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

Isobaric crystallisation paths obtained from phase equilibrium experiments show that, whereas in rhyolitic compositions melt fraction trends are disti nctly eutectic, dacitic and more mafic compositions have their crystallinit ies linearly correlated with temperatures. As a consequence, the viscositie s of the latter continuously increase on cooling, whereas for the former th ey remain constant or even decrease during 80% of the crystallisation inter val, which opens new perspectives for the fluid dynamical modelling of fels ic magma chambers. Given the typical dyke widths observed for basaltic magm as, results of analogue modelling predict that injection of mafic magmas in to crystallising intermediate to silicic plutons under pre-eruption conditi ons 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 co mposition. More generally, the potential for efficient mixing between silic ic and mafic magmas sharing large interfaces at upper crustal levels is gre ater for dry basalts than for wet ones. At the other extreme, small mafic e nclaves found in many granitoids behave essentially as rigid objects during a substantial part of the crystallisation interval of the host magmas, whi ch 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 h ost pluton. Mafic enclaves carried by granitic magmas behave as passive mar kers only at near solidus conditions, typically when the host granitic magm a shows near-solid behaviour. Thus they cannot be used as fossil indicators of direction of magmatic flow.