The low viscosities of F + H2O-bearing granitic melts and implications for melt extraction and transport

Melt viscosity affects virtually all aspects of igneous petrogenesis. Although addition of H2O is known to lower melt viscosities, addition of fluorine is controversial and may raise them. Because these two volatiles most commonly reach their highest concentrations in granitic rocks, the viscosities of peralkaline, strongly peraluminous, and weakly peraluminous granitic melts with approximately 1.5 wt% F and 6 wt% H2O were measured to investigate the combined effects of H2O and F on granitic melt viscosities when those volatiles are present at natural concentration levels. Viscosities were measured between 800 and 1000° C at 1.0 GPa with the falling sphere technique in a piston-cylinder apparatus. Viscosities of all melts studied behave in an Arrhenius manner and can be described by the following equations: Peralkaline η = 2.70 ± 0.21 × 10 − 6 exp ⁡ ( 181.9 ± 1.0 / RT ) Metalminous η = 4.54 ± 0.44 × 10 − 7 exp ⁡ ( 213.8 ± 1.0 / RT ) Peraluminous η = 2.60 ± 0.25 × 10 − 8 exp ⁡ ( 242.8 ± 1.0 / RT ) where viscosity is in units of Pa s and the activation energy is in kJ/mol;R is the gas constant, in J mol−1 K−1, andT is temperature in Kelvin. Uncertainties are 1 standard errors based upon estimated uncertainties, ±5 relative %, in the viscosity measurements. Viscosities of these melts with high volatile contents can be well estimated with existing viscosity models. Viscosities of the peraluminous melts studied are similar to those of hydrous granitic melts containing water concentrations equal to the total of H2O + F in the melts studied. All volatile-bearing per- and metaluminous granitic melts display a rapid (6 to 7 orders of magnitude) drop in viscosity with the first 3 wt% volatiles added followed by only an additional factor of two decrease in viscosity as the volatile concentration is increased from 3 to 12 wt%. The peralkaline melt displays viscosities significantly below those of all other volatile-bearing, per- and metaluminous melts previously studied. The lower viscosities of peralkaline granitic melts allow them to separate more rapidly from their source regions, be transported through conduits at a velocity approximately an order of magnitude faster, and experience more efficient crystallization-fractionation than per- and metaluminous granitic melts.