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| Titel |
Experimental solidification of an andesitic melt by cooling |
| VerfasserIn |
Gianluca Iezzi, Silvio Mollo, Guglielmo Torresi, Guido Ventura, Andrea Cavallo, Piergiorgio Scarlato |
| Konferenz |
EGU General Assembly 2011
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 13 (2011) |
| Datensatznummer |
250047134
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| Zusammenfassung |
| The transition from a silicate melt to a fully solidified magmatic rock is an important phase
transformation occurring on the Earth. The melt to rock transition involves vitrification and/or
crystallization, two processes related to the melt composition and to temperature/pressure
variation. The aim of this study is to investigate the crystallization behaviour of an
andesitic melt under dynamic cooling conditions. Solidification experiments at (a) five
different cooling rates (25, 12.5, 3, 0.5 and 0.125 Ë C/min) between 1300 and 800 Ë
C, and (b) variable quenching temperatures (1100, 1000, 900 and 800 Ë C) at a
fixed cooling rate of 0.5 Ë C/min were performed on an andesitic melt (SiO2=
58.52 wt.% and Na2O + K2O = 4.43 wt.%) at air conditions from high superheating
temperature. Results show that simultaneous and duplicated experiments with Pt-wire or
Pt-capsule produce identical run-products. Preferential nucleation on Pt-containers or
bubbles is lacking. Plagioclase and Fe-Ti oxide crystals nucleate firstly from the melt.
Clinopyroxene crystals form only at lower cooling rates (0.5 and 0.125 Ë C/min) and
quenching temperatures (900 and 800 Ë C). At higher cooling rates (25, 12.5 and
3 Ë C/min) and quenching temperature (1100 Ë C), plagioclase and Fe-Ti oxide
crystals are embedded in a glassy matrix; by contrast, at lower cooling rates (0.5
and 0.125 Ë C/min) and below 1100 Ë C they form an intergrowth texture. The
crystallization of plagioclase and Fe-Ti oxide starts homogeneously and then proceeds by
heterogeneous nucleation. The crystal size distribution (CSD) analysis of plagioclase
shows that crystal coarsening increases with decreasing cooling rate and quenching
temperature. At the same time, the average growth rate of plagioclases decrease
from 2.1Ã10-6 cm/s (25 Ë C/min) to 5.7Ã10-8 cm/s (0.125 Ë C/min) and crystals
tend to be more equant in habit. Plagioclases and Fe-Ti oxides depart from their
equilibrium compositions under the effect of dynamic crystallization conditions. The
anorthite (An) content of plagioclases increases as the cooling rate and quenching
temperature increase; consequently, plagioclases shift from labradorite-andesine to
anorthite-bytownite. Fe-Ti oxides are mainly close to the magnetite and/or Ti-magnetite
end-members. However, at higher cooling rates and quenching temperature some crystals are
solid solutions with the spinel s.s. end-member. The progressive decrease of the
crystal content with increasing cooling rate and quenching temperature produces
glasses characterized by higher CaO and Al2O3 and lower SiO2 and K2O amounts.
Therefore, kinetic effects due to cooling significantly change phase compositions
with remarkable petrological implications for the solidification of andesitic lavas
and dikes. The glass-forming ability (GFA) of the andesitic melt has been also
quantified in a critical cooling rate (Rc) of ~37 Ë C/min. This value is higher than those
measured for latitic (Rc ~1 Ë C/min) and trachytic (Rc |
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