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| Titel |
Transient rheology of crystallizing andesitic magmas |
| VerfasserIn |
L. J. de Biasi, M. O. Chevrel, J. B. Hanson, C. Cimarelli, Y. Lavallée, D. B. Dingwell |
| Konferenz |
EGU General Assembly 2012
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 14 (2012) |
| Datensatznummer |
250060473
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| Zusammenfassung |
| The viscosity of magma strongly influences its rheological behaviour, which is a key
determinant of magma transport processes and volcanic eruptions. Understanding the factors
controlling the viscosity of magma is important to our assessment of hazards posed by active
volcanoes. In nature, magmas span a very wide range in viscosity (10-1 to 1014 Pa s),
depending on chemical composition (including volatile content), temperature, and
importantly, crystal fraction, which further induces a complex strain rate dependence (i.e.
non-Newtonian rheology).
Here, we present results of transient viscosities of a crystallizing andesitic melt (57 wt.%
SiO2) from Tungurahua volcano (Ecuador). We followed the experimental method developed
by Vona et al. (2011) for the concentric cylinder apparatus, but optimized its implementation
by leaving the spindle in situ before quenching the experimental products, to preserve the
complete developed texture of the sample. The viscosity is investigated under super-liquidus
(1400 Ë C) and sub-liquidus temperatures (1162 and 1167 Ë C). For each temperature
increment, thermal equilibrium is achieved over a period of days while the spindle
constantly stirs the magma. Simultaneous monitoring of the torque is used to calculate
the apparent viscosity of the transient suspension. To get a better understanding
of the nucleation and crystal growth processes that are involved at sub-liquidus
conditions, further time-step experiments were carried out, where the samples were
quenched at various equilibration stages. The mineralogical assemblage, as well as the
crystal fraction, distribution and preferential alignment were then quantitatively
analyzed.
At temperatures below the liquidus, the suspension shows a progressive, but irregular
increase of the relative shear viscosity. First, the viscosity slightly increases, possibly due to
the crystallization of small, equant oxides and the formation of plagioclase nuclei.
After some time (1.5-2.5 days), crystallization of large, tabular plagioclase begins,
inducing a significantly stronger increase in apparent viscosity until thermo-chemical
equilibration is achieved. After continued stirring the apparent viscosity slightly decreases,
likely due to increasing crystal alignment. The analysis of pre-equilibrium quenched
samples indicates that crystals nucleate and grow preferentially in proximity to both
the spindle and the crucible wall. Furthermore, decreasing the stirring rate (aka
strain rate) results in an increase in the apparent viscosity, which evidences the
non-Newtonian characteristics of the magmatic suspension. In conclusion, these experiments
indicate that natural andesitic magmas undergo significant rheological changes at
the onset of crystallization. The observed thermo-chemical variations elucidate
a transient viscosity, which deserves consideration into all problems of magma
transport.
Reference:
Vona, A., Romano, C., Dingwell, D.B., Giordano, D. 2011. The rheology of
crystal-bearing basaltic magmas from Stromboli and Etna. Geochim. Cosmochim. Acta, 75,
3214-3236. |
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