EarthRef.org Reference Database (ERR) -- Beckett & Stolper 2000

The partitioning behavior of Na between melilite and liquid has been determined from the compositions of coexisting phases produced in slowly cooled (1 degrees 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 (D-Na(Mel/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., D-Na(Mel/L) = 0.4 at X-Ak(Mel) = 0.22) but compatible in melilite more magnesian than X-Ak(Mel) similar to 0.50 (e.g., D-NA(MeL) = 1.5 at X-Ak(Mel) = 0.65). Variations in D-Na(Mel/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 D-Na(Mel/L) and, with an empirical correction for Na2O in the melt, D-Na(Mel/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. Copyright (C) 2000 Elsevier Science Ltd.