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Titel |
Crystal clustering and non-Newtonian rheology of low-viscosity crystal-poor magmas |
VerfasserIn |
Silvia Campagnola, Alessandro Vona, Claudia Romano, Guido Giordano |
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 |
250129577
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Publikation (Nr.) |
EGU/EGU2016-9709.pdf |
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Zusammenfassung |
We have investigated the rheology of liquid and crystal-bearing tephriphonolite magmas from
the Colli Albani volcanic district. High (1124 - 1569 ˚ C) and low (690 - 800 ˚ C)
temperature anhydrous liquid viscosities were determined by a combination of
concentric cylinder (101.0 to 103.6 Pa s) and micropenetration (109.2 to 1012.1 Pa s)
viscometry. Comparison with literature data reveals that at high temperatures, viscosity
seems to be related to the melts degree of polymerization (NBO/T), while at low
temperatures the dependency is not linear with values of viscosity higher than expected.
Subliquidus isothermal crystallization experiments and viscosity determinations were
carried out at high temperature (1150 - 1240 ˚ C) in air using a concentric cylinder
apparatus at constant shear strain rate (γ’ = 0.1 s−1). The overall crystal fraction
varies between φ = 0.06 at 1240 ˚ C (leucite) and φ = 0.34 at 1150 ˚ C (leucite φ =
0.32 + plagioclase φ = 0.02), with a direct linear increase of crystal content with
decreasing temperature which parallels the viscosity increase. The inspection of
products quenched at the end of the crystallization stage, defined when viscosity
reaches a constant value, reveals strong evidence of leucite clustering. After the first
segment of the experiment, performed at a constant shear rate, a second stage of
experiments at variable shear rate was performed, comprised of an up-ramp (γ’ =
0.1 - 0.9 s−1) and a down-ramp (γ’= 0.9 - 0.1 s−1) segment. At the end of the
down-ramp, leucite crystals appear sub-spherical and unclustered. For the same
applied shear rate, the viscosity values of the up-ramp are not recovered within the
experimental time-scale, indicating strain and strain-rate dependent rheology for these
suspensions. While the down-ramp viscosity data are shown to be in perfect agreement
with literature models, discrepancies between the up-ramp data and pre-existing
predicting models have been observed. We suggest that this complex behavior is
related to the clustering of leucite crystals during the crystallization process, and
their breakage during the higher shear rate viscosity measurements. The kinetics of
crystallization (in terms of nucleation and growth rates) seems to be strongly affected by the
degree of undercooling and dynamic stirring conditions. We conclude that any
modeling of tephriphonolite magma flow (either in conduit or subaerial) should take
into account the effects of strain, strain-rate and internal textures (i.e. clustering). |
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