The effects of silica and water on the viscosity of hydrous quartzofeldspathic melts

The viscosities of hydrous melts (0.65 to 2.8 wt% H2O) with quartzofeldspat hic compositions corresponding to Ab, Ab(74)Qz(26), and Ab(48)Qz(52) (mole proportions calculated on the basis of eight oxygen atoms; Ab = NaAlSi3O8, Oz = Si4O8) have been determined between 980 and 1375 degrees C at pressure s between 190 and 360 MPa using the falling sphere technique. The use of la rge bubble-free hydrous glass cylinders (placed in internally heated pressu re vessels) previously prepared and already containing markers and platinum spheres allows falling distances up to several centimeters to be measured with a precision of +/-50 to 200 mu-m This results in a precision of +/-15% relative or less for most viscosity data (+/-10% relative or less if the t emperature is known within +/-5 degrees C). For a water content of 2.8 wt% H2O, viscosity increases with increasing Oz content. In the investigated viscosity range, no significant deviation from Arrhenian behavior is observed and the activation energy of viscous flow i ncreases slightly with decreasing water content of the melt (for Ab). Combi ning the experimental data obtained in this study with data for a haplogran itic composition investigated previously by Schulze et al. (1996) shows tha t the viscosities, and hence, the activation energies of viscous flow are s imilar for compositions with the same atom ratio (Si + AI)/(H + Na + K) (SA /HNK). Thus, melt viscosity is constant if Al, charge balanced by Na or K, is exchanged with Si + H (H incorporated as OH or H2O). The viscosities (in dPa.s) of all investigated hydrous haplogranite compositions with water co ntents ranging between 0.7 and 8.2 wt% H2O can be calculated to better than +/-0.15 log units using the expression: log eta = -1.8 + [940 + 5598.(SA/HNK)(0.3774)].1/T where T is expressed in Kelvin and varies from 1073 to 1650 K.