity. Starting materials having
variable vesicularity (φ = 0 - 66%) were synthesized by high-temperature foaming (T = 900 -
1050 ˚ C and P = 1 bar) of cores of natural rhyolitic obsidian from Hrafntinnuhryggur,
Krafla, Iceland. These cores were subsequently deformed using a high-temperature uniaxial
press at dry atmospheric conditions. Each experiment involved deforming vesicle-bearing
cores isothermally (T = 750 ˚ C), at constant displacement rates (strain rates between 0.5-1 x
10−4 s−1), and to total strains (ɛ) of 10–40%. The viscosity of the bubble-free melt (η0)
was measured by micropenetration and parallel plate methods and establishes a
baseline for comparing data derived from experiments on vesicle rich cores. At the
experimental conditions, the presence of vesicles has a major impact on the rheological
response, producing a marked decrease of bulk viscosity (maximum decrease of 2
log units Pa s) that is best described by a two-parameter empirical equation: log
ηBulk = log η0 – 1.47 * [φ/(1-φ)]0.48. Our model provides a means to compare the
diverse behaviour of vesicle-bearing melts reported in the literature and reflecting
material properties (e.g., analogue vs. natural), geometry and distribution of pores (e.g.
foamed/natural vs. unconsolidated/sintered materials), and flow regime. Lastly, we apply
principles of Maxwell relaxation theory, combined with our parameterization of
bubble-melt rheology, to map the potential onset of non-Newtonian behaviour (strain
localization) in vesiculated magmas and lavas as a function of melt viscosity, vesicularity,
strain rate, and geological condition. Increasing vesicularity in magmas can initiate
non-Newtonian behaviour at constant strain rates. Lower melt viscosity sustains
homogeneous Newtonian flow in vesiculated magmas even at relatively high strain rates. |
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