EVIDENCE FOR PRESSURE-DEPENDENCE OF THE PEAK POSITION IN THE REE MINERAL MELT PARTITION PATTERNS OF CLINOPYROXENE

Major element compositions, REE abundances, and strontium and neodymium isotope ratios were measured for three megacrysts of garnet, five of clinopyroxene, one of amphibole and the host alkali basalts from two volcanoes in northeastern China. Strontium and neodymium isotopic data and major elements (Fe and Mg) indicate a cognate origin of megacrysts and host lavas. Logarithmic values of REE partition coefficients for these minerals were found to vary parabolically with ionic radius, and hence the REEs in these three minerals are considered to substitute primarily into only one lattice site of each mineral. The ionic radius (optimum radius) defined by the position of peak top of the parabola is variable. Based on the REE partition coefficient data for both experimental and naturally existing clinopyroxene/melt pairs, the change of the optimum radius may be interpreted to be mainly pressure dependent.