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

Abstract

Analysis of silicate minerals equilibrated at high pressure with silicate- (clinopyroxene) and carbonatite-melts (olivine, orthopyroxene, clinopyroxene, 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 carbonatite melt—olivine (18 kb), DTi (<0.05), DNb (0.034), DZr (0.036), DTa (<0.06), DY <0.03), DSr (0.02); carbonatite melt-orthopyroxene (46 kb), DTi (0.13) DNb (<0.01), DZr (0.17), DTa (<0.01), DY (0.18), DLu (0.52), DSr (0.053); carbonatite melt-garnet (34 and 46 kb), DTi (1.2−0.99), DNb (0.086−0.014), DZr (1.42−0.83), DTa (0.14−.051), DY (1.91−1.53), DLu (5.3), DSr (0.062−0.008) ; carbonatite melt-clinopyroxene (46 kb), DTi (0.12), DNb (0.22), DZr (0.29), DTa (0.22), DY (0.27), DLu (0.33), DSr (0.26), and carbonatite melt-phlogopite (18 kb), DTi (2.1) DNb (0.09), DZr (0.05), DTa (0.21), DY (<0.04), DSr (0.056), and DRb (4.0). The partition coefficients are consistent with substitutions involving the main stoichiometric cations Mg, Ca, Fe, and K (phlogopite) in minerals. The effect of different D's and the trace element content of metasomatizing carbonatite and silicate melts will dictate the extent to which trace element ratios may be fractionated differently in residual mantle mineralogies by these two styles of metasomatism. The effect of partitioning differences is quantified using a model assuming equivalent abundances of incompatible minor and trace elements in silicate and carbonatite melts. This model shows that Ti/Na, Ti/Y, Ti/Lu, Ti/Nb, Ti/Ta, and Ti/Sr ratios would be lower in a peridotite affected by carbonatite metasomatism and therefore, in silicate melts derived from such a mantle, relative to a source affected by silicate metasomatism. From natural data (peridotite and carbonatite), element abundances in the metasomatizing carbonatite will also lower these ratios in residual minerals. In addition, the natural data show that carbonatite metasomatism would also increase LREE/HREE, LREE/Hf, LREE/Ti, and Sr/Hf ratios in residual mantle minerals.