EXPERIMENTALLY DETERMINED TRACE AND MINOR ELEMENT PARTITIONING BETWEEN CLINOPYROXENE AND CARBONATITE MELT UNDER UPPER-MANTLE CONDITIONS

Laser ablation microprobe analyses of clinopyroxenes equilibrated with carbonatite melt at 20-22 kbar and 1050-1100 degrees C are used to ca lculate partition coefficients for an extensive suite of trace element s. Our experiments were performed on a mixture of peridotite minerals and two types of carbonates with differing trace element contents (nat ural carbonatite and mixtures of sedimentary carbonates). Although tra ce element concentrations vary by an order of magnitude between the ca rbonate mixtures, the partition coefficients are similar. Some, howeve r, differ substantially from previously published values. Most of the measured elements have broadly similar partition coefficients of D-cpx /clq = 0.1-0.4, including Pr, Nd, Eu, Dy, Er and Hf, which are reporte d here for the first time. Considerably higher partition values are fo und for Ti and V which are compatible in clinopyroxene with D-cpx/lq = 1.4 +/- 0.6 for Ti and 2.9 +/- 0.9 for V (1 sigma). The clinopyroxene partitioning data for carbonatite (D-cpx/clq) are remarkably similar to published values for silicate melts (D-cpx/slq), with significant e xceptions for the behaviour of Ti, Pa, Nb and perhaps Ta, whereas valu es for other silicate minerals differ more strongly between carbonatit e and silicate melts. From our results, the most sensitive indicator o f mantle metasomatism by carbonatite melt as opposed to silicate melt infiltration should be low Ti/Eu in metasomatised peridotites assuming closed-system metasomatism. In contrast to suggestions from some stud ies of metasomatised mantle xenoliths Zr/Hf ratios appear to remain un changed during carbonatite metasomatism. These chemical effects only p artly agree with those described from natural mantle rocks interpreted to result from carbonatite metasomatism.