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Phase equilibrium constraints on the viscosity of silicic magmas II: implications for mafic–silicic mixing processes

Published online by Cambridge University Press:  03 November 2011

Bruno Scaillet
Affiliation:
Institut des Sciences de la Terre d'Orléans, CNRS. 1A rue de la Férollerie, 45071 Orléans cedex 02, France; e-mail:[email protected]
Alan Whittington*
Affiliation:
IPG, 4 place Jussieu, Paris cedex 5, 75252, France
Caroline Martel
Affiliation:
Bayerisches Geoinstitut, Universität Bayreuth, 95440 Bayreuth, Germany; and Institut des Sciences de la Terre d'Orléans, CNRS, 1A rue de la Férollerie, 45071 Orléans cedex 02, France
Michel Pichavant
Affiliation:
Institut des Sciences de la Terre d'Orléans, CNRS, 1A rue de la Férollerie, 45071 Orléans cedex 02, France
François Holtz
Affiliation:
Institut für Mineralogie, Welfengarten 1, D-30167 Hannover, Germany
*
Present address: Department of Geology, University of Illinois, 1301 W. Green Street, Urbana, IL 61801, USA

Abstract

Isobaric crystallisation paths obtained from phase equilibrium experiments show that, whereas in rhyolitic compositions melt fraction trends are distinctly eutectic, dacitic and more mafic compositions have their crystallinities linearly correlated with temperatures. As a consequence, the viscosities of the latter continuously increase on cooling, whereas for the former they remain constant or even decrease during 80% of the crystallisation interval, which opens new perspectives for the fluid dynamical modelling of felsic magma chambers. Given the typical dyke widths observed for basaltic magmas, results of analogue modelling predict that injection of mafic magmas into crystallising intermediate to silicic plutons under pre-eruption conditions cannot yield homogeneous composition. Homogenisation can occur, however, if injection takes place in the early stages of magmatic evolution (i.e. at near liquidus conditions) but only in magmas of dacitic or more mafic composition. More generally, the potential for efficient mixing between silicic and mafic magmas sharing large interfaces at upper crustal levels is greater for dry basalts than for wet ones. At the other extreme, small mafic enclaves found in many granitoids behave essentially as rigid objects during a substantial part of the crystallisation interval of the host magmas, which implies that finite strain analyses carried out on such markers can give only a minimum estimate of the total amount of strain experienced by the host pluton. Mafic enclaves carried by granitic magmas behave as passive markers only at near solidus conditions, typically when the host granitic magma shows near-solid behaviour. Thus they cannot be used as fossil indicators of direction of magmatic flow.

Type
Research Article
Copyright
Copyright © Royal Society of Edinburgh 2000

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