SODIUM PARTITIONING BETWEEN CLINOPYROXENE AND SILICATE MELTS

We present a quantitative model for Na partitioning between clinopyrox ene and silicate melt, applicable in the range 0.1 MPa to 6.0 GPa, 100 0-2000 degrees C over a wide span of composition. Our model is derived from the jadeite melting equilibrium, via the relationship Delta G(f( P,T)) = RT1nK(f) = RT1nD(Na), where Delta G(f(P,T)) is the Gibbs free energy of fusion of jadeite at the pressure (P) and temperature (T) o f interest, T is in kelvins, R is the gas constant, K-f is the equilib rium constant for the the melting reaction and D-Na, is a molar parti tion coefficient, defined here as the molar ratio of Na in crystal to Na in melt on a six-oxygen basis. An expression for Delta G(f(P,T)) is obtained from published experimental data on the jadeite melting curv e from 2.8 to 16.5 GPa, combined with available (or estimated) thermoc hemical data for jadeite crystal and jadeite melt. The model is tested against: (1) new experimental data in the system diopside-albite from 0.1 MPa to 6.0 GPa; (2) new analyses of clinopyroxene and glass from the mid-ocean ridge basalt-pyrolite sandwich experiments of Falloon an d Green (1988); and (3) published experimental data from natural and s ynthetic systems. The model is in good agreement with the experimental data over the entire P-T range investigated. The only systematic devi ations occur in oxidizing systems (f(O2)FMQ+1), due to stabilization o f the NaFe3+Si2O6 (acmite) component in clinopyroxene, and on the diop side-albite join at 0.1 MPa. The latter can be quantitatively ascribed to nonideality of melts on the diopside-silica join in the system CaM gSi2O6-NaAlSi2O6-Si3O6. The activity-composition relationships adopted for crystal and melt do not require prior knowledge of the aluminum c ontent of either phase. As D-Na is typically within 3% relative of th e weight partition coefficient (D-Na) our thermodynamic model provides a quantitative description of Na partitioning between clinopyroxene a nd melt in the upper mantle.