The albite fusion curve re-examined: New experiments and the high-pressure density and compressibility of high albite and NaAlSi₃O₈ liquid

Abstract

Experimental brackets on the melting temperature of high albite (NaAlSi₃O₈) were determined at 2.33 ± 0.03 GPa (1360-1370 °C) and 2.79 ± 0.03 GPa (1370-1389 °C) in a piston-cylinder apparatus. All run products that quenched to a glass were analyzed by Fourier-transform infrared spectroscopy and found to contain ≤500 ppm H₂O. In addition, new X-ray diffraction experiments on fully disordered albite are reported to 7.6 GPa; the fitted results lead to a zero-pressure bulk modulus (K₀) of 56.4 ± 0.7 and a pressure derivative (K₀′) of 3.9 ± 0.3 in a third-order Birch-Murnaghan equation of state. Revised values for the enthalpy and entropy of fusion of high albite at one bar and 1100 °C [Δ_HTf = 64.5 ± 2.1 kJ/mol and Δ_STf = 47.0 J/(mol·K)] are recommended on the basis of improved heat capacity equations for NaAlSi₃O₈ glass and liquid. On the basis of these new results on the fusion curve and thermodynamic data for high albite, the pressure dependence of the NaAlSi₃O₈ liquid compressibility (K₀′) is constrained to be 10.8 ± 1.5 in a third-order Birch-Murnaghan equation of state. The uncertainty in K₀ ′ of ±1.5 contributes an error to melt density at 3 GPa (2.543 ± 0.010 g/cm3 at 1500 °C) of ±0.4%.