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| Titel |
Oxidation-state dependence of rheology in peralkaline glasses of phonolitic composition |
| VerfasserIn |
M. C. Scherrer, K.-U. Hess, K. T. Fehr, 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 |
250063700
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| Zusammenfassung |
| The precise description of magmatic melts rheology at the glass transition is crucial in
understanding dynamic processes in volcanology. The glass transition has been described to
scale with the viscosity of the material according to Maxwell’s relaxation theory
for viscoelastic liquids (Dingwell and Webb, 1989). The temperature dependence
of the viscosity of multi-component systems can adequately be calculated using
empirical models such as Hess et al. (1996), Giordano et al. (2008) and Hui and Zhang
(2008); yet, within these calculations, the influence of oxidation state has been so far
considered minor and was consequently neglected. The rheological behavior of some
iron-rich silicate melts has shown noteworthy oxidation state-dependent variations
(Cukierman and Uhlmann 1974, Dingwell and Virgo 1987). The focus of our study is
to improve the viscosity models by investigating the necessity of an additional
redox-parameter.
Thirteen re-melted glass samples of natural phonolitic composition (peralkaline lavas
with 8.5 wt. % FeOtot) were produced under different oxygen fugacity (fO2) conditions in a
CO/CO2 gas-mixing furnace. Their oxidation-state (Fe3+/Fetot) ranges from 0.44 to
0.93 (±0.05). The viscosity above the liquidus was recorded via the concentric
cylinder technique at a constant temperature of 1186 Ë C. Additionally, viscosities
were measured in the interval of 107to 1011Pa swith temperatures up to 900 Ë C
at ambient pressure via a BAEHR micro-penetration viscometer. Glass transition
temperatures (Tg) have been determined with a constant heating/cooling rate of
10K/min on a SETARAM Sensys evo DSC using the peak of the specific heat capacity
curve.
Under a constant temperature in the super-liquidus state, the viscosity increases strongly
with increasing fO2. In the sub-liquidus state, the measured calorimetric Tgis shifted to lower
temperatures as the ratio of ferrous/total iron decreases from 638 Ë C to 610 Ë C. However,
there is no equivalent measurable effect of the oxidation-state on super-cooled melt viscosity
at Tg, within the precision of the micro-penetration experiment (all values are scattered
around 10.10 ± 0.2 Pa s). Our results show a large discrepancy of 0.5 log units
compared to the predicted viscosity at Tg using the kinetic model of Gottsmann et al.
(2002). We further investigate additional relatively iron-rich compositions aiming for
the development of an improved model for the viscosity prediction at the glass
transition.
References:
Cukierman, M. and Uhlmann D.R (1974) Effects of iron oxidation-state on viscosity,
lunar composition 15555. Journal of Geophysical Research, 79, 1594-1598.
Dingwell, D.B. and Virgo, D. (1987) The effect of oxidation-state on the viscosity of melts
in the system Na2O-FeO-Fe2O3-SiO2. Geochimica et Cosmochimica Acta, 51,
195-205.
Dingwell, D.B. and Webb, S.L. (1989) Structural relaxation in silicate melts and
non-newtonian melt rheology in geologic processes. Physics and Chemistry of Minerals, 16,
508-516.
Giordano, D.; Russel, J.K.; Dingwell, D.B. (2008) Viscosity of magmatic liquids: A
model. Earth and Planetary Science Letters, 271, 123-134.
Gottsmann, J.; Giordano, D.; Dingwell, D.B. (2002) Predicting shear viscosity during
volcanic processes at the glass transition: a calorimetric calibration. Earth and Planetary
Science Letters, 198, 417-427.
Hess, K.U. and Dingwell, D.B. (1996) Viscosities of hydrous leucogranitic melts: a
non-Arrhenian model. American Mineralogist, 81, 1297-1300.
Hui, H. and Zhang, Y. (2007) Toward a general viscosity equation for natural
anhydrous and hydrous silicate melts. Geochimica et Cosmochimica Acta, 71, 403- 416. |
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