Granitic melt viscosity and dike formation

Abstract

Melt viscosity is an important variable controlling the rates of granitic melt separation and transport from its source region. Viscosities of granitic melts are significantly affected by changes in temperature and volatile content. Addition of 2 wt% H 2O to anhydrous peraluminous and metaluminous granitic melts reduces viscosity by six orders of magnitude at 800°C which is equivalent to the effect of raising the temperature by 500°C. Addition of 10 wt% more H 2O only decreases melt viscosity by approximately 1.5 additional orders of magnitude. At higher temperatures the effect of H 2O addition on melt viscosity is less significant, at lower temperatures more significant. The effect of pressure on hydrous granitic melt viscosities at crustal conditions is insignificant in comparison. Between 0.5 and 1.5 GPa the viscosity at 1000°C increases by 0.4 log units, or less; this effect can be ignored in most petrological calculations. Using newly acquired viscosity data at low H 2O contents, the Baker model for the calculation of metaluminous and peraluminous granitic melt viscosities has been updated and expanded. The new model can predict hydrous (0.3–12.3 wt% H 2O), granitic (69–77 wt% SiO 2) melt viscosities between 577 and 1200°C at crustal pressures. The new granitic melt viscosities have been used to investigate the possibility of granitic melt transport via diking. Diking can be an efficient mechanism of damp granitic melt transport through the crust for granitic melts containing a minimum of 2 wt% H 2O if the lower crust has been sufficiently heated. Modeling the thermal history of basaltic intrusions and surrounding lower crust demonstrates that a sheet-like intrusion 5 km thick, or a suite of intrusions with the same aggregate thickness that were rapidly emplaced, can provide enough heat to allow granitic magmas to migrate through much of the crust in dikes. The generation of granitic dikes associated with thinner intrusions appears unlikely.