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

Abstract

A laser ablation microprobe coupled to an ICP-MS was used to analyse concentrations of K, Rb, Cs, Sr, Ba, Ti, Zr, Hf, Nb, La, Ce, Sm, Ho, Yb and Lu in experimentally produced garnet, clinopyroxene, amphibole and magnesiocarbonatitic melt. The experiment was conducted at 1050 °C and 2.5 GPa on a synthetic composition designed to simulate natural peridotite melting. Crystal/melt partition coefficients determined from the experiment were used to calculate minor and trace element concentrations in a hypothetical carbonatitic melt equilibrated with a garnet-bearing amphibole lherzolite of primitive mantle composition. The melt is strongly enriched in alkaline earths and rare earths relative to high-field-strength elements and alkalis, and plots within the compositional range of natural carbonatites. However, the fractionation of alkaline earths and REE from alkalis and HFSE is usually larger in natural carbonatites than in the calculated composition. The absolute concentrations of many incompatible elements are also much larger in some natural carbonatites. Because of these features, 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.