We present a quantitative model for Na partitioning between clinopyroxene and silicate melt, applicable in the range 0.1 MPa to 6.0 GPa, 1000-2000øC over a wide span of composition. Our model is derived from the jadeite melting equilibrium, via the relationship AGf(p,r) = -RTInKf= RTInDNa where AGf(p,r) is the Gibbs free energy of fusion of jadeite at the pressure (P) and temperature (T) of interest, T is in kelvins, R is the gas constant, Kfis the equilibrium constant for the the melting reaction and DNa is a molar partition coefficient, defined here as the molar ratio of Na in crystal to Na in melt on a six-oxygen basis. An expression for AGf(p,r)is obtained from published experimental data on the jadeite melting curve from 2.8 to 16.5 GPa, combined with available (or estimated) thermochemical 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 and Green (1988); and (3) published experimental data from natural and synthetic systems. The model is in good agreement with the experimental data over the entire P-T range investigated. The only systematic deviations occur in oxidizing systems 0Co2>FMQ+ 1), due to stabilization of the NaFe3+Si206 (acmite) component in clinopyroxene, and on the diopside-albite join at 0.1 MPa. The latter can be quantitatively ascribed to nonideality of melts on the diopside-silica join in the system CaMgSi206-NaA1Si206-Si30 6. The activity-composition relationships adopted for c, rystal and melt do not require prior knowledge of the aluminum content of either phase. As DNa is typically within 3 % relative of the weight partition coefficient (DNa) our thermodynamic model provides a quantitative description of Na partitioning between clinopyroxene and melt in the upper mantle. Recent advances in our understanding of melt extraction from the mantle [McKenzie, 1984] and the global geochemical systematics of mid-ocean ridge basalts (MORB)[Klein and Langmuir, 1987; Langmuir et al., 1992] have highlighted the need to fully understand the variation in composition of small degree melt fractions with pressure (P), temperature (T), bulk composition and volatile content. An element of particular significance is Na, as (1) its abundance in mantle melts is sensitive to the degree of melting and hence it displays significant variation in MORB; (2) the phase equilbria governing mantle melting are themselves sensitive to the Na content of the melt [Kinzler and Grove, 1992; Langmuir et al., 1992]; and (3) at the concentrations present in most basalt glasses Na is readily analyzable by electron-microprobe, thereby providing an extensive source of chemical data. Klein and Langmuir [1987] have demonstrated that MORB suites, when corrected for the effects of crystal fractionation, show systematic correlations between Na20 and FeO contents, Paper number 95JB00954. 0148-0227/95/95 JB -00954505.00...
