Origin of carbonatites by liquid immiscibility

Abstract

THE existence of carbonatite magmas has been generally accepted1, but their origin remains uncertain. The more favoured petrogenetic models include: (1) direct partial melting of the upper mantle2–5; (2) fractional crystallisation of CO2-rich alkaline silicate magma6; and (3) separation of an immiscible carbonate melt from an initially homogeneous CO2-rich alkaline silicate magma7–10. Experiments have shown all of these processes to be feasible5–7, and each may generate the geochemical characteristics of carbonatite, such as enrichment in rare earths and other incompatible trace and minor elements11,12, and low 87Sr/86Sr ratios13. Here we discuss the role of immiscibility, and report new experimental data which demonstrate for the first time that liquid immiscibility does occur between silicate and carbonate liquids of the compositions found in nature.