THERMODYNAMICS OF RARE-EARTH ELEMENT PARTITIONING BETWEEN CLINOPYROXENE AND MELT IN THE SYSTEM CAO - MGO - AL2O3 - SIO2

We use the experimental data of Gaetani and Grove (1995) on rare earth element (REE) partitioning between clinopyroxene and melt in the syst em CaO-MgO-Al2O3-SiO2 to constrain the energetics of the coupled subst itution REEAl[CaSi](-1).The data show that the partition coefficients D-Yb and D-Ce and the ratio D-Yb/D-Ce increase with the Ca-Tschermaks content of the clinopyroxene. The compositional dependence of D-Yb/D-C e can be reconciled with a simple model of lattice strain around the m isfit Ce3+ and Yb3+ cations in the crystal, while the compositional de pendence of D-Yb and D-Ce can be eliminated by accounting for configur ational entropy in pyroxene and melt using ionic activity models for t he component REEMgAlSiO(6). For pyroxene we assume ideal mixing on M1 and M2 sites with local charge balance on the T sites. For melt we ado pt an ideal two-lattice mixture of cations and pyroxene-like six-oxyge n anions. The ionic approach is validated by a more rigorous determina tion of the equilibrium constant for the formation of REEMgAlSiO(6) py roxene from its molten component oxides using available thermochemical data for MgO, Al2O3 and SiO2 and Henry's law for REEO(1.5). In comple x systems the simple ionic approach provides a useful means of predict ing the compositional dependence of partition coefficients.