SELECTED TRACE AND MINOR ELEMENT PARTITIONING BETWEEN PERIDOTITE MINERALS AND CARBONATITE MELTS AT 18-46 KB PRESSURE

Analysis of silicate minerals equilibrated at high pressure with silic ate- (clinopyroxene) and carbonatite-melts (olivine, orthopyroxene, cl inopyroxene, garnet, phlogopite) using proton- and electron-microprobe techniques, enabled the measurement of single mineral/melt partition coefficients for elements in minor and trace abundances. For carbonati te melt-olivine (18 kb), D-Ti (<0.05), D-Nb (0.034), D-Zr (0.036), D-T a (<0.06), D-y (<0.03), D-Sr (0.02); carbonatite melt-orthopyroxene (4 6 kb), D-Ti (0.13) D-Nb (<0.01), D-Zr (0.17), D-Ta (<0.01), D-y (0.18) , D-Lu (0.52), D-Sr (0.053); carbonatite melt-garnet (34 and 46 kb), D -Ti (1.2-0.99), D-Nb (0.086-0.014), D-Zr (1.42-0.83), D-Ta (0.14-.051) , D-y (1.91-1.53), D-Lu (5.3), D-Sr (0.062-0.008); carbonatite melt-cl inopyroxene (46 kb), D-Ti (0.12), D-Nb (0.22), D-Zr (0.29), D-Ta (0.22 ), D-y (0.27), D-Lu (0.33), D-Sr (0.26), and carbonatite melt-phlogopi te(18 kb), D-Ti (2.1) D-Nb (0.09), D-Zr (0.05), D-Ta (0.21), D-y (<0.0 4), D-Sr (0.056), and D-Rb (4.0). The partition coefficients are consi stent with substitutions involving the main stoichiometric cations Mg, Ca, Fe, and K (phlogopite) in minerals. The effect of different D's a nd the trace element content of metasomatizing carbonatite and silicat e melts will dictate the extent to which trace element ratios may be f ractionated differently in residual mantle mineralogies by these two s tyles of metasomatism. The effect of partitioning differences is quant ified using a model assuming equivalent abundances of incompatible min or and trace elements in silicate and carbonatite melts. This model sh ows that Ti/Na, Ti/Y, Ti/Lu, Ti/Nb, Ti/Ta, and Ti/Sr ratios would be l ower in a peridotite affected by carbonatite metasomatism and therefor e, in silicate melts derived from such a mantle, relative to a source affected by silicate metasomatism. From natural data (peridotite and c arbonatite), element abundances in the metasomatizing carbonatite will also lower these ratios in residual minerals. In addition, the natura l data show that carbonatite metasomatism would also increase LREE/HRE E, LREE/Hf, LREE/Ti, and Sr/Hf ratios in residual mantle minerals.