Trace-element partitioning between garnet and clinopyroxene in mantle-derived pyroxenites and eclogites: P-T-X controls

Trace-element abundances in coexisting clinopyroxenes and garnets from five suites of mantle-derived garnet pyroxenite and eclogite xenoliths and two suites of eclogite inclusion pairs in diamonds have been determined by electron microprobe (Ti) and proton microprobe (Ni, Zn, Ga, Sr, Y, Zr). The sample sets provide gamet-clinopyroxene pairs from a range of depths on contrasting geothermal gradients (40-90 mW m^-2) from South Africa, and western and eastern Australia. These data are used to assess the effects of phase composition, pressure (P) and temperature (T) on the partitioning of each element between garnet and clinopyroxene. Ni partitioning is moderately dependent on X_Mg and strongly dependent on T. D_Zn ( Zn_Cpx Zn_Gnt) is ∼ 0.9±0.1 and may show a slight increase with T, but is independent of P. Ga partitioning is strongly dependent on X_Jd ^Cpx as well as P and T. Sr and Y distributions are mainly controlled by X_Ca ^Gnt and are weakly dependent on T but not on P. Zr partitioning is mainly dependent on T; D_Zr decreases with increasing X_Jd ^Cpx and possibly X_Ca ^Gnt but shows no P effect. D_T decreases markedly with increasing P (and possibly T) and has a weak tendency to increase with increasing X_Jd ^Cpx. These results on trace-element partitioning in natural, equilibrated systems provide data for the modelling of melting and metasomatic processes in the upper mantle. They also provide a test dataset to evaluate experimental determinations of partition coefficients, and some relationships (e.g., D_Zr, D_Ni) may be used to cross-check thermobarometry calculations where spurious calculated Fe³⁺ Fe²⁺ ratios may give invalid temperature estimates.