THE PARTITIONING OF NA BETWEEN MELILITE AND LIQUID: PART I. THE ROLE OF CRYSTAL CHEMISTRY AND LIQUID COMPOSITION

Abstract

The partitioning behavior of Na between melilite and liquid has been determined from the compositions of coexisting phases produced in slowly cooled (1°C/h) dynamic crystallization experiments on a single bulk composition corresponding to an average type B inclusion from the Allende C3V carbonaceous chondrite. Distribution coefficients (DNaMel/L) were determined over the entire range of melilite + spinel crystallization. For low Na concentrations (0.2-0.33 wt% Na2O in the melts), sodium is incompatible in gehlenitic melilite (e.g., DNaMel/L = 0.4 at XAkMel = 0.22) but compatible in melilite more magnesian than XAkMel ~ 0.50 (e.g., DNaMel/L = 1.5 at XAkMel = 0.65). Variations in DNaMel/L with composition for our experiments can be understood in terms of a simple thermodynamic model for the partitioning of Na between melilite and melt. The compositions of both melt and solid are important in determining the value of DNaMel/L and, with an empirical correction for Na2O in the melt, DNaMel/L for experimentally determined partition coefficients in this study and in the literature can be predicted with an average deviation of 15% for melts containing up to 8 wt% Na2O. The partitioning of Na between melilite and plagioclase in low-Na, Fe-free systems can also be predicted using a semiempirical model for the exchange equilibrium.