It has been proposed that the alkali and CO2 enrichment of the melt during fractionation of alkali magma can ultimately result in the immiscibility between a carbonatitic magma and an evolved Si-undersaturated melt. Because alkaline igneous rocks, including carbonatites, are known for their associated critical minerals deposits, it is important to understand how trace elements behave along the liquid line of descent of alkaline magmas.

We investigate the trace element partitioning behavior during the differentiation of strongly silica-undersaturated magmas along two contrasted liquid lines of descent (LLD). One series illustrates the evolution of a Mg-nephelinite toward an alkali-rich phonolite, that is, a LLD similar to the one documented at Oldoinyo Lengai, the only active extrusive carbonatite volcano on Earth. In order to decouple the effect of composition from the effect of temperature, the second series explores the differentiation trend of a melilitite for which the melt SiO2 concentration stays relatively constant. Experiments were conducted at 1 GPa in a piston-cylinder apparatus at CRPG. The starting materials were doped with a series of 30 trace elements expected to have contrasted partitioning behaviors (REE, FRTE, LILE, HFSE) and we used double Au-Pd capsules to limit the loss of iron and volatiles.

Melt and solid phases were analyzed for major and trace elements using an electron microprobe and a laser ablation ICP-MS system. The solid phases along the Mg-nephelinite suite include clinopyroxene, plagioclase, titanite, titanomagnetite, and minor ilmenite, iron oxide and apatite. The melilitite suite includes clinopyroxene, melanite (Ti-rich andradite), spinel, perovskite and apatite. Among these phases, titanite and melanite are major HFSE-sinks. Titanite can also incorporate large amounts of REEs, along with perovskite and apatite. Analyses are ongoing to characterize the various mineral-glass compositional ratios. Our goal is to provide trace element partitioning models applicable along the differentiation trend of silica-undersaturated magmas.