Experimental partitioning of high field strength and rare earth elements between clinopyroxene and garnet in andesitic to tonalitic systems

Abstract

Partition coefficients of the rare earth elements (REE) and the high field strength elements (HFSE: Ti, Zr, Hf, Nb, Ta) among clinopyroxenes, garnets, and andesitic to granodioritic melts were experimentally determined at pressures ranging from 1 to 3 GPa and temperatures between 900 and 1150°C. Natural rocks of quartz–dioritic and basaltic composition were used as starting materials. Melt compositions covered a range from dioritic to quartz–dioritic and granodioritic. Partition coefficients obtained from experiments with different doping levels at the same run condition indicate that Henry’s Law is fulfilled. Partition coefficients were investigated as a function of temperature and phase compositions. Apparent correlations between the clinopyroxene partition coefficients of the REE and degree of depolymerization expressed as nonbridging oxygens per tetrahedron (NBO/T) are due to variations in the Na contents of the clinopyroxenes which are controlled by pressure or NBO/T. Based on the model of Blundy and Wood (1994), which accounts for the strain associated with placing a cation on a particular crystallographic site when the radius of the cation differs from the optimal radius for that site, significant differences between Zr− and Hf partition coefficients as well as correlations between Ti and REE partition coefficients can be explained in terms of differences of their ionic radii. For garnets D La/D Yb ratios change as a function of temperature much more than for clinopyroxenes. This is also consistent with the Blundy and Wood (1994) model.