Rheological Properties of Granitic Magmas in Their Crystallization Range

Abstract

Experimental phase equilibria carried out on three natural granites are used to constrain the rheological patterns of natural cooling felsic intrusions with initial H₂O contents between 4.5 wt.% and 7 wt.% and emplacement temperatures between 920°C and 750 °C. Crystallization paths of H₂O-bearing magmas show eutectic melt fraction trends, with only 20–30 wt.% crystals after 90% of cooling. Melt viscosities increase by a factor of 2 or less during crystallization while magma viscosities remain within one order of magnitude of the initial value during 90% of the crystallization interval. This behaviour is due to the build-up in melt H₂O content during crystallization of hydrous granitic magmas that counterbalances the effects of temperature drop and increase in crystallinity on magma viscosity. Crystallization paths of H₂O-CO₂-bearing magmas show that the presence of CO₂ counteracts the lowering effect of H₂O upon viscosity, except at very low fluid/melt mass ratios. As a consequence, magma viscosities of H₂O-CO₂-bearing magmas may increase much more rapidly during crystallization, in some instances by many orders of magnitude relative to H₂O-bearing magmas. Phase equilibrium results show that oxidation may significantly increase granitic magma viscosities, especially when occuring at constant melt H₂O content. Thus, changes in redox state may affect the fluid dynamics of granitic magmas either during their ascent or at their emplacement level. In particular, diapiric granitic magmas are particularly sensitive to oxidation and may have their level of emplacement ultimately controlled by an oxidation event. In contrast, none of the oxidation mechanisms considered (H₂ diffusive loss, H₂-H₂O fluid phase fractionation and Fe²⁺-rich crystal removal) is fast enough to affect granitic magmas ascending through dykes.