Experimental Simulation of Liquid Immiscibility in a Primitive Silicate-Carbonatite System

Despite being the most important source of rare earth elements (REE), the formation of carbonatites as well as the enrichment processes of rare metals in these rocks remain elusive. While direct partial melting from a carbonated mantle source may account for some carbonatite occurrences, most carbonatites are spatially associated with Si-undersaturated, peralkaline magmatic silicates. The latter form either by crystal fractionation from a silicate parental melt, by separation of immiscible carbonatite from a carbonated silicate magma or a combination thereof. Currently, most experimental studies on carbonatite formation focus on evolved or simplified systems. In existing experiments, rare metals partially favour silicate over carbonatite melts, contradicting proposed enrichment processes.

In this study, we report on experiments simulating a primitive silicate-carbonatite system. As a starting material we used a synthetic trace element doped silicate glass, modelled after perovskite-hosted melt inclusions from the Kola Alkaline Province. To generate liquid immiscibility, carbonatite powder was added in varying proportions and run with/without water. The glass-powder mixtures were heated to 1250 °C at 500 MPa in an internally heated pressure vessel. While high proportions of silicate over carbonatite led to the generation of a homogeneous, carbonated silicate glass, liquid immiscibility was reached at proportions of 1:2 (silicate:carbonatite) alongside crystallisation of calcite. Mixing silicate and carbonatite 1:1 with addition of water resulted in the separation of a silicate glass from quench carbonatite, the latter containing idiomorphic perovskite. The high compatibility of REE in perovskite can strongly affect the rare metal partitioning and mobility in the immiscible liquids.