The Continuum of Primary Carbonatitic-Kimberlitic Melt Compositions in Equilibrium with Lherzolite: Data from the System CaO-MgO-Al2O3-SiO2-CO2 at 6 GPa

Abstract

The compositions of liquids in equilibrium with lherzolite over a range of temperatures (1380–1505°C) above the carbonated lherzolite solidus have been determined in the system CaO–MgO–Al2O3–SiO2–CO2 at 6 GPa. Melt compositions show systematic variation with temperature from carbonatitic (Mg/Ca ratio 1; 5 wt % SiO2) at the solidus (1380°C) through intermediary compositions to kimberlitic (Mg/Ca ratios >2; >25 wt % SiO2) 70–100°C above the solidus. For melting of model lherzolite with a CO2 content of 0.15 wt %, this continuous change in melt composition from carbonatitic to kimberlitic takes place in the melting range 0–1%. Our data are thus consistent with an origin for group 1B kimberlites by low-degree partial melting of carbonated, garnet lherzolite at pressures of at least <10 GPa. Furthermore, the observed carbonatite–kimberlite continuum in melt compositions supports petrogenetic links between carbonatites and kimberlites in the mantle source region by small variations in the melt fraction. Carbonatites are associated with kimberlites in mobile belts adjacent to cratons, such as in the Sarfartoq region in west Greenland. Here, a continuum of rock compositions that range from kimberlite through ultramafic lamprophyres to dolomitic carbonatites is in very good agreement with the experimental data at 6 GPa, consistent with the variations in magma compositions in the Sarfartoq region being produced mainly by variations in the amount of melting at the source. Our data suggest that a similar origin may apply to other carbonatite–kimberlite–ultramafic alkaline rock associations.