The system CaO-MgO-SiO2-CO2 at 1 GPa, metasomatic wehrlites, and primary carbonatite magmas

New experimental data in CaO-MgO-SiO2-CO2 at 1GPa define the vapor-saturate d silicate-carbonate liquidus field boundary involving primary minerals cal cite, forsterite and diopside. The eutectic reaction for melting of model c alcite (1% MC)-wehrlite at 1 GPa is at 1100 degrees C, with liquid composit ion (by weight) 72% CaCO3 (CC), 9% MgCO3 (MC), and 18% CaMgSi2O6 (Di). Thes e data combined with previous results permit construction of the isotherm-c ontoured vapor-saturated liquidus surface for the calcite/dolomite field, a nd part of the adjacent forsterite and diopside fields. Nearly pure calcite crystals in mantle xenoliths cannot represent equilibrium liquids. We rece ntly determined the complete vapor-saturated liquidus surface between carbo nates and model peridotites at 2.7 GPa; the peritectic reaction for dolomit e (25% MC)-wehrlite at 2.7 GPa occurs at 1300 degrees C, with liquid compos ition 60% CC, 29% MC, and 11% Di. The liquidus field boundaries on these tw o surfaces provide the road-map for interpretation of magmatic processes in various peridotite-CO2 systems at depths between the Moho and about 100 km . Relationships among kimberlites, melilitites, carbonatites and the liquid us phase boundaries are discussed. Experimental data for carbonatite liquid protected by metasomatic wehrlite have been reported. The liquid trends di rectly from dolomitic towards CaCO3 with decreasing pressure. The 1.5 GPa l iquid contains 87% CC and 4% Di, much lower in silicate components than our phase boundary. However, the liquids contain approximately the same CaCO3 (90 +/- 1 wt%) in terms of only carbonate components. For CO2-bearing mantl e, all magmas at depth must pass through initial dolomitic compositions. Ri sing dolomitic carbonatite melt will vesiculate and may erupt as primary ma gmas through cracks from about similar to 70 km. If it percolates through m etasomatic wehrlite from 70 km toward the Moho at 35-40 km, primary calcic siliceous carbonatite magma can be generated with silicate content at least 11-18% (70-40 km) on the silicate-carbonate boundary.