The transition from carbonate to silicate melts in the CaO-MgO-SiO2-CO2 system

The compositions of melts in equilibrium with a lherzolite mineral assembla ge were determined in the analogue system CaO-MgO-SiO2-CO2 at 3 GPa. Carbon ate liquids coexist with olivine and two pyroxenes between the solidus for carbonated Iherzolite at 1250 degrees C and 1450 degrees C. The Ca/(Ca + Mg ) ratio of these melts is 0.64 and the main effects of rising temperature a re increasing SiO2 content (from <4.3 to 7.5 wt %) and decreasing CO2 conte nt. Between 1475 degrees C and similar to 1525 degrees C the SiO2 content o f the liquid Increases dramatically from 10 to 30% and, thereafter the CO2 content decreases rapidly as the CO2-absent invariant poind (at >1700 degre es C) is approached. The progression from carbonate to silicate liquids is, therefore, abrupt and the field of transitional compositions (10-30% SiO2) is restricted to very narrow temperature intervals at pressures greater th an the solidus ledge. All liquids appear to be miscible. In the context of upwelling magma, our results provide possible insight into the origins of c omplexes that are considered to contain primary carbonatites. The solidus ' ledge' between 2.5 and 3 GPa acts as a filter for both carbonatites and tra nsitional melt compositions. Carbonatites, which have a wide stability fiel d at 3 GPa, may rise through the mantle if they are isolated from lherzolit e by wallrock reaction and production of wehrlite. Transitional carbonate-s ilicate melts must also, however, react with the mantle at low pressures. T his fact, combined with the small range of physical conditions over which t hey are generated and their higher (than carbonatite) viscosity: means that they are not reach crustal levels. Low-CO2 silicate melts, in contrast, ar e not required to react extensively en route to the surface and are abundan t. We suggest that the binary nature of some carbonatite complexes may be c ontrolled by the compositions of primary mantle melts produced at pressures greater than the solidus ledge.