The continuum of primary carbonatitic-kimberlitic melt compositions in equilibrium with lherzolite: Data from the system CaO-MgO-Al2O3-SiO2-CO2 at 6 GPa

nle compositions of liquids in equilibrium with lherzolite oner a range of temperatures (1380-1505 degrees C) above the carbonated lherzolite solidus have been determined in the system CaO-MgO-Al2O3-SiO2-CO2 at 6 GPa. Melt co mpositions show systematic variation with temperature from carbonatitic (Mg /Ca ratio 1; 5 wt % SiO2) at the solidus (1380 degrees C) through intermedi ary compositions to kimberlitic (Mg/Ca ratios >2; >25 wt % SiO2) 70-100 deg rees C above the solidus. For melting of model lherzolite with a CO2 conten t of 0.15 wt %; this continuous change in melt composition from carbonatiti c to kimberlitic fakes place in the melting range 0-1%. Our data are thus c onsistent with an origin for group 1B kimberlites by low-degree partial mel ting of carbonated; garnet lherzolite at pressures of at least <10 GPa. Fur thermore, the observed carbonatite-kimberlite continuum in melt composition s supports petrogenetic links between carbonatites and kimberlites in the m antle source region by small variations in the melt fraction. Carbonatites are associated with kimberlites in mobile belts adjacent to cratons, such a s in the Sarfartoq region in west Greenland. Here, a continuum of rock comp ositions that range from kimberlite through ultramafic lamprophyres to dolo mitic carbonatites is in very good agreement with the experimental data at 6 GPa, consistent with the variations in magma compositions in the Sarfarto q region being Produced mainly by variations in the amount of melting at th e source. Our data suggest that a similar origin may apply to other carbona tite-kimberlite-ultramafic alkaline rock associations.