Experimentally determined partition coefficients for minor and trace elements in peridotite minerals and carbonatitic melt, and their relevance to natural carbonatites

A laser ablation microprobe coupled to an ICP-MS was used to determine K, Rb, Cs, Sr, Ba, Ti, Zr, Hf, Nb, La, Ce, Ho, Yb and Lu in garnet, clinopyroxene, amphibole and magnesiocarbonatitic melt produced at 1050 degrees C and 2.5 GPa from a synthetic composition designed to simulate the melting of natural peridotite. Crystal/melt partition coefficients were then used to calculate minor and trace element contents in a hypothetical carbonatitic melt equilibrated with a garnet-amphibole lherzolite of primitive mantle composition. The melt is strongly enriched in alkaline earths and REE relative to HFSE and alkalis, and plots within the compositional range of natural carbonatites. The fractionation of alkaline earths and REE from alkalis and HFSE, however, is usually larger in natural carbonatites than for the calculated composition. Processes in addition to peridotite melting (such as crystal fractionation, wall rock reactions and variations in source chemistry) are needed to fully account for the minor and trace element characteristics of carbonatites.