Trace element partitioning between phlogopite, clinopyroxene and leucite lamproite melt

Partition coefficients for Rb, Sr, Sm, Nd, U, Th, Pb, La, Ce, Ba, Nb, and L i between phlogopite, clinopyroxene and melt were determined in order to ch aracterize the effect of metasomatically grown minerals on the isotopic evo lution of the earth's lithospheric mantle. Experiments were performed in a piston-cylinder-apparatus at 15 kbar and 1040-1175 degrees C with leucite l amproite as starting composition. Three phlogopite/glass and one clinopyrox ene/glass pairs were analyzed by secondary ion mass spectrometry (SIMS). Pa rtition coefficients (D-values) for phlogopite/melt of Rb, Cs and Ba and Ti are greater than or equal to 0.5. D-Cs is significantly lower than D-Rb (0 .50/1.34); in contrast to these two elements, D-Ba varies between 0.3 and 1 .6 in phlogopites of different major element composition. A similar behavio r is suggested for the elements Sr (D = 0.02-0.06) and Pb (D approximate to 0.02). The partition coefficients measured for REE, Th and U are in the or der of 10(-4)-10(-6), those for Zr and Nb are 0.015 and 0.07 respectively, and D-Li for phlogopite and melt is 0.2. Partition coefficients for REE and Sr between clinopyroxene and melt are relatively high compared to other st udies, presumably because of the high wollastonite component in the clinopy roxenes of this work. The data for clinopyroxene as well as the data for ph logopite were used to calculate crystal lattice-site parameters for the M2- site in clinopyroxene and the X-site in phlogopite. Parameters for clinopyr oxene are in good agreement with data from other works. For the phlogopite interlayer site, an optimum ionic radius of 1.71 Angstrom and polyhedral Yo ung's moduli of 552 kbar (1(+)-cations) and 1087 kbar (2(+)-cations) were f ound. Recent data and the results of this study show that phlogopite is abl e 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 p hlogopite 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 c ontrolled by clinopyroxene, which significantly contributes to a less radio genic evolution of Sr relative to the one caused by phlogopite. This implie s that the evolution of Sr isotopes depends on the modal proportion of phlo gopite and clinopyroxene. (C) 1999 Elsevier Science B.V. All rights reserve d.