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Phase equilibrium constraints on the viscosity of silicic magmas with implications for mafic-silicic mixing processes

Abstract : Isobaric crystallization paths obtained from phase equilibrium experiments show that, whereas in rhyolitic compositions melt fraction trends are distinctly eutectic-like, dacitic and more mafic compositions have their crystallinities linearly correlated with temperature. 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 modeling of felsic magma chambers. Given the typical dike widths observed for basaltic magmas, results of analogue modelling predict that injection of mafic magmas into crystallizing intermediate to silicic plutons under pre-eruption conditions cannot yield homogeneous composition. Homogenization 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 crystallization 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 behavior. Thus they cannot be used as fossil indicators of direction of magmatic flow.
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Bruno Scaillet, Alan Whittington, Caroline Martel, Michel Pichavant, François Holtz. Phase equilibrium constraints on the viscosity of silicic magmas with implications for mafic-silicic mixing processes. Transactions of the Royal Society of Edinburgh : Earth Sciences, 2000, 91, pp.61-72. ⟨10.1130/0-8137-2350-7.61⟩. ⟨hal-00115221⟩

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