Changes in viscosity and structure of melt of NaAlSi₂O₆composition at high pressures

Abstract

The viscosities of melts of NaAlSi2O6 (Jadeite) and Na2Si3O7 compositions have been determined at pressures between 5 and 24 kbar, using the falling sphere method and graphite capsules 10 mm long in solid media piston-cylinder apparatus. The viscosity of NaAlSi2O6 melt decreases from 3.4×104 P at 5 kbar to 5.3×103 P at 24 kbar at 1350°C, whereas that of Na2Si3O7 melt decreases by a factor of less than 3 from 1 atm to 20 kbar at 1175°C. These results indicate that the remarkable decrease in viscosity of NaAlSi2O6 melt with increasing pressure is largely due to the presence of Al in the melt. It is most probable that a part of Al in the melt changes from four- to six-fold coordination at high pressures. The density of glass Of NaAlSi2O6 composition increases with increasing pressure of quenching from 2.42 g/c3 at 5 kbar to 2.58 at 21 kbar, indicating that the melt also changes to a denser structure with the coordination change of Al at high pressures. It is suggested that most magmas undergo similar structural changes in the upper mantle and have higher density and lower viscosity at greater depths.