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 betwe en high-Ca clinopyroxene and silicate melt, Experiments performed in t he system CaO-MgO-Al2O3-SiO2 at 1 atm pressure, over a small temperatu re range, differentiate crystal-chemical controls on mineral/melt part itioning from the effects of pressure and temperature. The experimenta l results demonstrate that the Ca-Tschermakite content of high-Ca clin opyroxene exercises an important control on rare earth element partiti oning for pyroxene coexisting with basaltic melt, A comparison of our experimental results with those from two-liquid partitioning and therm al diffusion studies demonstrates that melt structure has only a minor influence on clinopyroxene/melt partitioning for basaltic composition s, but becomes progressively more important as polymerization of the m elt increases. Melt structure exercises the dominant control on partit ioning for highly polymerized melts such as high-silica rhyolites, Sem iempirical expressions developed using equilibrium constants for pyrox ene/melt exchange reactions successfully predict the partitioning of C e3+ and Yb3+ for a broad range of synthetic and natural coexisting mel t 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 clino pyroxene and melt is adequate to account for the effects of varying pr essure and temperature.