Experimentally determined trace and minor element partitioning between clinopyroxene and carbonatite melt under upper mantle conditions

Abstract

Laser ablation microprobe analyses of clinopyroxenes equilibrated with carbonatite melt at 20–22 kbar and 1050–1100°C are used to calculate partition coefficients for an extensive suite of trace elements. Our experiments were performed on a mixture of peridotite minerals and two types of carbonates with differing trace element contents (natural carbonatite and mixtures of sedimentary carbonates). Although trace element concentrations vary by an order of magnitude between the carbonate mixtures, the partition coefficients are similar. Some, however, 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 reported here for the first time. Considerably higher partition values are found 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 (1σ). The clinopyroxene partitioning data for carbonatite ( D cpx clq ) are remarkably similar to published values for silicate melts ( D cpx slq ), with significant exceptions for the behaviour of Ti, Ba, Nb and perhaps Ta, whereas values for other silicate minerals differ more strongly between carbonatite and silicate melts. From our results, the most sensitive indicator of 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 studies of metasomatised mantle xenoliths Zr Hf ratios appear to remain unchanged during caatite metasomatism. These chemical effects only partly agree with those described from natural mantle rocks interpreted to result from carbonatite metasomatism.