PARTITIONING OF RARE-EARTH ELEMENTS BETWEEN CLINOPYROXENE AND SILICATE MELT - CRYSTAL-CHEMICAL CONTROLS

This study explores some of the effects of major element compositional variations on the partitioning of trivalent rare earth elements between high-Ca clinopyroxene and silicate melt, Experiments performed in the system CaO-MgO-Al2O3-SiO2 at 1 atm pressure, over a small temperature range, differentiate crystal-chemical controls on mineral/melt partitioning from the effects of pressure and temperature. The experimental results demonstrate that the Ca-Tschermakite content of high-Ca clinopyroxene exercises an important control on rare earth element partitioning for pyroxene coexisting with basaltic melt, A comparison of our experimental results with those from two-liquid partitioning and thermal diffusion studies demonstrates that melt structure has only a minor influence on clinopyroxene/melt partitioning for basaltic compositions, but becomes progressively more important as polymerization of the melt increases. Melt structure exercises the dominant control on partitioning for highly polymerized melts such as high-silica rhyolites, Semiempirical expressions developed using equilibrium constants for pyroxene/melt exchange reactions successfully predict the partitioning of Ce3+ and Yb3+ for a broad range of synthetic and natural coexisting melt and clinopyroxene compositions. The ability of our model to predict the partitioning behavior of trivalent rare earth elements over a wide range of experimental conditions (0.0001-3.0 GPa; 1234-1430 degrees C) indicates that consideration of the compositions of coexisting clinopyroxene and melt is adequate to account for the effects of varying pressure and temperature.