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3.6 h. Hydrous intermediate granitic (sensu lato) melt viscosities: Expansion of the hydrous melt viscosity database and model to intermediate compositions (K.-U. Hess, D.B. Dingwell and C. Romano/Rome)

The compositions of calcalkaline magmas, representing the most common forms of intrusion in the continental crust, range from rhyolites to gabbros. The most volumetrically important plutonic intrusions consist of granodiorites, monzonites and diorites. In order to parameterise the viscosities of these magmas within the Earth´s continental crust, the database for the viscosities of hydrous granitic melts has been extended in compositional range to include significant normative anorthite. The addition of the anorthite component to the haplogranitic system has been discussed for the dry system in last years Annual Report. In order to incorporate the levels of water content anticipated in intermediate intrusive complexes we have now investigated hydrous melt viscosities on the compositional join HPG8-anorthite. Melts with anorthite contents of 10 and 20 wt% have been investigated. Samples were hydrated in piston cylinder runs that were rapidly quenched. The water contents, determined by infrared spectroscopy, are in the range of 1-2 wt%. Viscosities have been determined using a micropenetration method. The stability boundaries of these glasses are more stringent than the equivalent hydrous HGP8 melts resulting in a smaller measurable viscosity range. The viscosities of the hydrous HPG810An and HPG820An melts are plotted in Fig. 3.6-9 and compared with the viscosities of the dry HPG810An and HPG820An melts. A typically strong nonlinear decrease in melt viscosity with added water content is observed. For comparison, the data for equivalent HPG8 melts, wet and dry, are also shown in Fig. 3.6-9.

Fig. 3.6-9: The influence of water on the viscosities of calcalkaline melts. The data are presented as log viscosity (Pa s) as a function of inverse temperature. Note that the viscosities of the hydrous intermediate melts (HPG8An10-triangle, HPG8An20-square) are very similar to the hydrous anorthite-free melt (HPG8-circle).

It is recalled and can be seen in Fig. 3.6-9, that the addition of An component to HPG8 has a minimal effect on melt viscosity at high viscosities. Extremely An-rich melts have higher viscosities than HPG8 in the high viscosity range. The addition of water yields a decrease in melt viscosity that is smaller for the An-bearing melts than for HPG8 melts with the result that the initial (dry) viscosity difference between HPG8 and HPG8 with An additions of 10 and 20 wt%, is apparently not preserved in the presence of 1 to 2 wt% H2O. Thus, in the high viscosity range, the viscosities of intermediate granitic melts appear to be well approximated by the existing model for hydrous calcalkaline melts (Hess and Dingwell, 1996). This comparison will be refined and extended in the next months. If the approximation survives further tests then the prediction of calcalkaline granitic melt viscosities is a step closer in simplicity. It will be very important to test the approximation at high temperatures and low viscosities where very different behavior for dry An-bearing melts is observed.

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