THE FUSION CURVE OF ALBITE REVISITED AND THE COMPRESSIBILITY OF NAALSI3O8 LIQUID WITH PRESSURE

Abstract

The I bar, thermodynamic properties of crystalline and liquid NaAlSi308 are used to calculate the fusion curve of albite to 10 kbar. The calculated temperatures (± 2sigma) of the melting reaction are 1152 (± 7) degrees C at 3 kbar and 1247 (± 24) degrees Cat 10 kbar. The location of the calculated fusion curve to 10 kbar is in excellent agreement with phase-equilibrium constraints on the maximum and minimum temperatures of the fusion curve at 3 and 15 kbar, respectively. Calculation of the melting reaction at pressures >10 kbar requires that the pressure dependence of the liquid compressibility (K0 = dKT,O/ dP, where KT,O = 1BetaT,0) be known. On the basis of five half-reversal, crystallization experiments in the literature, which collectively provide minimum temperatures of the fusion curve between 12 and 32 kbar, KO (derived from the Birch-Murnaghan relation) is constrained to be greater or more than 10 for liquid NaAlSi308A comparison with other silicate liquids shows that there is a strong, positive correlation between the compressibility at one bar (BetaT,O) and K0. In addition, data on the water-saturated fusion curve of albite are used to quantify the effect of small amounts of H20 (less than or equal to 1 wt%) on lowering the melting temperature of albite (less than or equal to 68 degrees).