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 mantle^2–5; (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 magma^7–10. Experiments have shown all of these processes to be feasible^5–7, and each may generate the geochemical characteristics of carbonatite, such as enrichment in rare earths and other incompatible trace and minor elements^11,12, 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.