Liquids, Glasses, and the Melting of Silicates to High Pressures

Abstract

Processes involving melting have been of fundamental importance in the origin and evolution of the Earth and other terrestrial planets, but thermodynamic data and physical properties of liquids necessary to calculate phase relationships are not well known, particularly at high pressures. We present revised fusion curves of the silicate diopside and the aluminosilicate albite to 30 kbars. The fusion curve of diopside is in agreement with that calculated with the Clapeyron equation. Conversely, we were able to calculate the fusion curve of albite only by considering the effects of pressure and temperature in lowering the activity of NaAlSi3O8 component in the liquid, which presumably results from partial dissociation of this component into other species. Our experimental data corroborate that thermodynamic and, presumably, structural changes occur in aluminosilicate liquids with changes in pressure as well as with temperature, and they provide evidence that the metastable persistence of high-enthalpy structures in aluminosilicate glasses quenched from temperatures above the solidus produces glasses that are unreliable models of liquids in stable equilibrium with crystalline solids. These experiments also reveal for the first time that metastable aluminosilicate glasses prepared at very high temperatures can persist when reheated above the liquidus temperature and fail to convert instantly to the structure of the stable liquid. These observations provide a basis for extracting thermodynamic data for liquids presently unobtainable by other methods.