TRACE ELEMENT PARTITIONING BETWEEN PHLOGOPITE, CLINOPYROXENE AND LEUCITE LAMPROITE MELT

Abstract

Partition coefficients for Rb, Sr, Sm, Nd, U, Th, Pb, La, Ce, Ba, Nb, and Li between phlogopite, clinopyroxene and melt were determined in order to characterize the effect of metasomatically grown minerals on the isotopic evolution of the earth's lithospheric mantle. Experiments were performed in a piston-cylinder-apparatus at 15 kbar and 1040-1175°C with leucite lamproite as starting composition. Three phlogopite/glass and one clinopyroxene/glass pairs were analyzed by secondary ion mass spectrometry (SIMS). Partition coefficients (D-values) for phlogopite/melt of Rb, Cs and Ba and Ti are =<0.5. DCs is significantly lower than DRb (0.50/1.34); in contrast to these two elements, DBa varies between 0.3 and 1.6 in phlogopites of different major element composition. A similar behavior is suggested for the elements Sr (D=0.02-0.06) and Pb (D~0.02). The partition coefficients measured for REE, Th and U are in the order of 10-4-10-6, those for Zr and Nb are 0.015 and 0.07 respectively, and DLi for phlogopite and melt is 0.2. Partition coefficients for REE and Sr between clinopyroxene and melt are relatively high compared to other studies, presumably because of the high wollastonite component in the clinopyroxenes of this work. The data for clinopyroxene as well as the data for phlogopite were used to calculate crystal lattice-site parameters for the M2-site in clinopyroxene and the X-site in phlogopite. Parameters for clinopyroxene are in good agreement with data from other works. For the phlogopite interlayer site, an optimum ionic radius of 1.71 #9 and polyhedral Young's moduli of 552 kbar (1+-cations) and 1087 kbar (2+-cations) were found. Recent data and the results of this study show that phlogopite is able to store significant amounts of Rb, Cs and Ba in the lithospheric mantle, thus causing a radiogenic evolution of Sr. In contrast, the influence of phlogopite on the Nd-isotope evolution is negligible because of the very low partition coefficients for Sm and Nd. The concentrations of Sr, Nd and Sm and the evolution of Nd isotopes in metasomatized lithospheric mantle are controlled by clinopyroxene, which significantly contributes to a less radiogenic evolution of Sr relative to the one caused by phlogopite. This implies that the evolution of Sr isotopes depends on the modal proportion of phlogopite and clinopyroxene.