The effects of silica and water on the viscosity of hydrous quartzofeldspathic melts
Résumé
The viscosities of hydrous melts (0.65 to 2.8 wt% H2O) with quartzofeldspathic compositions corresponding to Ab, Ab74Qz26, and Ab48Qz52 (mole proportions calculated on the basis of eight oxygen atoms; Ab 5 NaAlSi3O8, Qz 5 Si4O8) have been determined between 980 and 1375 8C at pressures between 190 and 360 MPa using the falling sphere technique. The use of large bubble-free hydrous glass cylinders (placed in internally heated pressure vessels) previously prepared and already containing markers and platinum spheres allows falling distances up to several centimeters to be measured with a precision of 650 to 200 mm. This results in a precision of 615% relative or less for most viscosity data (610% relative or less if the temperature is known within 65 8C). For a water content of 2.8 wt% H2O, viscosity increases with increasing Qz content. In the investigated viscosity range, no significant deviation from Arrhenian behavior is observed and the activation energy of viscous flow increases slightly with decreasing water content of the melt (for Ab). Combining the experimental data obtained in this study with data for a haplogranitic composition investigated previously by Schulze et al. (1996) shows that the viscosities, and hence, the activation energies of viscous flow are similar for compositions with the same atom ratio (Si 1 Al)/(H 1 Na 1 K) (SA/HNK). Thus, melt viscosity is constant if Al, charge balanced by Na or K, is exchanged with Si 1 H (H incorporated as OH or H2O). The viscosities (in dPa·s) of all investigated hydrous haplogranite compositions with water contents ranging between 0.7 and 8.2 wt% H2O can be calculated to better than 60.15 log units using the expression: logh 5 21.8 1 [940 1 5598·(SA/HNK)0.3774]·1/T where T is expressed in Kelvin and varies from 1073 to 1650 K.
Domaines
Volcanologie
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